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Time variation of spectral and wavefield features of volcanic tremor at Mt. Etna (January–June 1999)
Journal of Volcanology and Geothermal Research 161 (2007) 318 – 332 www.elsevier.com/locate/jvolgeores
Time variation of spectral and wavefield features of volcanic tremor at Mt. Etna (January–June 1999) S. Alparone a,⁎, A. Cannata b,1 , S. Gresta c b
a Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, P.zza Roma 2, 95123 Catania, Italy Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Via Ugo La Malfa 153, 90146 Palermo, Italy c Dipartimento di Scienze Geologiche, Università di Catania, Corso Italia 57, 95129 Catania, Italy
Received 5 July 2005; received in revised form 19 October 2006; accepted 7 December 2006 Available online 19 January 2007
Abstract We have studied the volcanic tremor recorded at Mt. Etna during January–June 1999, by using three-component seismic stations. This time period was characterised by both explosive and effusive eruptions occurring at the summit craters. We found significant time variations in the trend of the overall spectral amplitude of tremor, as well as in the dominant spectral peaks. Moreover, the tremor wavefield features (polarization, particle motion and ratios between the amplitude of the three components of the ground velocity) have been studied too, confirming significant time variations. This leads us to suggest the existence of at least two tremor sources: a shallow one, mainly characterised by relatively high frequencies (3.5–7 Hz), is linked to the upper portions of the active conduits, and directly related to the observed eruptive activity. The latter deeper source was active for only a few weeks, and characterised by frequencies lower than about 2.5 Hz. It had no relationship with the observed eruptive activity, and was speculatively related to the gas exsolution from fresh magma refilling a small batch. © 2007 Elsevier B.V. All rights reserved. Keywords: Mt. Etna; volcanic tremor; spectral and wavefield analysis; tremor source
1. Introduction Mt. Etna, located on the eastern side of Sicily, is a composite, Quaternary volcano characterised by Naalkaline magmatism (Monaco et al., 1997). The present complex picture of the summit area of the volcano is characterised by four upper vents, Voragine, Bocca Nuova, South-East Crater and North-East Crater (here⁎ Corresponding author. Tel.: +39 095 7165836; fax: +39 095 435801. E-mail address: [email protected] (S. Alparone). 1 Now at: Dipartimento di Scienze Geologiche, Università di Catania, Corso Italia 57, 95129 Catania, Italy. 0377-0273/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2006.12.012
after referred as VOR, BN, SEC and NEC, respectively) (Fig. 1). Current volcanic activity may be divided into two main types: persistent activity, which is generally related to the activity of the summit craters, and flank eruptions, that take place through fissures affected by magmatic intrusions. The activity at the summit vents can be of different and sometimes coexistent types: degassing, strombolian or hydromagmatic explosions, lava filling or collapses, and low rate lava emissions (Cristofolini et al., 1988). The flank eruptions are mainly characterised by lava flows (Harris et al., 2000), although during recent episodes the explosive activity is consistent (Behncke and Neri, 2003; Andronico et al., 2005).
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Fig. 1. Sketch map of Mt. Etna with location of the seismic stations used in the present work (triangles); the distribution of the summit craters is reported in the upper left corner (VOR = Voragine, BN = Bocca Nuova, SEC = South-East Crater, NEC = North-East Crater).
After a long quiet period following the December 1991–March 1993 lateral eruption, volcanic activity at Etna resumed on 30 July 1995, at first only in the BN then spreading to the NEC on 2 August (Coltelli et al., 1998). From November 1995 to August 1996, the NEC was the site of lava fountains, while BN had mild strombolian explosions and small lava overflows (Coltelli et al., 1998, 2000). In early November 1996, the eruptive activity (mild strombolian explosions and slow lava effusion) resumed at the SEC. In the summer 1998, eruptive activity at Etna's summit vents changed radically, when persistent strombolian activity and small intracrater lava flows were replaced by occasional shortlived eruptions of higher energy and larger volume, characterised by lava fountains (Rothery et al., 2001). In particular, in September 1998 an explosive eruptive period began at the SEC, characterised by 21 violent lava fountains (Alparone and Privitera, 2001), ending on February 4, 1999. Many authors have investigated the characteristics of volcanic tremor, a seismic signal observed worldwide on most volcanoes (McNutt, 1994). The dominant frequency in tremor waveforms was observed to vary between 0.1 and 10 Hz at various volcanoes (Kubotera, 1974). Many researchers found that these peaks must be
due to source rather than path or site effects (Fehler, 1983; Almendros et al., 1997; Hellweg, 1999; Hagerty et al., 2000). However, other authors (Gordeev, 1993; Benoit and McNutt, 1997; Chouet et al., 1997; Saccorotti et al., 2001) suggested that path effects could also be responsible for some of the observed spectral peaks. Volcanic tremor is particularly evident at Mt. Etna, where it has been studied for decades. Schick and Riuscetti (1973) suggested that the tremor energy could be associated with the velocity of gases escaping through the summit crater conduits. The observed steadiness of the frequency peaks was related to at least one oscillator, excited by gas bubbles rising in the magma (Riuscetti et al., 1977). A physical model for the source was then proposed (Seidl et al., 1981) and oscillators were identified as pipes along which magma rises. The relationship between volcanic tremor and eruptive activity was also studied by various authors (Gresta et al., 1991; Falsaperla et al., 1994; Gresta et al., 1996a,b; Alparone et al., 2003). Ereditato and Luongo (1994) have evidenced that volcanic tremor, during no-eruptive activity, is a stationary phenomenon, characterised by a small variability of its main parameters such as intensity, spectral contents, polarization and wavefield composition, whereas significant variations of its features occur
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during eruptive activity. Falsaperla et al. (2005) noted that different values of the frequency content are associated with diverse styles of eruptive activity; the frequency content of tremor decreases from the preeffusive to effusive stage. In order to locate the source of volcanic tremor, Schick and Riuscetti (1973) reported that, because of the nonimpulsive character of tremor, it is impossible to use techniques based on travel time inversion, similar to those used to calculate epicentres and hypocentres of earthquakes. They used amplitude–distance functions in their attempt to locate the tremor source. Results showed a possible migration of the tremor source, occurring between 1971 and 1972, from a few hundred metres beneath the summit toward the northern part of the Valle del Bove at a depth ranging from 2 to 4 km. Using spectral analysis, Riuscetti et al. (1977) located the tremor source in the Central Crater or beneath it. Two possible detailed sources were located in the magma column, a deep one (between 0.5 and 1 km) and a shallower one, probably at the free surface of the magma column within the Central Crater. A detailed analysis of the recorded tremor spectra along profiles, and their comparison with theoretical models graphs allowed Schick et al. (1982) to distinguish two different sources. The former was characterised by low frequency contents ( f b 1.5 Hz), and might have its origin in a flat magma chamber with a horizontal extent of about 4 km, located at about 2 km beneath the Central Crater. Conversely, the latter source, characterised by a higher frequency content, was associated with the upper portion of the active vents. During the various phases of the July–August 2001 flank eruption, Falsaperla et al. (2005), by using the tremor amplitude decay, estimated the time migration of the tremor source from about 5 km to surface. Di Grazia et al. (2006) suggested the upward migration of the tremor source from about 1700 to 2200 m a.s.l. before the 2004–2005 eruption. Carbone et al. (2006), comparing tremor and gravity data, supposed the activation of two tremor sources: the former associated with the fire-fountaining activity, and the latter deeper and characterised by energetic body-wave arrivals. Many researchers investigated the characteristics of the volcanic tremor wavefield at Mt. Etna, obtaining different results. Ferrucci et al. (1990) suggested the existence of an elongated source having a horizontal projection trending north–south and radiating mainly P waves, during paroxysmal summit eruptions. Conversely, Del Pezzo et al. (1993), using cross-correlation techniques on data collected at a small seismic array during non-eruptive stages, found a high content of surfacewaves originating from a source located in the summit crater area. Analysis of polarization and propagation
velocity (Ereditato and Luongo, 1994) has evidenced the presence of Love waves overlapping other seismic phases. Wassermann and Scherbaum (1994), using data acquired at a distance of 6.5 km from the main crater during a period characterised by strombolian activity at the summit craters, have evidenced that the dominant signal power is concentrated around 1 Hz with the maximum amplitudes observed on the horizontal components. Moreover, through polarization and dispersion analyses, they found a major content of Love waves in the tremor wavefield. More recently, Ripepe et al. (2001) found that the tremor wavefield, recorded at only 50 m from the summit craters during vigorous strombolian activity, indicated the presence of P waves generated by the superposition of small shallow point sources, acting at intervals of 1–2 s. Saccorotti et al. (2004), using data from two dense small aperture arrays, found a complex wavefield where body- and surfacewaves alternatively dominate, depending on both the frequency band and component of motion analysed. The aim of this work is to study the features of the volcanic tremor accompanying eruptive activity occurring at Mt. Etna during January–June 1999. The main goal is the study of the time evolution of both spectral and wavefield parameters in order to characterise the tremor source, as well as to evidence the relationship (if any) with the eruptive activities. 2. Volcanic and earthquakes framework The volcanic activity at Mt. Etna during January– June 1999 was characterised by both explosive and effusive eruptions at the summit craters. Until February 4, eight lava fountains at SEC and discontinuous strombolian activity at BN and VOR occurred. On February 4, during the last lava fountain at SEC, a fissure opened at the base of the SEC cone, starting an effusive eruption that lasted ten months (Fig. 2a). The related lava flow field was characterised by a complex tube network, skylights, ephemeral vents and tumuli (Calvari et al., 2002). During the 6-month studied period, 424 earthquakes were recorded on the volcano (Fig. 3). The seismic activity was composed of individual earthquakes, seismic swarms and small temporal clusters. In particular, the major seismic swarms occurred on January 23 and on February 16 (I and II in Fig. 3). The former was made of 47 earthquakes located on the summit area at about 500 m b.s.l.; the strongest earthquake belonging to this swarm had a magnitude of 2.5. The latter was composed of 54 events located on the southern side of the volcano at depth ranging from 1.5 to 2.5 km b.s.l. and showed a
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Fig. 2. (a) Eruptive activity from January to June 1999 shown for each of the four summit craters and for the eruptive fissure (EF) opened at the base of SEC. 1 = discontinuous strombolian activity; 2 = emissions of ash and lithic material; 3 = lava fountains; 4 = effusive activity. (b) Overall spectral amplitudes of TDF, MNT and ESP vertical component signals and their ratios (grey lines). The moving averages (black lines) are computed every 2.5 days, sliding 1 h. The vertical dashed lines indicate the occurrence of the most important time variations of the analysed tremor features (see text for details).
maximum magnitude of 2.8. Also the time period between March 9 and 20 was characterised by important seismic activity (III in Fig. 3); about 60 earthquakes (Mmax = 2.8), located east of summit craters at depth ranging from 1.5 to 3.5 km b.s.l., occurred. On June 2, a swarm composed of 10 earthquakes, located southeast of summit area at depth ranging from 5.5 to 7.5 km b.s. l., was recorded (IV in Fig. 3); the highest magnitude obtained for these events was 2.7. Finally, on June 24, a swarm composed of 18 events (Mmax = 2.6), affected the southern side of the volcano at depth more than 8.5 km b.s.l. (V in Fig. 3).
