Annals 47, 5, 2004 1597 ANNALS OF GEOPHYSICS, VOL. 47, N. 5, October 2004 Key words seismic attenuation – Southern Italy – Q 1. Introduction The main goal of this study is to find empir- ical attenuation functions that describe the spectral amplitude decay with distance using local and regional events located in Southern Italy. Southern Italy is a complex region, locat- ed at the boundary between the Apulia micro plate and the extensional Tyrrhenian Sea Basin (see e.g., Mantovani et al., 1996). Important volcanic activity has been related to the forma- tion of the Tyrrhenian-Apennine system (Mele et al., 1997). Southern Italy is also a region with high seismic activity. Intermediate and deep seismicity has been associated to the subduct- ing Apulia slab dipping toward the Tyrrhenian Basin (McKenzie, 1972; Gasparini et al., 1982). Shallow crustal seismicity is mainly concentrated along the Apenninic Chain, where normal faults trending NW-SE are separated by transfer zone with strike-slip seismicity (Valen- sise and Pantosti, 2001). Important historical events have also occurred, for instance the 1857 Val d’Agri (M = 7.0), 1980 Irpinia (M = 6.9) earthquakes. The Tyrrhenian region is charac- An attenuation study in Southern Italy using local and regional earthquakes recorded by seismic network of Basilicata Raúl Ramón Castro (1), Maria Rosaria Gallipoli (2) (3) and Marco Mucciarelli (3) (1) CICESE, División Ciencias de la Tierra, Departamento de Sismología, Ensenada, Baja California, México (2) Istituto di Metodologie per l’Analisi Ambientale (IMAA) – CNR, Tito Scalo (PZ), Italy (3) Di.S.G.G., Università degli Studi della Basilicata, Potenza, Italy Abstract We determined a set of empirical functions that describe the spectral amplitude decay of S-waves with distance in Southern Italy. We analyzed 32 earthquakes with magnitudes ML 2.0-5.4 and hypocentral distances ranging between 12 and 216 km. We obtained attenuation functions for 14 frequencies (1.0 < f < 20.0 Hz). We compared these functions with average non-parametric attenuation functions reported by Castro et al. (1999) for different regions of Italy, and we observe that at low frequencies ( f < 5.0 Hz) the spectral amplitudes from earthquakes in Southern Italy decay faster than the average. However, at high frequencies ( f > 5.0 Hz), the spectral amplitudes are above the average. At higher frequencies ( f > 10 Hz), the attenuation functions obtained for Southern Italy are slightly above the standard deviation of the average attenuation functions. It is possible that in this frequen- cy range (10-20 Hz) site effects may influence the amplitude decay. In order to quantify the attenuation of the S-waves, we estimated the quality factor Q modeling the empirical attenuation functions using the following parametric form: A( f , r)=10/r b⋅ e−π f R/Qβ; where 1.6 ≤ f ≤ 10.0 Hz is the frequency band with minimum effect of in- strument and site response, r ≤ 120 km is the distance range where the rate of decay of the spectral amplitudes is approximately constant, R=(r −10) and β = 3.2 km/s. We found that the exponent b =1.0 ± 0.2 in the frequency band analyzed and Q shows a frequency dependence that can be approximated by the function Q = 32.1 f 1.7. Mailing address: Dr. Raúl Ramón Castro, CICESE, Di- visión Ciencias de la Tierra, Departamento de Sismología, km 107 carratera Tijuana-Ensenada, Ensenada, Baja Ca- lifornia, 22860 México; e-mail: raul@cicese.mx 1598 Raúl Ramón Castro, Maria Rosaria Gallipoli and Marco Mucciarelli terized by high S-wave attenuation in the crust and mantle and early P-wave arrivals (Caputo et al., 1972; Del Pezzo et al., 1979). Because reliable