Vol52,2,2009 137 ANNALS OF GEOPHYSICS, VOL. 52, N. 2, April 2009 Key words LP – Seismology waveform correlation – source geometry – Volcano Sismology – Volcano Monitoring 1. Introduction The island of Vulcano (500 m a.s.l.) is a composite volcanic edifice located in the south- central sector of the Aeolian Archipelago (Tyrrhenian Sea, Italy). The island, together with Lipari and Salina Islands (fig. 1), repre- sents the emerged part of the Tindari-Letojanni system (TL), a NW-SE elongated volcanic ridge affected by a right-lateral strike slip, mov- ing in response to a N100E regional extension field (Mazzuoli et al., 1995). If the overall complex is mainly controlled by the TL system, the northern sector of Vul- cano island is characterized by NE-SW and N-S trending normal structures which accommodate the horizontal movements of the main system (fig. 1) (Mazzuoli et al., 1995). Along these two oblique trends are aligned the primary (dikes, vents and eruptive fissures) volcanic structures (Mazzuoli et al., 1995; Ventura et al., 1999). Recent eruptions on the island have taken place at Vulcanello (1550) and La Fossa crater (1888-1890), with volcanic products consisting mainly of pyroclastic material with lesser vol- umes of lava flows. La Fossa is a 391 m high cone with a base diameter of 1 km whose his- toric activity has been characterized by frequent transitions from phreatomagmatic to minor magmatic activity. After the last eruptive episode (1888-1890), volcanic activity has been restricted to fumarolic High precision locations of long-period events at La Fossa Crater (Vulcano Island, Italy) Salvatore Gambino, Laura Cammarata and Salvatore Rapisarda Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Italy Abstract Since the last eruption in 1888-90, the volcanic activity on Vulcano Island (Aeolian Archipelago, Italy) has been limited to fumarolic degassing. Fumaroles are mainly concentred at the active cone of La Fossa in the northern sector of the island and are periodically characterized by increases in temperature as well as in the amount of both CO2 and He. Seismic background activity at Vulcano is dominated by micro-seismicity originating at shal- low depth (<1-1.5 km) under La Fossa cone. This seismicity is related to geothermal system processes and com- prises long period (LP) events. LPs are generally considered as the resonance of a fluid-filled volume in response to a trigger. We analyzed LP events recorded during an anomalous degassing period (August-October 2006) ap- plying a high precision technique to define the shape of the trigger source. Absolute and high precision locations suggest that LP events recorded at Vulcano during 2006 were produced by a shallow focal zone ca. 200 m long, 40 m wide and N30-40E oriented. Their occurrence is linked to magmatic fluid inputs that by modifying the hy- drothermal system cause excitation of a fluid-filled cavity. Mailing address: Dr. Salvatore Gambino, Istituto Na- zionale di Geofisica e Vulcanologia (INGV), Sezione di Ca- tania, P.zza Roma 2, 95123 Catania, Italy; e-mail: gambi- no@ct.ingv.it Vol52,2,2009 17-06-2009 19:02 Pagina 137 138 S. Gambino, L. Cammarata and S. Rapisarda degassing, mainly at La Fossa with mean tem- peratures ranging between 200° and 300° C. The origin of the geothermal system at Vul- cano has been discussed by several authors using different approaches and chemical tracers (e.g. Chiodini et al., 1995; Todesco, 1997; Capasso et al., 1999; Paonita et al., 2002 Gambino and Guglielmino, 2008); there is a general agreement that the fumarole gas composition results from the mixing of a deeper magmatic source and a shallow boiling hydrothermal system. La Fossa crater is characterized by the oc- currence of periodical anomalous degassing episodes with increasing output of the fumarole and chemical changes caused by new magmat- ic fluid inputs (e.g. Granieri et al., 2006; Paoni- ta et al., 2002; Chiodini et al., 1995) from a deep pressurized stationary magma body (Granieri et al., 2006; Gambino et al., 2007). Earthquakes occurring in the