3. Tremor data analysis In 1999, two independent permanent seismic networks were operating at Mt. Etna, totalling about 40 stations. However, for the scope of our investigation, stations equipped with three-component seismometers are required; only 7 of these stations are available. Among them, we have excluded the discontinuously running stations and/or those with a poor signal to noise ratio. Then, we use the data acquired from TDF (Torre del Filosofo: 2919 m a.s.l.), MNT (La Montagnola: 2504 m a.s.l.) and ESP (Serra Pizzuta Calvarina: 1600 m a.s.l.) seismic
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Fig. 3. Daily earthquake number (left vertical axis) and strain release cumulative curve (right vertical axis) for earthquakes occurring at Mt. Etna during January–June 1999. The seismic energy (E) was computed using the relationship logE(erg) = 9.9 + 1.9 M–0.024 M2 (Richter, 1958).
stations, located on the southern flank of the volcano at a distance of about 1.1, 3 and 6 km from SEC, respectively (Fig. 1). TDF was equipped with a short period geophone (Lennartz LE-3D/1s), while MNT and ESP were equipped with broadband geophones (Lennartz LE-3D/20s). Data were acquired at a sampling rate of 100 Hz and radio link transmitted to the data acquisition centre in Catania. 3.1. Overall spectral amplitude The overall spectral amplitude, defined as the cumulative of spectral amplitudes, of the vertical component of the ground velocity (OSAV) was calculated at TDF, MNT and ESP in the following way: the long tremor time series was divided in non-overlapping roughly 40-s-long windows (4096 points) and the spectrum was calculated for each window by means of a Fast Fourier Transform. The OSAV was obtained with hourly frequency by the hourly average spectra (90 spectra for each hour) in the frequency range 1–10 Hz (Fig. 2b). The values of the spectral amplitude are not corrected for path, site and instrumental effects. At all stations it is possible to distinguish some periods characterised by different trends, some of which are related to changes in eruptive activity. A first period was characterised by short sudden increases of the OSAV, occurred coinciding with lava fountains at the SEC. From February 4 to 23, we noticed increases in the OSAV, more evident at TDF, occurring in coincidence with the first phases of the summit effusive activity. Afterward, the tremor amplitude decreased until March 12, when a new increasing trend, more evident
in the signal acquired from ESP, was recorded. On March 20, the OSAV at ESP and MNT reached the highest values for the entire studied period. From March 20, the OSAV calculated at TDF showed a strong decrease in its values, lasting until March 23; this variation was less evident at MNT and ESP. On April 16, the OSAV at the seismic stations were sharply reduced, reaching successively the lowest values observed during the studied period. In order to investigate the dynamics of the tremor sources, we calculated the ratios between the OSAV at the stations (Fig. 2b). Variations in the trends of the ratios allowed us to suppose spatial and temporal changes of the tremor sources. During the first period, the ratios were characterised by a stationary trend, suddenly interrupted by short oscillations in coincidence with the lava fountains. A mild increase can be recognised after February 4, in coincidence with the initial phase of the effusive activity. Afterward, from February 23 to March 3, the ratios TDF/MNT and TDF/ESP showed a marked increase, reaching the highest values obtained during the whole 6-month period. It is noteworthy that this trend is due to the greater decrease of the OSAV at MNT and ESP than at TDF. From March 3 on, we noticed an inversion of the ratio pattern, lasting until March 23. Until April 16, the ratio settled around very low and almost stable values. The maximum steadiness was found in the ratio MNT/ ESP, in which the variations were less than 5%. On April 16, the ratios increased sharply (by about 45%). The variability, showed by OSAV during the following 2.5 months, was not considered significant because of very low tremor amplitude.
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3.2. Spectral analysis The spectral analysis of volcanic tremor was performed on the vertical component of the ground velocity acquired at TDF, MNT and ESP seismic stations. The
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daily average spectra, obtained by averaging 24 hourly average spectra (calculated in the same way described in Section 3.1), were calculated. In order to verify the time variations of the frequency content, using the daily average spectra we
Fig. 4. Normalised spectrograms at TDF, MNT and ESP seismic stations from January to June 1999. Black triangles indicate the days when representative spectra of Fig. 5 were collected. Roman numbers (and related vertical lines) indicate the most significant time variations in the spectrogram contents (see text for details).
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achieved the spectrograms showing the whole studied period (Fig. 4). At TDF station, most of the tremor energy was radiated in the frequency band 1–7 Hz, whereas at MNT and ESP in the frequency bands 1.0– 3.5, and 1.0–3.0 Hz, respectively. TDF is located at a shorter distance (ca. 1.1 km) from the summit craters with respect to MNT and ESP (ca. 3 and 6 km, respectively); therefore an additional contribution of relatively high frequencies may be expected owing to propagation effects. The spectrograms allowed distinguishing of four significant time intervals characterised by different frequency content (Fig. 4): I) The first time interval (January 1–February 4; I in Fig. 4) was marked by short sudden increases of tremor amplitude, more evident at TDF, in coincidence with the lava fountains that occurred at the SEC. Representative amplitude spectra of the signals, recorded by TDF, MNT and ESP stations during the lava fountains (Fig. 5a), showed that the dominant frequency band was 1.0–2.5 Hz. The tremor spectra obtained at TDF showed that a
significant amount of energy was also radiated in the band 3.5–7.0 Hz. II) From February 4 to March 23 (II in Fig. 4), we noticed an increase of tremor amplitude, mainly evident in the signal acquired from TDF in the band 3.5–7.0 Hz (Fig. 5b–c). Besides, since February 4, a significant energy contribution was recognisable also in the band 1.0–2.2 Hz, which was reduced from February 23 (Fig. 5c). III) Between March 12 and April 16, the radiated tremor energy was mainly bounded in the range 1.2–2.5 Hz (III in Fig. 4). This feature was well evident at MNT and above all at ESP, where the spectra showed exclusively this frequency range, with amplitude higher than that obtained during the other time intervals, excluding the lava fountains (Fig. 5e). In particular, two dominant peaks with frequencies of 1.7 and 2.0 Hz were observed. No variation in the observed eruptive activity occurred in coincidence with these sharp variations of the tremor features. In the time period March 12–23, tremor simultaneously showed the features observed during the
Fig. 5. Amplitude tremor spectra collected during the significant phases of the whole studied period (see Fig. 4) at TDF (continuous black line), MNT (dashed black line) and ESP (continuous grey line) seismic stations. Each spectrum was obtained by 24 hourly average spectra, excluding the spectrum “a”; in fact, it is representative of a short time interval (1 h) during a lava fountain at SEC. Spectral amplitudes are normalised to the maximum amplitude values observed in the spectrum “a”.
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phases II and III. In particular, at TDF two dominant frequency bands (1.2–2.5 and 3.5– 7.0 Hz) can clearly be seen (Fig. 5d). IV) Between June 1 and 8, an increase of the tremor amplitude occurred in the band 0.5–1.5 Hz. Typical amplitude spectra of the signal, recorded by TDF, MNT and ESP during this short time interval, were very similar (Fig. 5f ). Also these spectral variations cannot be related to variation in the eruptive activity. The comparison among the spectral amplitudes obtained at the three stations used has confirmed that amplitudes did not change uniformly. In fact, with the exception of the period I, when the maximum amplitudes were observed during the lava fountains, the highest values were recorded at TDF during period II (Figs. 4, 5b–c) and at MNT and ESP during period III (Figs. 4, 5e). We can exclude path and site effects for the observed spectral peaks because of their similarity at all three stations (e.g. band 1.2–2.5 Hz in Fig. 5e; band 0.5– 1.5 Hz in Fig. 5f). Consequently, also the time variations of the main spectral peaks at an individual station (Fig. 5c–f) cannot be attributed to site effects, but rather to source migration effects (and/or to the activation of a latter source). 3.3. Wavefield analysis In order to investigate the wavefield composition of the volcanic tremor, three different analyses were performed on the signals acquired from TDF, MNT and ESP stations: (i) computation of ratios between the amplitudes of the three components of the ground velocity, (ii) polarization analysis based on the evaluation of the covariance matrix, and (iii) study of the particle motion. 3.3.1. Ratios OSAH /OSAV In order to verify the main time variations of the wavefield, we performed a general analysis obtaining the values of ratios between the OSA of the three components of ground velocity. With this aim, we calculated the OSA for the frequency range 1–10 Hz with hourly frequency, in the same way described in Section 3.1, also at TDF, MNT and ESP horizontal components. We obtained the OSA of the horizontal component of the ground velocity (OSAH) in the following way: OSAH ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi OSA2NS þ OSA2EW
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where: OSANS and OSAEW are the OSA values of the NS and EW components of the ground velocity, respectively. Fig. 6 shows that the OSAH was about 1.3–2 times larger than the OSAV. It was also possible to note that the ratios OSAH/OSAV, calculated at TDF, MNT and ESP, showed similar trends: in fact, the time period from March 12 to April 16 (period III, see Fig. 4) was characterised, at all the considered stations, by a slight decrease in the ratio (about 10%). Moreover, it is noteworthy that TDF, the nearest station to the summit area, showed the largest values of OSAH/OSAV. 3.3.2. Polarization The analysis of polarization was performed by following the method proposed by Montalbetti and Kanasewich (1970) and Kanasewich (1981), analysing 2-hour-long daily samples, bandpass filtered in various frequency bands. The covariance matrix of the three components of ground displacement was calculated over successive windows with a time step of 0.01 s (one sample of the time series). The values of azimuth, incidence angle (with respect to the vertical direction) and rectilinearity were calculated in the frequency band 1.2–2.5 Hz, with 1.7-s-long windows, at TDF, MNT and ESP and in the frequency band 3.5–7.0 Hz, with 0.6-s-long windows (Fig. 7), at TDF; in fact, the poor signal to noise ratio, observed in the frequency band 3.5–7.0 Hz at MNT and ESP, did not allow us to obtain reliable values of the polarization parameters. In the frequency band 1.2–2.5 Hz, at ESP the polarization parameters showed a greater variability than at MNT and TDF. At all used stations it was possible to distinguish a first time interval (whole period I and part of period II, see Fig. 4) characterised by almost steady values: polarization azimuth was clustered mainly in the N70°E–N100°E range, the incidence angle was linked to the horizontal particle motion and the rectilinearity coefficient exhibited stable values of about 0.6. From March 12 to April 16 (period III, see Fig. 4), the polarization parameters changed sharply: the azimuth values were equal to about N100°E at TDF, N110°E at MNT and N125°E at ESP; the rectilinearity coefficient settled around almost stable values of about 0.5 at all three stations; the incidence angle decreased, especially at MNT and ESP, showing intermediate values from March 12 to 23 (about 80° at MNT and 75° at ESP) and from March 24 to April 16 the lowest values obtained for the studied period (about 75° at MNT and 55° at ESP). Since April 16, the polarization parameters returned to the values observed before March 12. At ESP, a major
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Fig. 6. Time variation of the ratios OSAH/OSAV at TDF, MNT and ESP stations (see text, for details).