characterization of seismic attenuation is important for seismic hazards and risk assessment, we study the characteristics of the spectral amplitude decay of S-waves with distance using small to moderate earthquakes recorded at stations of the Basilicata seismic network of Southern Italy. In a previous study, Castro et al. (1999) ob- tained average Nonparametric Attenuation Func- tions (hereafter referred to as NAF) using seismic records from other regions of Italy. In this paper we will use those NAF as a reference frame to compare the attenuation characteristics in South- ern Italy. 2. Data We analyzed horizontal component records from 32 earthquakes with magnitudes ML = 2 .0- 5.4 recorded by four stations of the Basilicata Table I. Earthquake coordinates and hypocentral distances. No. Date Origin Lat. Long. ML Depth VEN MAT TIT VIL D.M.Yr Hr.Min.S Deg. Deg. km km km km km 2 14.09.01 08.01.49 40.664 15.741 2.0 10 73.00 12.00 34.00 5 20.11.01 18.29.03 41.468 15.676 3.4 11 58.00 120.00 97.00 123.00 6 21.11.01 06.10.40 40.507 15.799 2.8 20 55.00 72.00 23.00 25.00 7 21.11.01 06.20.54 40.520 15.809 2.7 11 69.00 16.00 20.00 8 30.11.01 03.17.37 41.751 15.860 3.4 12 88.00 138.00 129.00 153.00 9 09.12.01 12.15.21 40.786 15.348 2.8 9 41.00 103.00 36.00 12 02.01.02 02.17.08 40.753 15.429 2.8 11 39.00 30.00 52.00 13 15.01.02 00.07.07 40.737 15.801 2.6 7 27.00 69.00 18.00 14 15.01.02 01.29.16 40.734 15.796 2.4 9 27.00 69.00 18.00 19 06.04.02 06.35.23 39.224 16.544 2.9 5 201.00 159.00 139.00 20 05.04.02 14.54.07 39.144 16.804 3.8 10 216.00 168.00 156.00 22 13.04.02 17.04.12 40.569 16.394 3.3 6 62.00 26.00 51.00 50.00 23 18.04.02 20.56.47 40.612 15.594 4.1 5 43.00 84.00 11.00 31.00 24 18.04.02 21.00.34 40.583 15.516 3.2 9 49.00 18.00 33.00 25 18.04.02 21.36.52 40.569 15.524 3.1 5 50.00 16.00 31.00 26 18.04.02 22.58.31 40.577 15.547 3.0 7 48.00 87.00 18.00 31.00 27 18.04.02 23.19.06 40.572 15.524 3.1 8 50.00 15.00 32.00 29 21.04.02 23.39.19 40.569 15.580 3.5 10 85.00 15.00 28.00 36 31.10.02 11.34.42 41.760 14.940 5.4 10 110.00 181.00 142.00 167.00 37 31.10.02 11.56.22 41.710 14.825 3.5 10 112.00 184.00 167.00 38 31.10.02 13.04.11 41.670 14.890 3.7 10 105.00 177.00 134.00 160.00 41 01.11.02 15.09.15 41.690 14.830 5.3 10 109.00 184.00 139.00 164.00 42 01.11.02 15.20.04 41.740 14.850 4.1 10 113.00 143.00 168.00 43 01.11.02 15.42.23 41.720 14.840 3.4 10 111.00 184.00 166.00 45 01.11.02 17.21.15 41.710 14.810 4.3 10 112.00 141.00 166.00 46 01.11.02 22.30.26 41.720 14.850 3.1 10 111.00 183.00 166.00 47 01.11.02 22.44.13 41.690 14.800 3.8 10 111.00 184.00 140.00 165.00 48 02.11.02 02.36.27 41.720 14.810 3.7 10 113.00 186.00 167.00 49 02.11.02 06.22.06 41.690 14.810 3.8 10 110.00 183.00 164.00 50 03.02.03 11.24.24 40.760 15.538 3.1 10 33.00 89.00 25.00 48.00 51 03.02.03 12.18.39 40.760 15.538 2.7 10 33.00 89.00 25.00 48.00 52 04.02.03 08.31.33 40.740 16.240 2.5 10 41.00 38.00 43.00 54.00 1599 An attenuation study in Southern Italy using local and regional earthquakes recorded by the seismic network of Basilicata seismic network. The network was set up by the Institute for Methodology of Environmental Analysis of the National Research Council and the Department of Structure, Soil Mechanics and Engineering Geology of the University of Basilicata in the framework of a multiparamet- ric geophysical observatory (Balasco et al., 2001). All the stations are equipped with 1 Hz tridirectional seismometers (Lennartz 3d Lite at VEN, VIL, MAT and Mark L4C at TIT) con- nected to 24 bit A/D converter (PRAXS) with an instrument response approximately constant between 1 and 25 Hz. Table I lists the epicentral coordinates of the events used and the hypocentral