area of Vul- cano are associated with both fracturing (spo- radic swarms of low magnitude shocks) (Aubert and Alparone, 2000) and degassing processes of the geothermal system (Montalto, 1994). La Fossa shallow (<1-1.5 km) micro- seismicity is linked to hydrothermal activities at Vulcano Island and an increase in its occur- rence accompanies periods of anomalous de- gassing (Chiodini et al., 1992). Microseismic background activity comprises two main groups of events: M-type and N-type, as exten- sively discussed by Montalto (1994). Long-pe- Fig. 1. Map of the Vulcano permanent and temporary seismic network. Structural lineaments from Mazzuoli et al., (1995). Vol52,2,2009 17-06-2009 19:02 Pagina 138 139 High precision locations of long-period events at La Fossa Crater (Vulcano Island, Italy) riod (LP) events at Vulcano have been reported by Godano and Vilardo (1991) who using 1987- 1988 data recorded by a temporary network lo- cated this seismicity on the SE part of the cone. LPs are generally related to vibrations and/or volumetric changes of a fluid-filled res- onator, in which the fluid has a hydrothermal or magmatic origin (Chouet, 1996). The shape of a resonant structure may vary in relation to the feeding/hydrothermal system features; precise hypocentre locations may help to image this shape. Table I. Hypoellipse location parameters of the analyzed earthquakes. n Date Origin time Latitude Long. Depth GAP No RMS ERZ ERH km b.s.l. km km 1 060809 10:23:44.66 38.4042 14.9702 0.52 174 7 0.07 0.2 0.7 2 060821 01:56:39.88 38.4078 14.9652 0.56 190 6 0.04 0.2 0.5 3 060823 06:12:29.89 38.4028 14.9713 0.49 167 7 0.14 0.2 0.8 4 060827 02:54:44.49 38.4023 14.9695 0.72 156 7 0.12 0.2 0.8 5 060828 03:04:10.34 38.4023 14.9643 0.84 123 7 0.06 0.3 0.6 6 060901 01:23:54.54 38.4030 14.9663 0.78 144 6 0.03 0.3 0.8 7 060901 21:22:11.80 38.4030 14.9653 0.75 138 7 0.14 0.3 0.6 8 060903 00:11:57.58 38.4005 14.9672 0.77 132 7 0.14 0.3 0.6 9 060905 05:57:41.62 38.4043 14.9677 0.93 163 6 0.03 0.3 0.8 10 060905 17:40:14.68 38.4008 14.9683 0.76 139 7 0.03 0.3 0.6 11 060906 23:07:43.17 38.4023 14.9685 0.66 151 7 0.04 0.2 0.7 12 060907 05:53:49.72 38.4030 14.9643 0.76 129 7 0.08 0.3 0.6 13 060909 15:56:09.24 38.4043 14.9628 0.78 118 7 0.06 0.2 0.5 14 060910 23:51:42.95 38.4065 14.9627 0.74 133 6 0.02 0.1 0.6 15 060919 20:02:26.14 38.4018 14.9682 0.83 146 6 0.01 0.4 0.9 16 060921 03:29:21.70 38.4077 14.9655 0.60 189 7 0.22 0.1 0.5 17 060921 05:32:08.16 38.4028 14.9713 0.54 167 7 0.11 0.2 0.8 18 060929 20:56:29.57 38.4052 14.9647 0.70 150 7 0.08 0.2 0.5 19 061006 00:37:20.09 38.4038 14.9673 0.70 201 6 0.02 0.5 0.8 20 061011 02:09:47.41 38.4020 14.9647 0.91 124 7 0.06 0.3 0.6 21 061013 04:17:08.26 38.4060 14.9612 0.73 112 6 0.07 0.2 0.6 GAP = azimuthal gap (degrees); RMS = travel-time residual root mean square (s); No = number of P arrivals; ERZ, ERH = vertical and horizontal location error. Vol52,2,2009 17-06-2009 19:02 Pagina 139 140 S. Gambino, L. Cammarata and S. Rapisarda Vulcano long periods events are character- ized by nearly identical waveforms (multi- plets); the application of cross-spectral/cross- correlation techniques on multiplets enables obtaining precise relative locations with accura- cy within 5-20 m (e.g. Poupinet et al., 1984; Haase et al., 1995). Concerning tectonic events, these techniques help in reconstructing the seismogenic fault plane (e.g. Alparone and Gambino, 2003) while relocation of LPs does not define fault planes but rather volumes (Battaglia et al., 2003). LP precise relocations have been performed on Ki- lauea Volcano (e.g. Battaglia et al., 2003; Wolfe et. al., 2003), Soufrière Hills Volcano Montser- rat (Rowe et al., 2004; Green and Neuberg, 2006) and Mt. Etna (Gambino, 2006). This paper considers 21 LP multiplets by in- tegrating permanent and temporary seismic sta- tion data recorded during the anomalous de- gassing period August-October 2006. We ap- plied a cross-spectrum technique derived from Fig. 2 a-c. Occurrence of LP during 2006 (a), waveforms (b) and spectra (c) of the events analyzed (and relo- cated). Each event number is referred to tab. I. Vol52,2,2009 17-06-2009 19:02 Pagina 140 141 High precision locations of long-period events at La Fossa Crater (Vulcano Island, Italy) Frèmont and Malone (1987) to a subset of 14 long-period events, located on SE part of La Fossa crater at Vulcano. The aim of this paper is to obtain a picture of the source volume shape for these earthquakes. 