difference concerned the high variability of the azimuth values that ranged between N80°E and N120°E. In the frequency band 3.5–7.0 Hz at TDF, azimuth directions were clustered mainly in the N80°E–N110°E range and the incidence angle values were representative of a horizontal particle motion. The values of rectilinearity ranged between 0.55 and 0.65, reaching its lowest value in February, during the first phases of effusive activity at SEC (period II, see Fig. 4). The analysis of polarization was also performed over successive 2-s-long windows on the signal bandpass filtered in the frequency band 0.5–1.5 Hz, recorded at the beginning of June (period IV, see Fig. 4), when an increase of the tremor amplitude occurred. The values of the moving average over 75 min are reported in the insets of Fig. 7. Significant variations of trends of these parameters can be noted from June 1: at MNT and ESP polarization azimuths changed respectively from N100°E to N120°E and from N90°E to N125°E; the incidence angles decreased reaching in the next days minimum values of about 50° at MNT, and 40° at ESP; the rectilinearity coefficient changed from about 0.6 to 0.5. 3.3.3. Particle motion The particle motion analysis was performed on samples of signal, bandpass filtered into narrow
frequency bands (bandwidth of about 2 Hz). During most of the studied period, the tremor was characterised by horizontal polarization, mainly toward east–west (Fig. 8a–d). We noted some periods characterised by variations of the particle motion. In February, in coincidence with the first phase of the effusive activity at SEC, we also noticed an elliptical retrograde motion, vertically polarized, at TDF in the band 3.5–7 Hz (Fig. 8b). From March 12 to April 16, the particle motion was elliptical/chaotic in all three planes (NS–Z, EW–Z and NS–EW), in the frequency band 1.2–2.5 Hz (Fig. 8c). Also the signal acquired at the beginning of June at MNT and ESP, in the band 0.5–1.5 Hz, was characterised by elliptical/chaotic particle motion (Fig. 8e). Considering all features of the tremor wavefield, obtained by the three different analyses described above, we can state that the wavefield was dominated by transverse, horizontally propagating waves (SH waves or Love waves), during most of the studied period. In February, Rayleigh waves were also recorded at TDF in the frequency band 3.5–7 Hz, as confirmed by the lowest values of rectilinearity. From March 12 to April 16, the complexity of wavefield in the frequency band 1.2– 2.5 Hz may be due to the combination of different waves (such as P, SH and SV waves). Also at the beginning of June, the observed wavefield features in the band 0.5–
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Fig. 7. Polarization parameters obtained by analysing 2-hour-long daily samples, bandpass filtered in the frequency bands 3.5–7.0 Hz at TDF and 1.2–2.5 Hz at TDF, MNT and ESP. Triangles, squares and circles indicate azimuth, incidence angle (calculated with respect to the vertical direction) and rectilinearity coefficient, respectively. More details of the polarization parameters at MNT and ESP in the band 0.5–1.5 Hz, concerning June 1–3, are reported in the insets of the figure; (a), (b) and (c) series indicate azimuth, incidence angle and rectilinearity coefficient, respectively. The arrows indicate the beginning of the most important variations of the polarization parameters.
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Fig. 8. Some examples of particle motion plots for signals acquired at TDF, MNT and ESP seismic stations highlighting the remarkable time variation of the tremor wavefield features during the studied time period (see text for details).
1.5 Hz can be explained with the coexistence in the tremor wavefield of various wave-types. The dominant signal power of the horizontal components and very low deviations of the polarization directions from the horizontal, which characterised the volcanic tremor recorded during most of the studied period, agree with results already found by Wassermann and Scherbaum (1994), using data acquired during a period characterised by strombolian activity at the summit craters. 4. Discussion Time variations of the volcanic tremor features have been observed in different volcanoes such as Ruapehu (Bryan and Sherburn, 1999), Stromboli (Falsaperla et al., 1998), Vatnajökull (Konstantinou et al., 2000) and Shishaldin (Thompson et al., 2002), and interpreted as due to changes in tremor sources. The analysis of the volcanic tremor recorded at Mt. Etna during January–June 1999 allowed us to find significant time variations in some parameters, to com-
pare them with the evolution of volcanic phenomena, and to suggest the activation of different tremor sources. Some time intervals having different tremor features have been distinguished. During the lava fountains period, a general short increase of the OSAs and changes of the ratios among the OSAs at different stations occurred (Fig. 2). Several variations of the tremor features were recognisable after the last lava fountain, occurring on February 4, until February 23 (the first part of the period II, see Fig. 4): i) an increase of the OSAs, most evident at TDF station (Fig. 2b); ii) an increase in the ratios between pairs of OSAs: TDF/MNT, TDF/ESP and MNT/ESP (Fig. 2b); iii) variations in the spectral content (Fig. 4). Both increases of tremor amplitude and spectral changes have frequently been observed at Mt. Etna in coincidence with the onset of flank eruptions, as in December 1991 (Falsaperla et al., 1994) and in July 2001 (Falsaperla et al., 2005). These evidences have been associated with the activation of tremor sources linked to the feeding system of the eruptive fractures.