distances to the recording stations. The distribution of the epi- centers and the location of the seismic stations are also displayed in fig. 1. The source-station paths cover mainly the southern end of the Apennines and northeast of Calabria, Italy. The Fig. 1. Location of earthquakes and seismic stations used. The circles with identification number are the epi- centers (see also table I) and the triangles are the location of the stations. We also draw the source-station paths. Fig. 2. Hypocentral distance versus magnitude dis- tribution of velocity records used. 1600 Raúl Ramón Castro, Maria Rosaria Gallipoli and Marco Mucciarelli Fig. 3. Examples of acceleration spectra and the frequency band used. The frequency band used for further analysis is shown with brackets. tion spectra were instrument corrected and smoothed around 14 pre-selected frequencies (1.0 < f < 20 Hz). We visually selected, for each spectral record, the frequency band with the highest signal to noise ratio. The frequency at which the spectra became flat (in a semi-log- arithmetic plot) was chosen as the end of the frequency band. The same procedure was used to estimate NAF in other regions of Italy (Cas- tro et al., 1999). Figure 3 shows four examples of the frequency window selected for further analysis. For most of the records the spectral amplitude flattens beyond 25 Hz. Figure 4 displays velocity records from an earthquake M = 3.0 used in the analyses. This figure shows the time windows selected to cal- culate the acceleration spectra of pre-event noise and S-wave signal as well. In general, the signal to noise ratio is at least a factor of 10 be- tween 1-10 Hz. At higher frequencies the ratio remains high for stations VIL and MAT, and for stations TIT and VEN the spectral amplitude of the noise approaches that of the S-wave at f > 18 hypocentral coordinates were taken from the Seismic Bulletin of the Italian National Institute of Geophysics and Volcanology (INGV). Events 36 to 49 in table I are events of the Molise seismic sequence of 2002 that caused damage in San Giuliano (Mucciarelli et al., 2003a). In particular, events 36 and 41 are the main events of the sequence that generated in- tensity up to VIII-IX MCS in San Giuliano. Ac- cording to the information reported by the IN- GV Bulletin the focal depth of these events and events 50-52 was fixed at 10 km. The distance- magnitude distribution of the records, plotted in fig. 2, shows that most of the data is in the ranges of M < 4.0 and r < 180 km. The records were base line corrected sub- tracting the average of all the points of the record. Then we calculated the acceleration spectra using a time window containing the first S-wave arrivals. The beginning of the window was chosen at the first S arrival and the end when 80 % of the total energy was reached, the window was also cosine tapered. The accelera- 1601 An attenuation study in Southern Italy using local and regional earthquakes recorded by the seismic network of Basilicata Hz. We can infer from the spectral inspection of the spectral records that the quality of the data set is very good up to 10 Hz for all the stations used and up to 25 Hz for VIL and MAT. 3. Method We model the spectral amplitude decay with hypocentral distance r for each sampled frequen- cy using a nonparametric approach. We assumed that the observed spectral amplitude Ui(r, f ) at frequency f depends on the size of the earthquake that can be represented by a scalar Si, and the at- tenuation due to geometrical spreading and anelasticity of the medium. Thus, we can write Ui(r, f ) = Si ⋅ A(r, f ). (3.1) In eq. (3.1) the attenuation function A(r, f ) is the same for