2. Data Since the late 70’s, continuous seismic mon- itoring activity on Vulcano has been performed by a permanent seismic network composed of four analogical 3C (three components) stations (fig. 1). Recently, new digital 24-bit seismic sta- tions equipped with broad-band (0.2-40 sec) Nanometrics Trillium instruments have been in- stalled at 3 of the 4 stations of the permanent net- work. In addition, at the end of 2005, three tem- porary broadband stations were installed on the northern rim of the volcano crater (La Fossa). During 2006, we recorded about 65 LP events with similar waveforms, 44 of which oc- curred during August-October (fig. 2a). We fo- cus our analyses on this period in which an in- creasing release of CO2-He rich magmatic fluids was recorded at the rim fumaroles (A. Paonita pers. comm.) and no recording problems affect- ed permanent and temporary stations. Vulcano LPs show an emergent onset, short duration and spectra ranging about 1-6 Hz with a single dom- inant peak at about 2.5 Hz (figs. 2b, 2c). We recognized a P wave on LP first arrival by high values of the «rectilinearity» (fig. 3) lasting about 0.7 seconds obtained by a three compo- nent polarization analysis (Jurkevics, 1988). 3. Location and Q estimation We obtained suitable locations for 21 micro- earthquakes (table I, fig. 4) using the HYPOEL- LIPSE program (Lahr, 1989) which accommo- dates the difference in altitude of the seismic stations with a local one-dimensional velocity crustal model derived by Falsaperla et al. (1985) and previously used by Montalto (1994) and Aubert and Alparone (2000). Hypocenters are located on the eastern and south-eastern sector of La Fossa 0.5-0.9 km depth. Mean epicentre error (ERH) is about 0.7 km and 0.3 km for the focal depth errors (ERZ); RMS values are lower than 0.2 s. We further analyzed LP waveforms (event n. 18 in table I) using the Sompi method (Kumaza- wa et al., 1990) that performs a spectral analysis based on an autoregressive (AR) model and de- termines the complex frequencies (frequency and quality factor Q) of decaying oscillations. Figure 5 reports the diagram frequency (f), ver- sus growth rate (g), (g= -2Q/f,) where the com- plex frequencies for different trial AR orders be- tween 20-60 are plotted (Kumagai and Chouet, 2000). Areas of the diagram that are densely populated represent stably determined complex frequencies and indicate Q-values near 20, while scattered points indicate incoherent noise. 4. Re-location method The presence of multiplets suggests spatial- ly close sources whose geometry may be in- ferred using cross-spectral/cross-correlation re- location techniques. The application of these techniques on the P-wave furnish, for tectonic events, the positions above the fault radiating most of the energy, thereby allowing the fault plane reconstruction; indeed their application on LP events provides the relative event starting points allowing to define volumes (Battaglia et al., 2003; Wolfe et al., 2003). If we assume that at La Fossa LPs corre- spond to the resonance of a fluid filled volume, then their precise relocation may furnish an im- age of the container geometry. We used a cross-spectrum method based on that discussed by Frèmont and Malone (1987). This method permits very accurate relative tim- ing (dt) for pairs of earthquakes with very sim- ilar waveforms (doublets) and subsequently to perform precise relative relocations. Each dou- blet comprises a reference event (master event) and one of the other events belonging to the same multiplet. The differences dt in first arrival time be- tween seismograms from the same station of a doublet have been computed in the frequency