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We have interpreted the variation observed on February 4, 1999 as due to the activation of a shallow tremor source related to the effusive activity that began at SEC (Calvari et al., 2002). The tremor radiated by this source was characterised, at the closest station TDF, by a large band of frequency (1–7 Hz), and a wavefield mainly composed by transversal waves propagating horizontally (SH or Love waves) and sporadically by Rayleigh waves. The contribution of energy in the band of frequency 3.5–7.0 Hz was poorly evident at MNT and ESP stations, probably owing to propagation effects. Since February 23, the increase of the ratios TDF/ MNT and TDF/ESP (Fig. 2b), was related to the decrease of the tremor amplitude in the band 1–2 Hz (Fig. 4). In fact, the amplitude reduction in this frequency band mainly affected the OSAs at MNT and ESP, because at TDF there was also a significant energy contribution in the band 3.5–7.0 Hz. On March 12 (the beginning of period III, see Fig. 4), several features of the tremor sharply changed without any variation in the observed eruptive activity. We observed: i) an increase of the OSAs, more evident at MNT and ESP, related to the increase of the tremor amplitude in the band 1.2–2.5 Hz (Figs. 2b, 4); ii) a decrease of the ratios TDF/MNT, TDF/ESP and MNT/ ESP (Fig. 2b); iii) a decrease in the incidence angle of the seismic rays calculated with respect to the vertical direction (Fig. 7). All these evidences allowed us to hypothesize that since March 12 a “deep” tremor source was activated, mainly radiating tremor-waves in the frequency range of 1.2–2.5 Hz. A steepening of the incidence angle of tremor waves, similar to the one found in the present study, was observed at Stromboli, without any concurrent variation in the volcanic activity; it was interpreted as due to changes affecting a deep part of the magma feeder, probably due to the replenishment of fluids (Falsaperla et al., 2002). A similar variation of the ratio between the tremor amplitude recorded at the topmost station and the intermediate/peripheral ones, was found during a huge explosive eruption at NEC, and interpreted as the activation of a ca. 2 km deep sub-vertically planar source (Gresta et al., 1996a). The wavefield variations, occurring during the time period March 12–April 16, may be due to the combination of different wave types, causing the decrease of rectilinearity and the ellipticity and complexity of the particle motion (Figs. 7, 8c). These wavefield features cannot be explained by supposing the existence of a radially symmetric source, which radiates only P and SV waves, but can be due to the activation of a rectangular crack-like resonator, that radiates P, SV and SH
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waves, as was supposed by Chouet et al. (1997) at Stromboli volcano and by Hagerty et al. (2000) at Arenal volcano. From March 12 to 23, the volcanic tremor showed spectral content (Fig. 4) and wavefield features, such as OSAH/OSAV ratios (Fig. 6) and incidence angle (Fig. 7), representing the superposition of the wavefield characteristics of the two observed sources. This characteristic was due to the coexistence of the two different tremor sources (the shallow one and the deep one) described above. On March 23, the energy radiated by the shallow source was reduced and the deep tremor source became dominant. On April 16 (the end of period III, see Fig. 4), the tremor energy, released by the deep source, was also sharply reduced. In particular, we observed: i) a decrease of the OSAs, related to the sharp reduction of the tremor amplitude in the band 1.2–2.5 Hz (Figs. 2b, 4); ii) a marked increase of the ratios TDF/MNT, TDF/ESP and MNT/ESP (Fig. 2b); iii) an increase of the incidence angle of the seismic rays (Fig. 7); iv) a particle motion comparable to that observed usually before March 12 (Figs. 8 a–d). Finally, during a few days at the beginning of June (period IV, see Fig. 4), we observed an increase of the tremor amplitude in the band 0.5–1.5 Hz and a decrease of the incidence angle of the seismic rays (Figs. 4, 7). In this case, we supposed that the presence of a deep source, like the previous deep source, produced a complex wavefield. In order to roughly locate the deeper tremor source, active from March 12 to April 16, we considered the parameters obtained by the polarization analysis. During this time period, because of the coexistence of at least two tremor sources located at different depths, polarization parameters showed values affected by the wavefield produced by both sources. Anyway, taking into account the intersection point between the azimuth directions obtained at MNT and ESP, and the incidence angle value at ESP, we roughly located the tremor source south-west of the summit area at depth of a few kilometres below sea level. The polarization parameters obtained at TDF were not considered, because the seismic signal at this station is strongly influenced by the tremor produced at the summit craters. We well know that the used method is oversimplified, because it does not take into account the effects induced by the medium heterogeneity (as the layering) and the topography; in fact, Neuberg and Pointer (2000) demonstrated that the incidence angle values may be significantly affected by volcano topography. However, we would like to stress that, regardless of its correct
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location, we observed tremor feature variations related to a secondary source that was active at Mt. Etna from March 12 to April 16, 1999. Looking at the results of the other disciplines during the entire analysed period, the only significant variation in ground deformation was given by a tiltmeter located at Pizzi Deneri (see Fig. 1); a tilt of about 2–3 μrad was measured from March 2 to 10, suggesting a slight general uplift of the southern slope of the volcano (Gambino, personal communication), just a few days before the activation of the deeper tremor source. On the contrary, SO2 flux measurements showed a strongly irregular trend until mid-February, with the estimated daily release varying from ca. 2000 to 14,000 t. These values are in agreement with the occurrence of lava fountains and the first stages (with larger magma output rate) of the effusive eruption. Afterwards a slightly decreasing trend began (from ca. 5500 t/d to 1000 t/d) lasting to the beginning of June. During this generally decreasing trend only two significantly larger amounts have been found (ca. 7000 t/d and 9000 t/d), both in the second half of March (Caltabiano, personal communication), a few days after the activation of the “deep” tremor source. Even if slight, the observed variation in ground deformation and SO2 flux, respectively a few days before and after the observed variation of volcanic tremor features starting on March 12, supports the hypothesis of the activation of a tremor source, roughly located south-west of the summit area a few kilometres below sea level. At Mt. Etna, most SO2 and water exsolve into the gas phase from 2 kbar to 0.5 kbar, (Métrich et al., 1993), that is between about 2.5 km b.s.l. and 2.0 km a.s.l. We suggest that during the first days of March 1999 a refilling of magma occurred from depth into a relatively small and shallow batch. This successively produced: i) a slight inflation of the southern upper part of the volcano body; ii) the beginning of gas exsolution from the new magma and consequently the generation of tremor; iii) the emission of the exsolved gases from the central plumbing system of the volcano some days later. Finally, the rough location of the deep source is obtained by considering a point-like source. Some previous works have provided evidence for a horizontally (i.e. Schick et al., 1982) or vertically (i.e. Ferrucci et al., 1990) extended source of tremor at Mt. Etna volcano. This does not conflict with our results. We suggest that the inferred deep source is to be considered as the active part (where the gas exsolution was able to generate the radiation of seismic energy) of a larger magma batch.
5. Concluding remarks We have studied the spectral and wavefield features of the volcanic tremor recorded at Mt. Etna during January–June 1999. This time period was characterised by both explosive and effusive eruptions in the summit area. We performed spectral and wavefield (polarization, particle motion and ratios between amplitudes) analysis at three seismic stations, and a good agreement among the results obtained by the different used approaches was found. Two tremor sources have been identified. The former, characterised mainly by relatively high frequencies (3.5–7.0 Hz), was linked to the upper portions of the active conduits and was directly related to the observable eruptive activity. The latter source, radiating frequencies lower than 2.5 Hz, was highly active from March 12 to April 16 and deeper than the first one. It could be due to the gas exsolution of fresh magma refilling a small batch from depth. However, these results need to be proved with additional data. This work, such as other recent papers (e.g. Falsaperla et al., 2005; Carbone et al., 2006; Di Grazia et al., 2006), shows that the location and the features of the tremor source at Mt. Etna strongly change depending on considered time period and volcanic activity. Acknowledgments We thank Ferruccio Ferrari for useful technical support and Steve Conway for the English revision of the text. Salvo Gambino and Tommaso Caltabiano are kindly acknowledged for providing us unpublished data. We are indeed grateful to two anonymous reviewers for their useful comments and suggestions which helped us to significantly improve the quality of the paper. This research has been supported by funds INGV-DPC (2005–2006). References Almendros, J., Ibanez, J.M., Alguacil, G., Del Pezzo, E., Ortiz, R., 1997. Array tracking of the volcanic tremor source at Deception Island, Antarctica. Geophys. Res. Lett. 24, 3069–3072. Alparone, S., Privitera, E., 2001. Characteristics of the intermittent volcanic tremor at Mt. Etna, Italy, during the 15 September 1998–4 February 1999 eruptive episode. Proceedings of the Cities on Volcanoes 2 Conference, Auckland, New Zealand, 12–14 February 2001, Earthquake Commission, Inst. Geol. Nucl. Sci. Inf. Ser., vol. 49, Lower Hutt, New Zealand. Alparone, S., Andronico, D., Lodato, L., Sgroi, T., 2003. Relationship between tremor and volcanic activity during the Southeast Crater eruption on Mount Etna in early 2000. J. Geophys. Res. 108 B (5), 2241. doi:10.1029/2002JB001866.
S. Alparone et al. / Journal of Volcanology and Geothermal Research 161 (2007) 318–332 Andronico, D., Branca, S., Calvari, S., Burton, M., Caltabiano, T., Corsaro, R.A., Del Carlo, P., Garfì, G., Lodato, L., Miraglia, L., Murè, F., Neri, M., Pecora, E., Pompilio, M., Salerno, G., Spampinato, L., 2005. A multi-disciplinary study of the 2002– 2003 Etna eruption: insights into a complex plumbing system. Bull. Volcanol. 67, 314–330. Behncke, B., Neri, M., 2003. The July–August 2001 eruption of Mt. Etna (Sicily). Bull. Volcanol. 65, 461–476. Benoit, J.P., McNutt, S.R., 1997. New constraints on source processes of volcanic tremor at Arenal Volcano, Costa Rica, using broadband seismic data. Geophys. Res. Lett. 24, 449–452. Bryan, C.J., Sherburn, S., 1999. Seismicity associated with the 1995– 1996 eruptions of Ruapehu volcano, New Zealand: narrative and insights into physical processes. J. Volcanol. Geotherm. Res. 90, 1–18. Calvari, S., Neri, M., Pinkerton, H., 2002. Effusion rate estimations during the 1999 summit eruption on Mount Etna, and growth of two distinct lava flow fields. J. Volcanol. Geotherm. Res. 119, 107–123. Carbone, D., Zuccarello, L., Saccorotti, G., Greco, F., 2006. Analysis of simultaneous gravity and tremor anomalies observed during the 2002–2003 Etna eruption. Earth Planet. Sci. Lett. 245, 616–629. Chouet, B.A., Saccorotti, G., Martini, M., Dawson, P., De Luca, G., Milana, G., Scarpa, R., 1997. Source and path effects in the wave fields of tremor and explosions at Stromboli Volcano, Italy. J. Geophys. Res. 102, 15129–15150. Coltelli, M., Pompilio, M., Del Carlo, P., Calvari, S., Pannucci, S., Scribano, V., 1998. Mt. Etna: the eruptive activity (1993–1995). Acta Vulcanol. 10, 141–148. Coltelli, M., Del Carlo, P., Pompilio, M., 2000. Mt. Etna: the eruptive activity (1996). Acta Vulcanol. 12, 63–67. Cristofolini, R., Gresta, S., Imposa, S., Patanè, G., 1988. Feeding mechanism of eruptive activity at Mt. Etna based on seismological and petrological data. In: King, C.Y., Scarpa, R. (Eds.), Modelling of Volcanic Process. Springer, Berlin, pp. 73–93. Del Pezzo, E., De Martino, S., Gresta, S., Martini, M., Milana, G., Patanè, D., Sabbarese, C., 1993. Velocity and spectral characteristics of the volcanic tremor at Etna deduced by a small seismometer array. J. Volcanol. Geotherm. Res. 56, 369–378. Di Grazia, G., Falsaperla, S., Langer, H., 2006. Volcanic tremor location during the 2004 Mount Etna lava effusion. Geophys. Res. Lett. 33, L04304. doi:10.1029/2005GL025177. Ereditato, D., Luongo, G., 1994. Volcanic tremor wave field during quiescent and eruptive activity at Mt. Etna (Sicily). J. Volcanol. Geotherm. Res. 61, 239–251. Falsaperla, S., Privitera, E., Spampinato, S., Cardaci, C., 1994. Seismic activity and volcanic tremor related to the December 14, 1991 Mt. Etna eruption. Acta Vulcanol. 4, 63–73. Falsaperla, S., Langer, H., Spampinato, S., 1998. Statistical analyses and characteristics of volcanic tremor on Stromboli Volcano (Italy). Bull. Volcanol. 60, 75–88. Falsaperla, S., Wassermann, J., Scherbaum, F., 2002. Polarization analyses of broadband seismic data recorded on Stromboli Volcano (Italy) from 1996 to 1999. Geophys. Res. Lett. 29, 1–4. Falsaperla, S., Alparone, S., D'Amico, S., Di Grazia, G., Ferrari, F., Langer, H., Sgroi, T., Spampinato, S., 2005. Volcanic tremor at Mt. Etna, Italy, preceding and accompanying the eruption of July– August, 2001. PAGEOPH 162, 1–22. Fehler, M., 1983. Observations of volcanic tremor at Mount St. Helens Volcano. J. Geophys. Res. 88, 3476–3484. Ferrucci, F., Godano, C., Pino, N.A., 1990. Approach to the volcanic tremor by covariance analysis: application to the 1989 eruption of Mt. Etna (Sicily). Geophys. Res. Lett. 17, 2425–2428.