all source-station paths and is not limited to have a particular functional form. The site effects are eliminated using a smooth- ing constraint and by making A(0, f ) = 1.0, since at r = 0 only the source term controls the value of the spectral amplitude. We calculated inde- pendent inversions for 14 different frequencies between 1.0 and 20 Hz. The distance range Fig. 4. The frames on the left show the velocity records from event 26 as recorded for the four stations used. The time series are order from minimum distance (top frame) to maximum hypocentral distance. It is also shown the window use to calculate the Fourier transform. The frames on the right are the corresponding acceleration spectra of the S-wave signal (solid line) and the pre-event noise (dashed lines). 1602 Raúl Ramón Castro, Maria Rosaria Gallipoli and Marco Mucciarelli from 5 km to 200 km was discretized into 40 bins, 5 km wide. Anderson and Quaas (1988) and Castro et al. (1990, 1999) used this non- parametric model before to describe the ampli- tude decay in other regions. 4. Results Figure 5 shows a sample of 6 attenuation functions obtained between 1 and 20 Hz. For comparison we also plotted with different sym- bols the observed amplitudes, circles for event 5 (M = 3.4) and asterisks for event 8 (M = 3.4). The curves shown are scaled accordingly with the corresponding value of Si. The functions ob- tained show a change in the rate of amplitude decay at r > 100 km probably due to the effect of geometrical spreading, which for that dis- tance range is less severe. It is also noticeable that at f > 10 Hz the scatter of the data tends to increase, perhaps as a result of site effects. For some of the sites used, Mucciarelli et al. (2003b) observed that site effect is a remark- able and stable feature. At high frequencies ( f > 10 Hz) the near-site attenuation (κ 0) can al- so explain the observed scatter. In order to evaluate how the NAF of South- ern Italy compare with those for other regions of Italy, we plot them in fig. 6 (with circles) to- gether with the average attenuation functions obtained by Castro et al. (1999) (solid lines) ± 1 standard deviation (dashed lines). The NAF reported for Castro et al. (1999) were obtained averaging attenuation functions from the re- gions of Lombardy, Piedmont, Eastern Sicily, Friuli and Marche. At 1 Hz the NAF of South- ern Italy (circles) are slightly below the stan- dard deviation of the average, but in general at low frequencies ( f < 5Hz) the attenuation is stronger than the average. At high frequencies (f > 5 Hz) attenuation in Southern Italy is minor than the average, and at higher frequencies (f > 10 Hz) the NAF are slightly above the stan- dard deviation of the average. The frequency dependence of the attenua- tion can be appreciated in fig. 7, where we com- pared NAF between 2 and 10 Hz. It can be seen in this figure that for a given distance r the S- wave attenuation factor tends to decrease with frequency. The NAF obtained can be also quantified in terms of the quality factor Q and the geometri- cal spreading. For this purpose, we can parame- Fig. 5. Nonparametric attenuation functions obtained (solid lines) and observed spectral amplitudes from event 5 (circles) and from event 8 (asterisks). 1603 An attenuation study in Southern Italy using local and regional earthquakes recorded by the seismic network of Basilicata Fig. 6. Average NAF (solid lines) and ± 1 standard deviation (dashed lines) obtained by Castro et al. (1999) from different regions of Italy and the attenuation functions obtained for Southern Italy (this study). terize the attenuation functions as ( ) .A r r e 10 / b fR Q $= -r b ,f (4.1) Where G(r) = 10/r b accounts for the geometri- cal spreading, R = (r −10), Q is the quality fac- tor of the S-waves and β = 3 . 