domain using the phase of the cross spectrum obtained on a short window containing the whole P phase. In particular, dt is proportional Vol52,2,2009 17-06-2009 19:02 Pagina 141 142 S. Gambino, L. Cammarata and S. Rapisarda to the slope of the phase of the cross spectrum which can be written as Φ(f) = 2πdtf, plotted versus the frequency (f). The degree of success of this procedure depends on the similarity of the waveforms and the signal to noise ratio on all traces. A parameter measuring the similarity de- gree between two waveforms is the coherency C(f), defined as the ratio of cross-spectrum modulus over the product of the spectra of the two signals (Frèmont and Malone 1987): C(f) = |γs1s2(f)| / [(γs1)1/2(f)(γs2)1/2(f)] (4.1) where γs1(f) = S1(f)S1*(f) and γs2(f) = S2(f)S2*(f) are the spectra of the first and second signals, re- Fig. 3. Example of three component waveforms recorded at STZ2 (Event n. 13 in tab. I) and calculated tempo- ral trend of the rectilinearity. Vol52,2,2009 17-06-2009 19:02 Pagina 142 143 High precision locations of long-period events at La Fossa Crater (Vulcano Island, Italy) spectively, and γs1s2(f) = S1(f)S2*(f) is the cross- spectrum (* denoting complex conjugate). Mean coherency calculated above the P- wave frequency interval defines the quality (Qw) of comparison between two waveforms. Two very similar signals have a Qw>90. A de- creasing value of Qw indicates a poorer similar- ity and generally 80 is the threshold below which it is difficult to obtain dt with acceptable errors (Frèmont and Malone 1987). Pairs of events that do not correlate highly may be caused by a low signal to noise ratio; generally smaller magnitude events recorded at distant stations are more difficult to match. The position and difference in origin time of the second with respect to the master event have been determined using a decomposition in singu- lar values technique discussed by Aki and Richards (1980) (volume II, chapter n. 12) and realized by Frèchet (1985). The parameters needed to obtain a suitable relocation of an event are: the P-wave velocity in the source volume, the take-off angles and azimuths of stations from the reference event. The method requires a min- imum number of 5 time differences to obtain the four unknowns (relative position and difference in origin times) with an associated error. 5. Results The application of the cross-spectrum method (Frèmont and Malone, 1987) on LP Vulcano events limited the definitive set to 14 events; this factor was mainly caused by low signal to noise ratios at IVLP, IVLT and IVUG stations. For each event, we considered the waveforms of the 7 stations of permanent and temporary networks. Event n. 18 (table I) characterized by the larger peak to peak amplitude was chosen as the master event; the relative timing (dt) for pairs of Table II. Results of the event relocation. n master n event An. St. Mean Qw T. err. (ms) Dx (m) Dy (m) Dz (m) Ril. err. (m) 18 01 6 96.6 2.5 40.40 23.81 -9.57 16.25 18 02 6 96.0 1.6 52.10 67.53 1.56 6.1 18 03 7 91.4 2.7 86.01 152.02 2.36 9.33 18 04 6 95.0 1.5 80.29 147.11 -8.58 6.79 18 05 7 94.4 2.7 53.75 69.73 5.55 9.56 18 07 6 94.3 1.9 52.15 79.54 -1.03 8.19 18 08 6 97.3 0.7 -4.45 -10.95 -17.20 2.95 18 11 6 96.5 3.0 26.02 37.15 -0.85 11.57 18 13 6 97.2 1.6 24.13 34.48 2.01 3.12 18 14 6 95.6 0.9 24.88 12.34 0.96 4.71 18 15 6 97.6 1.7 11.47 -3.56 1.70 4.83 18 17 6 97.2 0.8 32.57 36.70 -15.01 8.36 18 20 6 94.7 2.6 16.22 12.19 3.72 3.98 An. St. = number of stations used in recolation; Mean Qw = mean of the quality factor at different stations; T. err. = mean relative time error; Dx, Dy, Dz. location differences with associated error (Ril. err.) respect to the master event. Vol52,2,2009 17-06-2009 19:02 Pagina 143 144 S. Gambino, L. Cammarata and S. Rapisarda earthquakes was obtained on a 1.28 s window of signal (128 points) starting about 0.24 s be- fore the first arrival, thereby allowing the com- plete sampling of the P wave train as also per- formed by Wolfe et al., (2003). The quality of comparisons (Qw) between the events at