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Gordeev, E., 1993. Modeling of volcanic tremor as explosive point sources in a single-layered, elastic half-space. J. Geophys. Res. 98, 19687–19703. Gresta, S., Montalto, A., Patanè, G., 1991. Volcanic tremor at Mt. Etna (January 1984–March 1985): its relationship to the eruptive activity and modelling of the summit feeding system. Bull. Volcanol. 53, 309–320. Gresta, S., Lombardo, G., Cristofolini, R., 1996a. Characteristics of volcanic tremor accompanying the September 24th, 1986 explosive eruption of Mt. Etna (Italy). Ann. Geofis. 39, 411–420. Gresta, S., Privitera, E., Leotta, A., Gasperini, P., 1996b. Analysis of the intermittent volcanic tremor observed at Mt. Etna (Sicily), during March–May 1987. Ann. Geofis. 39, 421–428. Hagerty, M.T., Schwartz, S.Y., Garcés, M.A., Protti, M., 2000. Analysis of seismic and acoustic observations at Arenal Volcano, Costa Rica, 1995–1997. J. Volcanol. Geotherm. Res. 101, 27–65. Harris, A.J.L., Murray, J.B., Aries, S.E., Davies, M.A., Flynn, L.P., Wooster, M.J., Wright, R., Rothery, D.A., 2000. Effusion rate trends at Etna and Krafla and their implications for eruptive mechanisms. J. Volcanol. Geotherm. Res. 102, 237–269. Hellweg, M., 1999. Seismic signals from Lascar Volcano. J. South Am. Earth Sci. 12, 123–133. Kanasewich, E.R., 1981. Time Sequence Analysis in Geophysics. University of Alberta Press, Edmonton, pp. 1–532. Konstantinou, K.I., Nolet, G., Morgan, W.J., Allen, R.M., Pritchard, M.J., 2000. Seismic phenomena associated with the 1996 Vatnajökull eruption, central Iceland. J. Volcanol. Geotherm. Res. 102, 169–187. Kubotera, A., 1974. Volcanic tremors at Aso volcano. In: Civetta, L., Gasparini, P., Luongo, G., Rapolla, A. (Eds.), Physical Volcanology. Elsevier, Amsterdam, pp. 29–47. McNutt, S.R., 1994. Volcanic tremor from around the world: 1992 update. Acta Vulcanol. 5, 197–200. Métrich, N., Clocchiatti, R., Mosbah, M., Chaussidon, M., 1993. The 1989–1990 activity of Etna magma mingling and ascent of H2O– Cl–S rich basaltic magma. Evidence for melt inclusions. J. Volcanol. Geotherm. Res. 59, 131–144. Monaco, C., Tapponnier, P., Tortorici, L., Gillot, P.Y., 1997. Late Quaternary slip rates on the Acireale–Piedimonte normal faults and tectonic origin of Mt. Etna (Sicily). Earth Planet. Sci. Lett. 147, 125–139. Montalbetti, J.F., Kanasewich, E.R., 1970. Enhancement of teleseismic body phases with a polarization filter. Geophys. J. R. Astron. Soc. 21, 119–129. Neuberg, J., Pointer, T., 2000. Effects of volcano topography on seismic broad-band waveforms. Geophys. J. Int. 143, 239–248. Richter, C.F., 1958. Elementary Seismology. Freeman and Company, San Francisco. 768 pp. Ripepe, M., Coltelli, M., Privitera, E., Gresta, S., Moretti, M., Piccinini, D., 2001. Seismic and infrasonic evidences for an impulsive source of the shallow volcanic tremor at Mt. Etna, Italy. Geophys. Res. Lett. 28, 1071–1074. Riuscetti, M., Schick, R., Seidl, D., 1977. Spectral parameters of volcanic tremors at Etna. J. Volcanol. Geotherm. Res. 2, 289–298. Rothery, D.A., Coltelli, M., Pirie, D., Wooster, M.J., Wright, R., 2001. Documenting surface magmatic activity at Mount Etna using ATSR remote sensing. Bull. Volcanol. 63, 387–397. Saccorotti, G., Chouet, B., Dawson, P., 2001. Wavefield properties of a shallow long-period event and tremor at Kilauea Volcano, Hawaii. J. Volcanol. Geotherm. Res. 109, 163–189. Saccorotti, G., Zuccarello, L., Del Pezzo, E., Ibanez, J., Gresta, S., 2004. Quantitative analysis of the tremor wavefield at Etna Volcano, Italy. J. Volcanol. Geotherm. Res. 136, 223–245.
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Time variation of spectral and wavefield features of volcanic tremor at Mt. Etna (January–June 1999) S. Alparone a,⁎, A. Cannata b,1 , S. Gresta c b
a Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, P.zza Roma 2, 95123 Catania, Italy Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Via Ugo La Malfa 153, 90146 Palermo, Italy c Dipartimento di Scienze Geologiche, Università di Catania, Corso Italia 57, 95129 Catania, Italy
Received 5 July 2005; received in revised form 19 October 2006; accepted 7 December 2006 Available online 19 January 2007
Abstract We have studied the volcanic tremor recorded at Mt. Etna during January–June 1999, by using three-component seismic stations. This time period was characterised by both explosive and effusive eruptions occurring at the summit craters. We found significant time variations in the trend of the overall spectral amplitude of tremor, as well as in the dominant spectral peaks. Moreover, the tremor wavefield features (polarization, particle motion and ratios between the amplitude of the three components of the ground velocity) have been studied too, confirming significant time variations. This leads us to suggest the existence of at least two tremor sources: a shallow one, mainly characterised by relatively high frequencies (3.5–7 Hz), is linked to the upper portions of the active conduits, and directly related to the observed eruptive activity. The latter deeper source was active for only a few weeks, and characterised by frequencies lower than about 2.5 Hz. It had no relationship with the observed eruptive activity, and was speculatively related to the gas exsolution from fresh magma refilling a small batch. © 2007 Elsevier B.V. All rights reserved. Keywords: Mt. Etna; volcanic tremor; spectral and wavefield analysis; tremor source
1. Introduction Mt. Etna, located on the eastern side of Sicily, is a composite, Quaternary volcano characterised by Naalkaline magmatism (Monaco et al., 1997). The present complex picture of the summit area of the volcano is characterised by four upper vents, Voragine, Bocca Nuova, South-East Crater and North-East Crater (here⁎ Corresponding author. Tel.: +39 095 7165836; fax: +39 095 435801. E-mail address: [email protected] (S. Alparone). 1 Now at: Dipartimento di Scienze Geologiche, Università di Catania, Corso Italia 57, 95129 Catania, Italy. 0377-0273/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2006.12.012
after referred as VOR, BN, SEC and NEC, respectively) (Fig. 1). Current volcanic activity may be divided into two main types: persistent activity, which is generally related to the activity of the summit craters, and flank eruptions, that take place through fissures affected by magmatic intrusions. The activity at the summit vents can be of different and sometimes coexistent types: degassing, strombolian or hydromagmatic explosions, lava filling or collapses, and low rate lava emissions (Cristofolini et al., 1988). The flank eruptions are mainly characterised by lava flows (Harris et al., 2000), although during recent episodes the explosive activity is consistent (Behncke and Neri, 2003; Andronico et al., 2005).
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Fig. 1. Sketch map of Mt. Etna with location of the seismic stations used in the present work (triangles); the distribution of the summit craters is reported in the upper left corner (VOR = Voragine, BN = Bocca Nuova, SEC = South-East Crater, NEC = North-East Crater).