2 km/s. Note that eq. (4.1) is normalized at 10 km, because we do not have information at closer hypocentral dis- tances (see fig. 2). We only considered the NAF at frequencies where the instrument response is constant ( f ≥ 1.6 Hz) and the specific near-sur- face attenuation at each site is less severe ( f ≤ 10 Hz). We also used only the distance 1604 Raúl Ramón Castro, Maria Rosaria Gallipoli and Marco Mucciarelli Where a (r) = Log A( f, r) – Log G(r) and m = π f⋅ ⋅ Log e/Qβ. For each frequency f, we searched for values of b and m that give the best least- squares solution of eq. (4.2) (e.g., Castro et al., 2003). Figure 8 and table II show the resulting val- ues of the exponent b and Q for the frequency band analyzed (1.6 ≤ f ≤ 10.0 Hz). The values of Q (triangles in fig. 8) show stronger frequency dependence than the exponent b of the geomet- rical spreading function. For comparison, we also plotted in fig. 8 (asterisks) recent estimates of coda Q (Qc) obtained by Bianco et al. (2002) for the Southern Apennine zone. Between 3 Hz and 5 Hz the values of Qc are similar to those obtained using S-waves (triangles); at higher frequencies ( f >5 Hz) Qc has lower values (56- 59 % lower). b is approximately 1.0 ± 0.2, so that G(r) ≈ 1/r, as is expected for body waves. The following relation can approximate the fre- quency dependence of Q: Q = 32.1 f 1.7. (4.3) Because of the trade-off between Q and the geometrical spreading function, it is more meaningful to compare the combined effect of Q and G(r) together. In fig. 9 we compare the nonparametric attenuation functions (open cir- cles) with the spectral amplitude decay predict- ed by eq. (4.1) using the values listed in table II range where the effect of site response and re- flected waves is minimum (r ≤ 120 km). We linearized eq. (4.1) by taking loga- rithms, thus we can write, a(r) = − mr. (4.2) Fig. 7. A sample of NAF obtained for frequencies between 2 and 10 Hz. Fig. 8. Left frame shows the values of the exponent b obtained for the geometrical spreading function assumed (see eq. (4.1)). The right frame shows the estimates of the quality factor Q- of the S-waves (triangles) as func- tion of frequency. The asterisks are the values of coda Q reported by Bianco et al. (2002) for the Southern Apen- nine zone. 1605 An attenuation study in Southern Italy using local and regional earthquakes recorded by the seismic network of Basilicata Fig. 9. Parametric Attenuation Functions (PAF) reported in different regions of Italy. The nonparametric functions obtained in this study are plotted with open circles. The solid lines are the attenuation functions calculated with eq. (4.1) using the values listed in table III. The dashed lines are the PAF calculated with the Q and G(r) obtained by Malagnini et al. (2000) (see table III). The dotted curves are the PAF reported by Rovelli et al. (1988) (eq. (4.4)).The asterisks are the PAF of Castro et al. (2003b). The squares and the tri- angles are the PGA and PGV attenuation functions, respectively, of Sabetta and Pugliese (1987) (eqs. (4.5) and (4.6)). Table II. Resulting parameters of eq. (4.1). f (Hz) b Q 1.6 1.2 80 2.0 1.2 118 2.5 1.1 121 3.2 1.0 188 4.0 1.0 302 5.0 1.0 413 6.3 0.9 643 7.9 0.9 1112 10.0 0.9 1608 (solid line in fig. 9). In this figure we also com- pare other parametric relations reported in the literature and that describe the amplitude decay with distance of S-waves (see also table III). In table III we did not list estimates of coda Q, as those reported by Del Pezzo and Zollo (1984), Del Pezzo