different stations always exceeded Qw = 90 somewhat being Qw = 95 between most events (table II); the relative timing be- tween the dt shows errors less than ± 0.003 s. We performed the relative locations using the parameters (take-off angles and azimuths) ob- tained by analytical location of the master event and a velocity of 1.5 km/s in the source region Falsaperla et al. (1985). Location differences between events range between several meters to several tens of meters with associated errors smaller than 10-15 meters. LPs relocation (fig. 6) shows the relative po- sitions of the events starting point inside the resonator and clearly describes an elongated shape located 700 m b.s.l. (depth of master event) in a N30-40E direction, 200 m long 40 m wide, only roughly 25 m deep. We also notice that LPs occurring in the pe- riod September 3-October are located in the SW part of the fluid-filled volume with respect to August-September 1 events (fig. 6). Howev- er, locations do not show a progressive migra- tion of the source and it is not easy to recognize a possible mechanism. 6. Discussion and conclusions The magmatic system at La Fossa volcano has been studied through petrologic and geo- chemical investigations. Analyses on CO2 fluid inclusions in quartz xenoliths (Clocchiatti et al., 1994; Zanon et al., 2003) suggest that the plumbing system beneath La Fossa consists of small dykes and magma pockets cutting through intrusive rocks at 1.6-2.0 km depth; Nuccio and Paonita (2001) inferred a magma body at depth of 2.5 km by means of geochem- ical analyses. We analyzed 21 LPs recorded during an anomalous degassing period (August-October Fig. 4. Epicentral map, NS and EW cross-sections of the located events. Vol52,2,2009 17-06-2009 19:02 Pagina 144 145 High precision locations of long-period events at La Fossa Crater (Vulcano Island, Italy) 2006) in which an increasing release of CO2-He rich magmatic fluids was recorded at the rim fumaroles. These events appear to have highly similar waveforms, indicating repetitive and non-destructive source mechanisms occurring within a very limited source volume. We ob- tained reliable absolute locations of a source positioned under the crater/the SE part of the cone at depth of 0.6-0.9 km (± 0.3 km) b.s.l. (fig. 4). LPs are shallow and this factor leaves little doubt on the hydrothermal origin of a possible fluid involved in LP occurrence. Moreover, if we consider a fluid-filled crack model, observed frequencies and low Q-values obtained by Sompi analysis (Q=ca. 20) are ex- plainable by fluids in the form of bubbly water (Kumagai and Chouet, 2000) or alternatively as steam (Kumagai et al., 2005). It is reasonable that at La Fossa, magmatic fluid inputs modify the hydrothermal system, causing a gradual build-up of steam pressure at depth that may excite hydrothermal fluid-filled containers (LP) and on the surface, geochemi- cal variations. Results of high precise relocations suggest a trigger region of about 200 m (long) x 40 m (wide) oriented along the NE-SW structural system (fig. 6). The limited variations in depth (all events are confined within 25 m) may indicate a hori- zontal source. These kinds of sources are not unusual at other volcanoes such as Kusatsu- Shirane in Japan (Nakano et al., 2003) or Ki- Fig. 5. Frequency, versus growth rate diagram where the complex frequencies for different trial AR orders be- tween 20-60 are plotted. Areas of the diagram that are densely populated represent stably determined complex frequencies and indicate Q-values near 20, while scattered points indicate incoherent noise. Vol52,2,2009 17-06-2009 19:02 Pagina 145 146 S. Gambino, L. Cammarata and S. Rapisarda lauea volcano (Kumagai et al., 2005). The presence of a hundred-meter long hori- zontal source suggests some complexity of La Fossa fluid circulation system, generally thought to comprise only vertical ducts; more- over NE-SW structural trend seems to play a role in the fluid circulation system controlling the fluid-filled cavity trend. 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