After a long quiet period following the December 1991–March 1993 lateral eruption, volcanic activity at Etna resumed on 30 July 1995, at first only in the BN then spreading to the NEC on 2 August (Coltelli et al., 1998). From November 1995 to August 1996, the NEC was the site of lava fountains, while BN had mild strombolian explosions and small lava overflows (Coltelli et al., 1998, 2000). In early November 1996, the eruptive activity (mild strombolian explosions and slow lava effusion) resumed at the SEC. In the summer 1998, eruptive activity at Etna's summit vents changed radically, when persistent strombolian activity and small intracrater lava flows were replaced by occasional shortlived eruptions of higher energy and larger volume, characterised by lava fountains (Rothery et al., 2001). In particular, in September 1998 an explosive eruptive period began at the SEC, characterised by 21 violent lava fountains (Alparone and Privitera, 2001), ending on February 4, 1999. Many authors have investigated the characteristics of volcanic tremor, a seismic signal observed worldwide on most volcanoes (McNutt, 1994). The dominant frequency in tremor waveforms was observed to vary between 0.1 and 10 Hz at various volcanoes (Kubotera, 1974). Many researchers found that these peaks must be
due to source rather than path or site effects (Fehler, 1983; Almendros et al., 1997; Hellweg, 1999; Hagerty et al., 2000). However, other authors (Gordeev, 1993; Benoit and McNutt, 1997; Chouet et al., 1997; Saccorotti et al., 2001) suggested that path effects could also be responsible for some of the observed spectral peaks. Volcanic tremor is particularly evident at Mt. Etna, where it has been studied for decades. Schick and Riuscetti (1973) suggested that the tremor energy could be associated with the velocity of gases escaping through the summit crater conduits. The observed steadiness of the frequency peaks was related to at least one oscillator, excited by gas bubbles rising in the magma (Riuscetti et al., 1977). A physical model for the source was then proposed (Seidl et al., 1981) and oscillators were identified as pipes along which magma rises. The relationship between volcanic tremor and eruptive activity was also studied by various authors (Gresta et al., 1991; Falsaperla et al., 1994; Gresta et al., 1996a,b; Alparone et al., 2003). Ereditato and Luongo (1994) have evidenced that volcanic tremor, during no-eruptive activity, is a stationary phenomenon, characterised by a small variability of its main parameters such as intensity, spectral contents, polarization and wavefield composition, whereas significant variations of its features occur
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during eruptive activity. Falsaperla et al. (2005) noted that different values of the frequency content are associated with diverse styles of eruptive activity; the frequency content of tremor decreases from the preeffusive to effusive stage. In order to locate the source of volcanic tremor, Schick and Riuscetti (1973) reported that, because of the nonimpulsive character of tremor, it is impossible to use techniques based on travel time inversion, similar to those used to calculate epicentres and hypocentres of earthquakes. They used amplitude–distance functions in their attempt to locate the tremor source. Results showed a possible migration of the tremor source, occurring between 1971 and 1972, from a few hundred metres beneath the summit toward the northern part of the Valle del Bove at a depth ranging from 2 to 4 km. Using spectral analysis, Riuscetti et al. (1977) located the tremor source in the Central Crater or beneath it. Two possible detailed sources were located in the magma column, a deep one (between 0.5 and 1 km) and a shallower one, probably at the free surface of the magma column within the Central Crater. A detailed analysis of the recorded tremor spectra along profiles, and their comparison with theoretical models graphs allowed Schick et al. (1982) to distinguish two different sources. The former was characterised by low frequency contents ( f b 1.5 Hz), and might have its origin in a flat magma chamber with a horizontal extent of about 4 km, located at about 2 km beneath the Central Crater. Conversely, the latter source, characterised by a higher frequency content, was associated with the upper portion of the active vents. During the various phases of the July–August 2001 flank eruption, Falsaperla et al. (2005), by using the tremor amplitude decay, estimated the time migration of the tremor source from about 5 km to surface. Di Grazia et al. (2006) suggested the upward migration of the tremor source from about 1700 to 2200 m a.s.l. before the 2004–2005 eruption. Carbone et al. (2006), comparing tremor and gravity data, supposed the activation of two tremor sources: the former associated with the fire-fountaining activity, and the latter deeper and characterised by energetic body-wave arrivals. Many researchers investigated the characteristics of the volcanic tremor wavefield at Mt. Etna, obtaining different results. Ferrucci et al. (1990) suggested the existence of an elongated source having a horizontal projection trending north–south and radiating mainly P waves, during paroxysmal summit eruptions. Conversely, Del Pezzo et al. (1993), using cross-correlation techniques on data collected at a small seismic array during non-eruptive stages, found a high content of surfacewaves originating from a source located in the summit crater area. Analysis of polarization and propagation
velocity (Ereditato and Luongo, 1994) has evidenced the presence of Love waves overlapping other seismic phases. Wassermann and Scherbaum (1994), using data acquired at a distance of 6.5 km from the main crater during a period characterised by strombolian activity at the summit craters, have evidenced that the dominant signal power is concentrated around 1 Hz with the maximum amplitudes observed on the horizontal components. Moreover, through polarization and dispersion analyses, they found a major content of Love waves in the tremor wavefield. More recently, Ripepe et al. (2001) found that the tremor wavefield, recorded at only 50 m from the summit craters during vigorous strombolian activity, indicated the presence of P waves generated by the superposition of small shallow point sources, acting at intervals of 1–2 s. Saccorotti et al. (2004), using data from two dense small aperture arrays, found a complex wavefield where body- and surfacewaves alternatively dominate, depending on both the frequency band and component of motion analysed. The aim of this work is to study the features of the volcanic tremor accompanying eruptive activity occurring at Mt. Etna during January–June 1999. The main goal is the study of the time evolution of both spectral and wavefield parameters in order to characterise the tremor source, as well as to evidence the relationship (if any) with the eruptive activities. 2. Volcanic and earthquakes framework The volcanic activity at Mt. Etna during January– June 1999 was characterised by both explosive and effusive eruptions at the summit craters. Until February 4, eight lava fountains at SEC and discontinuous strombolian activity at BN and VOR occurred. On February 4, during the last lava fountain at SEC, a fissure opened at the base of the SEC cone, starting an effusive eruption that lasted ten months (Fig. 2a). The related lava flow field was characterised by a complex tube network, skylights, ephemeral vents and tumuli (Calvari et al., 2002). During the 6-month studied period, 424 earthquakes were recorded on the volcano (Fig. 3). The seismic activity was composed of individual earthquakes, seismic swarms and small temporal clusters. In particular, the major seismic swarms occurred on January 23 and on February 16 (I and II in Fig. 3). The former was made of 47 earthquakes located on the summit area at about 500 m b.s.l.; the strongest earthquake belonging to this swarm had a magnitude of 2.5. The latter was composed of 54 events located on the southern side of the volcano at depth ranging from 1.5 to 2.5 km b.s.l. and showed a
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Fig. 2. (a) Eruptive activity from January to June 1999 shown for each of the four summit craters and for the eruptive fissure (EF) opened at the base of SEC. 1 = discontinuous strombolian activity; 2 = emissions of ash and lithic material; 3 = lava fountains; 4 = effusive activity. (b) Overall spectral amplitudes of TDF, MNT and ESP vertical component signals and their ratios (grey lines). The moving averages (black lines) are computed every 2.5 days, sliding 1 h. The vertical dashed lines indicate the occurrence of the most important time variations of the analysed tremor features (see text for details).
maximum magnitude of 2.8. Also the time period between March 9 and 20 was characterised by important seismic activity (III in Fig. 3); about 60 earthquakes (Mmax = 2.8), located east of summit craters at depth ranging from 1.5 to 3.5 km b.s.l., occurred. On June 2, a swarm composed of 10 earthquakes, located southeast of summit area at depth ranging from 5.5 to 7.5 km b.s. l., was recorded (IV in Fig. 3); the highest magnitude obtained for these events was 2.7. Finally, on June 24, a swarm composed of 18 events (Mmax = 2.6), affected the southern side of the volcano at depth more than 8.5 km b.s.l. (V in Fig. 3).
3. Tremor data analysis In 1999, two independent permanent seismic networks were operating at Mt. Etna, totalling about 40 stations. However, for the scope of our investigation, stations equipped with three-component seismometers are required; only 7 of these stations are available. Among them, we have excluded the discontinuously running stations and/or those with a poor signal to noise ratio. Then, we use the data acquired from TDF (Torre del Filosofo: 2919 m a.s.l.), MNT (La Montagnola: 2504 m a.s.l.) and ESP (Serra Pizzuta Calvarina: 1600 m a.s.l.) seismic
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Fig. 3. Daily earthquake number (left vertical axis) and strain release cumulative curve (right vertical axis) for earthquakes occurring at Mt. Etna during January–June 1999. The seismic energy (E) was computed using the relationship logE(erg) = 9.9 + 1.9 M–0.024 M2 (Richter, 1958).
stations, located on the southern flank of the volcano at a distance of about 1.1, 3 and 6 km from SEC, respectively (Fig. 1). TDF was equipped with a short period geophone (Lennartz LE-3D/1s), while MNT and ESP were equipped with broadband geophones (Lennartz LE-3D/20s). Data were acquired at a sampling rate of 100 Hz and radio link transmitted to the data acquisition centre in Catania. 3.1. Overall spectral amplitude The overall spectral amplitude, defined as the cumulative of spectral amplitudes, of the vertical component of the ground velocity (OSAV) was calculated at TDF, MNT and ESP in the following way: the long tremor time series was divided in non-overlapping roughly 40-s-long windows (4096 points) and the spectrum was calculated for each window by means of a Fast Fourier Transform. The OSAV was obtained with hourly frequency by the hourly average spectra (90 spectra for each hour) in the frequency range 1–10 Hz (Fig. 2b). The values of the spectral amplitude are not corrected for path, site and instrumental effects. At all stations it is possible to distinguish some periods characterised by different trends, some of which are related to changes in eruptive activity. A first period was characterised by short sudden increases of the OSAV, occurred coinciding with lava fountains at the SEC. From February 4 to 23, we noticed increases in the OSAV, more evident at TDF, occurring in coincidence with the first phases of the summit effusive activity. Afterward, the tremor amplitude decreased until March 12, when a new increasing trend, more evident
in the signal acquired from ESP, was recorded. On March 20, the OSAV at ESP and MNT reached the highest values for the entire studied period. From March 20, the OSAV calculated at TDF showed a strong decrease in its values, lasting until March 23; this variation was less evident at MNT and ESP. On April 16, the OSAV at the seismic stations were sharply reduced, reaching successively the lowest values observed during the studied period. In order to investigate the dynamics of the tremor sources, we calculated the ratios between the OSAV at the stations (Fig. 2b). Variations in the trends of the ratios allowed us to suppose spatial and temporal changes of the tremor sources. During the first period, the ratios were characterised by a stationary trend, suddenly interrupted by short oscillations in coincidence with the lava fountains. A mild increase can be recognised after February 4, in coincidence with the initial phase of the effusive activity. Afterward, from February 23 to March 3, the ratios TDF/MNT and TDF/ESP showed a marked increase, reaching the highest values obtained during the whole 6-month period. It is noteworthy that this trend is due to the greater decrease of the OSAV at MNT and ESP than at TDF. From March 3 on, we noticed an inversion of the ratio pattern, lasting until March 23. Until April 16, the ratio settled around very low and almost stable values. The maximum steadiness was found in the ratio MNT/ ESP, in which the variations were less than 5%. On April 16, the ratios increased sharply (by about 45%). The variability, showed by OSAV during the following 2.5 months, was not considered significant because of very low tremor amplitude.