and Scarcella (1986), Bindi et al. (2001) and others, but those are compared and discussed by Castro et al. (2002). The dotted lines in fig. 9 correspond to the amplitude decay calculated using the attenuation model of Rov- elli et al. (1988) , .exp expA f r r f Q r1 0 0 $ $= - -rl b r ^ ^ eh h o (4.4) Where κ 0 = 0.07, β = 3.2 km/s, and Q0 = 100. The dashed lines correspond to the attenuation curves proposed by Malagnini et al. (2000) for the entire Apennines (see table III). This curves are characterized for having significant discon- tinuities at 30 and 80 km that are possibly relat- ed to the complex crustal structure of the Apen- nines. It is interesting to note that at 3.2 Hz and r > 80 km the attenuation curves of Malagnini et al. (2000) are the same as those calculated 1606 Raúl Ramón Castro, Maria Rosaria Gallipoli and Marco Mucciarelli with eq. (4.1). The asterisks in fig. 9 are the at- tenuation functions calculated using the rela- tion obtained by Castro et al. (2004) for Central Italy, namely Q = 31.2 f 1.2 in the frequency band of 0.3-9.5 Hz and G(r) = 1/r. These curves are similar to those calculated with the model of Rovelli et al. (1988) at short distances (r < 40 km) in the frequency band of 1.6-3.2 Hz. It is important to keep in mind, when comparing these attenuation functions, that the data used to obtain them sampled not only different frequen- cy bands but also different crustal volumes. For instance, while the data set used by Malagnini et al. (2000) sampled the entire Apennines, our re- sults are only valid for Southern Italy. Because these attenuation functions may have an important impact in the estimation of Peak Ground Acceleration (PGA) and Peak Ground Velocity (PGV), we also compare the empirical relations obtained by Sabetta and Pugliese (1987) to estimate PGA and PGV in Italy from magnitude (M), site type (S = 0 for rock and S=1 for soil) and the closest distance to surface projection of the fault rupture (R) LogPGA= − 1.562 − Log(R2+5.82)1/2+ 0.169 S + + 0.306 M (4.5) LogPGV= − 0.710 − Log(R2+3.62)1/2+ 0.133 S + + 0.455 M. (4.6) We plotted in fig. 9 the first 3 terms of eqs. (4.5) and (4.6), with triangles and squares respective- ly, using both S = 1 and S = 0. The PGA relation (triangles) gives higher attenuation factors at low frequencies relative to the other curves. However, at 10 Hz and r > 80 km the PGA curve approaches the moel of Rovelli et al. (1988). On the other hand, the PGV curve approaches the model of Rovelli et al. (1988) between 3.2 and 5.0 Hz. It is important to point out that Sa- betta and Pugliese (1987) and Rovelli et al. (1988) used similar data sets, namely, records from the earthquakes of Valerina, 1979; Irpinia, 1980; Umbria, 1984 and Lazio-Abruzzo, 1984. Sabetta and Pugliese (1987) also included events from the regions of Friuli and Sicily. The result of comparing the parametric at- tenuation functions reported (fig. 9 and table III) indicates that a significant spatial variabil- ity of the attenuation parameters Q and G(r) must exist in Italy. This variability must be the result of a heterogeneous complex crustal structure. Spatial variability of Q can be signif- icant, particularly near seismogenic zones (Castro et al., 2000). Li et al. (1997), for in- stance, found a reduction of S-wave velocity (30-50%) and Q within tens of meters of the fault trace at Parkfield, California. Changes in the geologic characteristics of the rocks are al- so responsible for the spatial variability of Q. Thus, we can expect different behavior of Q and G(r) for different S-wave propagation crustal volumes. Table III. S-wave attenuation relations reported for Italy. Region Distance Frequency Q Geometrical Reference range (km) range (Hz) spreading Central and r ≤ 100 0.1≤ f ≤ 20. Q =100 r 1 Rovelli et al. (1988) Central-Southern and κ 0 = 0.07 Apennines Entire Apennines r ≤ 400 0.24≤ f ≤ 5.0 Q = 130 f 0.1 r − 0.9, r ≤ 30 km Malagnini et al. (2000) r 0, 30 ≤ r ≤ 80 km r − 0.5, r ≥ 80 km Umbria-Marche r ≤ 100 0.3≤ f ≤ 9.5 Q = 31.2 f 1.2 r 1 Castro et al. (2003a) Southern Italy r ≤ 120 1.6≤ f ≤ 10 Q = 32.1 f 1.7 r 10 b , b = 1.0 ± 0.2 This study 1607 An attenuation study in Southern Italy using local and regional earthquakes recorded by the seismic network of Basilicata 5. Conclusions We determined nonparametric attenuation functions for 14 frequencies (1.0 < f < 20.0 Hz). At low frequencies (f < 5.0 Hz) these functions show a faster decay than those obtained by Cas- tro et al. (1999) averaging NAF from different regions of Italy. However, at intermediate fre- quencies (f > 5.0 Hz) the spectral amplitudes are above the average, and at higher frequencies (f > 10 Hz) the attenuation functions obtained for Southern Italy are slightly above the stan- dard deviation of the average functions. The da- ta set used in this frequency band (f > 10 Hz) may be affected by stronger site effects than ex- pected, in particular near-surface attenuation may have a strong effect on the spectral ampli- tudes at high frequencies. We also estimated the quality factor Q of the S-waves using a parametric approach and we found that Q shows a frequency dependence that can be approximated by the function Q = 32.1 f 1.7, (1.6 ≤ f ≤ 10.0 Hz) and G(r) =10/r b, (b = 1.0 ± 0.2). This parametric model of S-wave attenuation shows significant differences with respect to attenuation models reported for neighboring regions (Sabetta and Pugliese, 1987; Rovelli et al., 1988; Malagnini et al., 2000). This indicates that a significant spatial variability of the attenuation parameters Q and G(r) exists in Italy. Further developments of this work will in- clude separation of ray-paths crossing mainly the Apennines and those crossing the Apulian plate. When enough data are available, it will be interesting to verify whether the Apulian plate is less attenuating than the Apennines, as geo- dynamic models predict. Acknowledgements The Basilicata network was set up and maintained thanks to grants of the Italian Min- istry for University and Research. Many thanks to Dr. Enzo Lapenna from the IMAA-CNR for his constant help. We also acknowledge Luca Malagnini and the anonymous reviewer for their comments and suggestions. Luis Inzunza help us preparing the figures. REFERENCES ANDERSON, J.G. and R. QUAAS (1988): The Mexico earth- quake of September 19, 1985: effect of magnitude on the character of strong ground motion: an example from the Guerrero, Mexico strong motion network, Earthquake Spectra, 4, 635-646. BALASCO, M., D. CHIANESE, V. CUOMO, G. DI BELLO, M.R. GALLIPOLI, V. LAPENNA and M. MUCCIARELLI (2001): De- sign of a new prototype of remote station to detect elec- trical and seismometric parameters in a seismic area of Southern Italy, Phys. Chem. Earth, 26 (10/12), 787-791. BIANCO, F., E. DEL PEZZO, M. CASTELLANO, J. IBANES and F. DI LUCCIO (2002): Separation of intrinsic and scat- tering seismic attenuation in the Southern Apennine zone, Italy, Geophys. J. Int., 150, 10-22. BINDI, D., D. SPALLAROSSA, P. AUGLIERA and M. CATTANEO (2001): Source parameters estimated from the after- shocks of the 1997 Umbria-Marche (Italy) seismic se- quence, Bull. Seismol. Soc. Am., 91, 448-455. CAPUTO, M., G.F. PANZA and D. POSTPISCHL (1972): New evidences about deep structure of the Lipari Arc, Tectonophysics, 15, 219-231. CASTRO, R.R., J.G. ANDERSON and