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3.2. Spectral analysis The spectral analysis of volcanic tremor was performed on the vertical component of the ground velocity acquired at TDF, MNT and ESP seismic stations. The
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daily average spectra, obtained by averaging 24 hourly average spectra (calculated in the same way described in Section 3.1), were calculated. In order to verify the time variations of the frequency content, using the daily average spectra we
Fig. 4. Normalised spectrograms at TDF, MNT and ESP seismic stations from January to June 1999. Black triangles indicate the days when representative spectra of Fig. 5 were collected. Roman numbers (and related vertical lines) indicate the most significant time variations in the spectrogram contents (see text for details).
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achieved the spectrograms showing the whole studied period (Fig. 4). At TDF station, most of the tremor energy was radiated in the frequency band 1–7 Hz, whereas at MNT and ESP in the frequency bands 1.0– 3.5, and 1.0–3.0 Hz, respectively. TDF is located at a shorter distance (ca. 1.1 km) from the summit craters with respect to MNT and ESP (ca. 3 and 6 km, respectively); therefore an additional contribution of relatively high frequencies may be expected owing to propagation effects. The spectrograms allowed distinguishing of four significant time intervals characterised by different frequency content (Fig. 4): I) The first time interval (January 1–February 4; I in Fig. 4) was marked by short sudden increases of tremor amplitude, more evident at TDF, in coincidence with the lava fountains that occurred at the SEC. Representative amplitude spectra of the signals, recorded by TDF, MNT and ESP stations during the lava fountains (Fig. 5a), showed that the dominant frequency band was 1.0–2.5 Hz. The tremor spectra obtained at TDF showed that a
significant amount of energy was also radiated in the band 3.5–7.0 Hz. II) From February 4 to March 23 (II in Fig. 4), we noticed an increase of tremor amplitude, mainly evident in the signal acquired from TDF in the band 3.5–7.0 Hz (Fig. 5b–c). Besides, since February 4, a significant energy contribution was recognisable also in the band 1.0–2.2 Hz, which was reduced from February 23 (Fig. 5c). III) Between March 12 and April 16, the radiated tremor energy was mainly bounded in the range 1.2–2.5 Hz (III in Fig. 4). This feature was well evident at MNT and above all at ESP, where the spectra showed exclusively this frequency range, with amplitude higher than that obtained during the other time intervals, excluding the lava fountains (Fig. 5e). In particular, two dominant peaks with frequencies of 1.7 and 2.0 Hz were observed. No variation in the observed eruptive activity occurred in coincidence with these sharp variations of the tremor features. In the time period March 12–23, tremor simultaneously showed the features observed during the
Fig. 5. Amplitude tremor spectra collected during the significant phases of the whole studied period (see Fig. 4) at TDF (continuous black line), MNT (dashed black line) and ESP (continuous grey line) seismic stations. Each spectrum was obtained by 24 hourly average spectra, excluding the spectrum “a”; in fact, it is representative of a short time interval (1 h) during a lava fountain at SEC. Spectral amplitudes are normalised to the maximum amplitude values observed in the spectrum “a”.
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phases II and III. In particular, at TDF two dominant frequency bands (1.2–2.5 and 3.5– 7.0 Hz) can clearly be seen (Fig. 5d). IV) Between June 1 and 8, an increase of the tremor amplitude occurred in the band 0.5–1.5 Hz. Typical amplitude spectra of the signal, recorded by TDF, MNT and ESP during this short time interval, were very similar (Fig. 5f ). Also these spectral variations cannot be related to variation in the eruptive activity. The comparison among the spectral amplitudes obtained at the three stations used has confirmed that amplitudes did not change uniformly. In fact, with the exception of the period I, when the maximum amplitudes were observed during the lava fountains, the highest values were recorded at TDF during period II (Figs. 4, 5b–c) and at MNT and ESP during period III (Figs. 4, 5e). We can exclude path and site effects for the observed spectral peaks because of their similarity at all three stations (e.g. band 1.2–2.5 Hz in Fig. 5e; band 0.5– 1.5 Hz in Fig. 5f). Consequently, also the time variations of the main spectral peaks at an individual station (Fig. 5c–f) cannot be attributed to site effects, but rather to source migration effects (and/or to the activation of a latter source). 3.3. Wavefield analysis In order to investigate the wavefield composition of the volcanic tremor, three different analyses were performed on the signals acquired from TDF, MNT and ESP stations: (i) computation of ratios between the amplitudes of the three components of the ground velocity, (ii) polarization analysis based on the evaluation of the covariance matrix, and (iii) study of the particle motion. 3.3.1. Ratios OSAH /OSAV In order to verify the main time variations of the wavefield, we performed a general analysis obtaining the values of ratios between the OSA of the three components of ground velocity. With this aim, we calculated the OSA for the frequency range 1–10 Hz with hourly frequency, in the same way described in Section 3.1, also at TDF, MNT and ESP horizontal components. We obtained the OSA of the horizontal component of the ground velocity (OSAH) in the following way: OSAH ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi OSA2NS þ OSA2EW
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where: OSANS and OSAEW are the OSA values of the NS and EW components of the ground velocity, respectively. Fig. 6 shows that the OSAH was about 1.3–2 times larger than the OSAV. It was also possible to note that the ratios OSAH/OSAV, calculated at TDF, MNT and ESP, showed similar trends: in fact, the time period from March 12 to April 16 (period III, see Fig. 4) was characterised, at all the considered stations, by a slight decrease in the ratio (about 10%). Moreover, it is noteworthy that TDF, the nearest station to the summit area, showed the largest values of OSAH/OSAV. 3.3.2. Polarization The analysis of polarization was performed by following the method proposed by Montalbetti and Kanasewich (1970) and Kanasewich (1981), analysing 2-hour-long daily samples, bandpass filtered in various frequency bands. The covariance matrix of the three components of ground displacement was calculated over successive windows with a time step of 0.01 s (one sample of the time series). The values of azimuth, incidence angle (with respect to the vertical direction) and rectilinearity were calculated in the frequency band 1.2–2.5 Hz, with 1.7-s-long windows, at TDF, MNT and ESP and in the frequency band 3.5–7.0 Hz, with 0.6-s-long windows (Fig. 7), at TDF; in fact, the poor signal to noise ratio, observed in the frequency band 3.5–7.0 Hz at MNT and ESP, did not allow us to obtain reliable values of the polarization parameters. In the frequency band 1.2–2.5 Hz, at ESP the polarization parameters showed a greater variability than at MNT and TDF. At all used stations it was possible to distinguish a first time interval (whole period I and part of period II, see Fig. 4) characterised by almost steady values: polarization azimuth was clustered mainly in the N70°E–N100°E range, the incidence angle was linked to the horizontal particle motion and the rectilinearity coefficient exhibited stable values of about 0.6. From March 12 to April 16 (period III, see Fig. 4), the polarization parameters changed sharply: the azimuth values were equal to about N100°E at TDF, N110°E at MNT and N125°E at ESP; the rectilinearity coefficient settled around almost stable values of about 0.5 at all three stations; the incidence angle decreased, especially at MNT and ESP, showing intermediate values from March 12 to 23 (about 80° at MNT and 75° at ESP) and from March 24 to April 16 the lowest values obtained for the studied period (about 75° at MNT and 55° at ESP). Since April 16, the polarization parameters returned to the values observed before March 12. At ESP, a major
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Fig. 6. Time variation of the ratios OSAH/OSAV at TDF, MNT and ESP stations (see text, for details).
difference concerned the high variability of the azimuth values that ranged between N80°E and N120°E. In the frequency band 3.5–7.0 Hz at TDF, azimuth directions were clustered mainly in the N80°E–N110°E range and the incidence angle values were representative of a horizontal particle motion. The values of rectilinearity ranged between 0.55 and 0.65, reaching its lowest value in February, during the first phases of effusive activity at SEC (period II, see Fig. 4). The analysis of polarization was also performed over successive 2-s-long windows on the signal bandpass filtered in the frequency band 0.5–1.5 Hz, recorded at the beginning of June (period IV, see Fig. 4), when an increase of the tremor amplitude occurred. The values of the moving average over 75 min are reported in the insets of Fig. 7. Significant variations of trends of these parameters can be noted from June 1: at MNT and ESP polarization azimuths changed respectively from N100°E to N120°E and from N90°E to N125°E; the incidence angles decreased reaching in the next days minimum values of about 50° at MNT, and 40° at ESP; the rectilinearity coefficient changed from about 0.6 to 0.5. 3.3.3. Particle motion The particle motion analysis was performed on samples of signal, bandpass filtered into narrow
frequency bands (bandwidth of about 2 Hz). During most of the studied period, the tremor was characterised by horizontal polarization, mainly toward east–west (Fig. 8a–d). We noted some periods characterised by variations of the particle motion. In February, in coincidence with the first phase of the effusive activity at SEC, we also noticed an elliptical retrograde motion, vertically polarized, at TDF in the band 3.5–7 Hz (Fig. 8b). From March 12 to April 16, the particle motion was elliptical/chaotic in all three planes (NS–Z, EW–Z and NS–EW), in the frequency band 1.2–2.5 Hz (Fig. 8c). Also the signal acquired at the beginning of June at MNT and ESP, in the band 0.5–1.5 Hz, was characterised by elliptical/chaotic particle motion (Fig. 8e). Considering all features of the tremor wavefield, obtained by the three different analyses described above, we can state that the wavefield was dominated by transverse, horizontally propagating waves (SH waves or Love waves), during most of the studied period. In February, Rayleigh waves were also recorded at TDF in the frequency band 3.5–7 Hz, as confirmed by the lowest values of rectilinearity. From March 12 to April 16, the complexity of wavefield in the frequency band 1.2– 2.5 Hz may be due to the combination of different waves (such as P, SH and SV waves). Also at the beginning of June, the observed wavefield features in the band 0.5–
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Fig. 7. Polarization parameters obtained by analysing 2-hour-long daily samples, bandpass filtered in the frequency bands 3.5–7.0 Hz at TDF and 1.2–2.5 Hz at TDF, MNT and ESP. Triangles, squares and circles indicate azimuth, incidence angle (calculated with respect to the vertical direction) and rectilinearity coefficient, respectively. More details of the polarization parameters at MNT and ESP in the band 0.5–1.5 Hz, concerning June 1–3, are reported in the insets of the figure; (a), (b) and (c) series indicate azimuth, incidence angle and rectilinearity coefficient, respectively. The arrows indicate the beginning of the most important variations of the polarization parameters.