S.K. SINGH (1990): Site response, attenuation and source spectra of S-waves along the Guerrero, Mexico, subduction zone, Bull. Seismol. Soc. Am., 80, 1481-1503. CASTRO, R.R., M. MUCCIARELLI, G. MONACHESI, F. PACOR and R. BERARDI (1999): A review of nonparametric at- tenuation functions computed for different regions of Italy, Ann. Geofis., 42 (4), 735-748. CASTRO, R.R., L. TROJANI, G. MONACHESI, M. MUCCIAREL- LI and M. CATTANEO (2000): The spectral decay param- eter in the region of Umbria-Marche, Italy, J. Geophys. Res., 105, 23,811-23,823. CASTRO, R.R., M. MONACHESI, L. TROJANI, M. MUCCIAREL- LI and M. FRAPICCINI (2002): An attenuation study us- ing earthquakes from the 1997 Umbria-Marche se- quence, J. Seismol., 6, 43-59. CASTRO, R.R., H. FABRIOL, M. BOUR and B. LE BRUN (2003): Attenuation and site effects in the region of Guade- loupe, Lesser Antilles, Bull. Seismol. Soc. Am., 93 (2), 612-626. CASTRO, R.R., F. PACOR, D. BINDI, G. FRANCESCHINA and L. LUZI (2004): Site response of strong motion stations in the Umbria, Central Italy, Region, Bull. Seismol. Soc. Am., 94 (2), 576-590. DEL PEZZO, E. and G. SCARCELLA (1986): Three component coda Q in the Abruzzi-Molise region, Central Ap- penines, Ann. Geophysicae, 5, 589-592. DEL PEZZO, E. and A. ZOLLO (1984): Attenuation of coda waves and turbidity coefficient in Central Italy, Bull. Seismol. Soc. Am., 74, 2665-2659. DEL PEZZO, E., G. LUONGO and R. SCARPA (1979): Seismic wave transmission in Southern Tyrrhenian Sea, Boll. Geofis. Teor. Appl., 21, 53-65. GASPARINI, C., G. IANNACCONE, P. SCANDONE and R. SCARPA (1982): Seismotectonics of the Calabrian Arc, Tectono- physics, 84, 267-286. LI, Y.G., W.L. ELLSWORTH, C.H. THURBER, P.E. MALIN and K. AKI (1997): Fault-zone guided waves from explosions in the San Andreas Fault at Parkfield and Cienega Valley, California, Bull. Seismol. Soc. Am., 87, 210-221. 1608 Raúl Ramón Castro, Maria Rosaria Gallipoli and Marco Mucciarelli MALAGNINI, L., R.B. HERRMANN and M. DI BONA (2000): Ground-motion scaling in the Apennines (Italy), Bull. Seismol. Soc. Am., 90 (4), 1062-1081. MANTOVANI, E., D. ALBARELLO, C. TAMBURELLI and D. BABBUCCI (1996): Evolution of Tyrrhenian Basin and surrounding regions as a result of the Africa-Eurasia convergence, J. Geodyn., 21 (1), 35-72. MCKENZIE, D. (1972): Active tectonics of the Mediterranean region, Geophys. J. R. Astron. Soc., 30, 109-185. MELE, G., A. ROVELLI, D. SEBER and M. BARAZANGI (1997): Shear wave attenuation in the lithosphere beneath Italy and surrounding regions: tectonic implications, J. Geo- phys. Res., 102, 11, 863-11,875. MUCCIARELLI, M., A. MASI, M. VONA, M.R. GALLIPOLI, P. HARABAGLIA, R. CAPUTO, S. PISCITELLI, E. RIZZO, M. PICOZZI, D. ALBARELLO and C. LIZZA (2003a): Quick survey of the possible causes of damage enhancement observed in San Giuliano after the 2002 Molise, Italy seismic sequence, J. Earthquake Eng., 7, 1-17. MUCCIARELLI, M., M.R. GALLIPOLI and M. ARCIERI (2003b): Stability of horizontal-to-vertical spectral ra- tio by triggered noise and earthquake recordings, Bull. Seismol. Soc. Am., 93 (3), 1407-1412. ROVELLI, A., O. BONAMASSA, M. COCCO, M. DI BONA and S. MAZZA (1988): Scaling laws and spectral parameters of the ground motion in active extensional areas in Italy, Bull. Seismol. Soc. Am., 78 (2), 530-560. SABETTA, F., and A. PUGLIESE (1987): Attenuation of peak horizontal acceleration and velocity from Italian strong-motion records, Bull. Seismol. Soc. Am., 77 (5), 1491-1511. VALENSISE, G. and D. PANTOSTI (2001): The investigation of potential earthquake sources in peninsular Italy: a re- view, J. Seismol., 5, 287-306. (received October 13, 2003; accepted February 12, 2004)