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Fig. 8. Some examples of particle motion plots for signals acquired at TDF, MNT and ESP seismic stations highlighting the remarkable time variation of the tremor wavefield features during the studied time period (see text for details).
1.5 Hz can be explained with the coexistence in the tremor wavefield of various wave-types. The dominant signal power of the horizontal components and very low deviations of the polarization directions from the horizontal, which characterised the volcanic tremor recorded during most of the studied period, agree with results already found by Wassermann and Scherbaum (1994), using data acquired during a period characterised by strombolian activity at the summit craters. 4. Discussion Time variations of the volcanic tremor features have been observed in different volcanoes such as Ruapehu (Bryan and Sherburn, 1999), Stromboli (Falsaperla et al., 1998), Vatnajökull (Konstantinou et al., 2000) and Shishaldin (Thompson et al., 2002), and interpreted as due to changes in tremor sources. The analysis of the volcanic tremor recorded at Mt. Etna during January–June 1999 allowed us to find significant time variations in some parameters, to com-
pare them with the evolution of volcanic phenomena, and to suggest the activation of different tremor sources. Some time intervals having different tremor features have been distinguished. During the lava fountains period, a general short increase of the OSAs and changes of the ratios among the OSAs at different stations occurred (Fig. 2). Several variations of the tremor features were recognisable after the last lava fountain, occurring on February 4, until February 23 (the first part of the period II, see Fig. 4): i) an increase of the OSAs, most evident at TDF station (Fig. 2b); ii) an increase in the ratios between pairs of OSAs: TDF/MNT, TDF/ESP and MNT/ESP (Fig. 2b); iii) variations in the spectral content (Fig. 4). Both increases of tremor amplitude and spectral changes have frequently been observed at Mt. Etna in coincidence with the onset of flank eruptions, as in December 1991 (Falsaperla et al., 1994) and in July 2001 (Falsaperla et al., 2005). These evidences have been associated with the activation of tremor sources linked to the feeding system of the eruptive fractures.
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We have interpreted the variation observed on February 4, 1999 as due to the activation of a shallow tremor source related to the effusive activity that began at SEC (Calvari et al., 2002). The tremor radiated by this source was characterised, at the closest station TDF, by a large band of frequency (1–7 Hz), and a wavefield mainly composed by transversal waves propagating horizontally (SH or Love waves) and sporadically by Rayleigh waves. The contribution of energy in the band of frequency 3.5–7.0 Hz was poorly evident at MNT and ESP stations, probably owing to propagation effects. Since February 23, the increase of the ratios TDF/ MNT and TDF/ESP (Fig. 2b), was related to the decrease of the tremor amplitude in the band 1–2 Hz (Fig. 4). In fact, the amplitude reduction in this frequency band mainly affected the OSAs at MNT and ESP, because at TDF there was also a significant energy contribution in the band 3.5–7.0 Hz. On March 12 (the beginning of period III, see Fig. 4), several features of the tremor sharply changed without any variation in the observed eruptive activity. We observed: i) an increase of the OSAs, more evident at MNT and ESP, related to the increase of the tremor amplitude in the band 1.2–2.5 Hz (Figs. 2b, 4); ii) a decrease of the ratios TDF/MNT, TDF/ESP and MNT/ ESP (Fig. 2b); iii) a decrease in the incidence angle of the seismic rays calculated with respect to the vertical direction (Fig. 7). All these evidences allowed us to hypothesize that since March 12 a “deep” tremor source was activated, mainly radiating tremor-waves in the frequency range of 1.2–2.5 Hz. A steepening of the incidence angle of tremor waves, similar to the one found in the present study, was observed at Stromboli, without any concurrent variation in the volcanic activity; it was interpreted as due to changes affecting a deep part of the magma feeder, probably due to the replenishment of fluids (Falsaperla et al., 2002). A similar variation of the ratio between the tremor amplitude recorded at the topmost station and the intermediate/peripheral ones, was found during a huge explosive eruption at NEC, and interpreted as the activation of a ca. 2 km deep sub-vertically planar source (Gresta et al., 1996a). The wavefield variations, occurring during the time period March 12–April 16, may be due to the combination of different wave types, causing the decrease of rectilinearity and the ellipticity and complexity of the particle motion (Figs. 7, 8c). These wavefield features cannot be explained by supposing the existence of a radially symmetric source, which radiates only P and SV waves, but can be due to the activation of a rectangular crack-like resonator, that radiates P, SV and SH
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waves, as was supposed by Chouet et al. (1997) at Stromboli volcano and by Hagerty et al. (2000) at Arenal volcano. From March 12 to 23, the volcanic tremor showed spectral content (Fig. 4) and wavefield features, such as OSAH/OSAV ratios (Fig. 6) and incidence angle (Fig. 7), representing the superposition of the wavefield characteristics of the two observed sources. This characteristic was due to the coexistence of the two different tremor sources (the shallow one and the deep one) described above. On March 23, the energy radiated by the shallow source was reduced and the deep tremor source became dominant. On April 16 (the end of period III, see Fig. 4), the tremor energy, released by the deep source, was also sharply reduced. In particular, we observed: i) a decrease of the OSAs, related to the sharp reduction of the tremor amplitude in the band 1.2–2.5 Hz (Figs. 2b, 4); ii) a marked increase of the ratios TDF/MNT, TDF/ESP and MNT/ESP (Fig. 2b); iii) an increase of the incidence angle of the seismic rays (Fig. 7); iv) a particle motion comparable to that observed usually before March 12 (Figs. 8 a–d). Finally, during a few days at the beginning of June (period IV, see Fig. 4), we observed an increase of the tremor amplitude in the band 0.5–1.5 Hz and a decrease of the incidence angle of the seismic rays (Figs. 4, 7). In this case, we supposed that the presence of a deep source, like the previous deep source, produced a complex wavefield. In order to roughly locate the deeper tremor source, active from March 12 to April 16, we considered the parameters obtained by the polarization analysis. During this time period, because of the coexistence of at least two tremor sources located at different depths, polarization parameters showed values affected by the wavefield produced by both sources. Anyway, taking into account the intersection point between the azimuth directions obtained at MNT and ESP, and the incidence angle value at ESP, we roughly located the tremor source south-west of the summit area at depth of a few kilometres below sea level. The polarization parameters obtained at TDF were not considered, because the seismic signal at this station is strongly influenced by the tremor produced at the summit craters. We well know that the used method is oversimplified, because it does not take into account the effects induced by the medium heterogeneity (as the layering) and the topography; in fact, Neuberg and Pointer (2000) demonstrated that the incidence angle values may be significantly affected by volcano topography. However, we would like to stress that, regardless of its correct
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location, we observed tremor feature variations related to a secondary source that was active at Mt. Etna from March 12 to April 16, 1999. Looking at the results of the other disciplines during the entire analysed period, the only significant variation in ground deformation was given by a tiltmeter located at Pizzi Deneri (see Fig. 1); a tilt of about 2–3 μrad was measured from March 2 to 10, suggesting a slight general uplift of the southern slope of the volcano (Gambino, personal communication), just a few days before the activation of the deeper tremor source. On the contrary, SO2 flux measurements showed a strongly irregular trend until mid-February, with the estimated daily release varying from ca. 2000 to 14,000 t. These values are in agreement with the occurrence of lava fountains and the first stages (with larger magma output rate) of the effusive eruption. Afterwards a slightly decreasing trend began (from ca. 5500 t/d to 1000 t/d) lasting to the beginning of June. During this generally decreasing trend only two significantly larger amounts have been found (ca. 7000 t/d and 9000 t/d), both in the second half of March (Caltabiano, personal communication), a few days after the activation of the “deep” tremor source. Even if slight, the observed variation in ground deformation and SO2 flux, respectively a few days before and after the observed variation of volcanic tremor features starting on March 12, supports the hypothesis of the activation of a tremor source, roughly located south-west of the summit area a few kilometres below sea level. At Mt. Etna, most SO2 and water exsolve into the gas phase from 2 kbar to 0.5 kbar, (Métrich et al., 1993), that is between about 2.5 km b.s.l. and 2.0 km a.s.l. We suggest that during the first days of March 1999 a refilling of magma occurred from depth into a relatively small and shallow batch. This successively produced: i) a slight inflation of the southern upper part of the volcano body; ii) the beginning of gas exsolution from the new magma and consequently the generation of tremor; iii) the emission of the exsolved gases from the central plumbing system of the volcano some days later. Finally, the rough location of the deep source is obtained by considering a point-like source. Some previous works have provided evidence for a horizontally (i.e. Schick et al., 1982) or vertically (i.e. Ferrucci et al., 1990) extended source of tremor at Mt. Etna volcano. This does not conflict with our results. We suggest that the inferred deep source is to be considered as the active part (where the gas exsolution was able to generate the radiation of seismic energy) of a larger magma batch.
5. Concluding remarks We have studied the spectral and wavefield features of the volcanic tremor recorded at Mt. Etna during January–June 1999. This time period was characterised by both explosive and effusive eruptions in the summit area. We performed spectral and wavefield (polarization, particle motion and ratios between amplitudes) analysis at three seismic stations, and a good agreement among the results obtained by the different used approaches was found. Two tremor sources have been identified. The former, characterised mainly by relatively high frequencies (3.5–7.0 Hz), was linked to the upper portions of the active conduits and was directly related to the observable eruptive activity. The latter source, radiating frequencies lower than 2.5 Hz, was highly active from March 12 to April 16 and deeper than the first one. It could be due to the gas exsolution of fresh magma refilling a small batch from depth. However, these results need to be proved with additional data. This work, such as other recent papers (e.g. Falsaperla et al., 2005; Carbone et al., 2006; Di Grazia et al., 2006), shows that the location and the features of the tremor source at Mt. Etna strongly change depending on considered time period and volcanic activity. Acknowledgments We thank Ferruccio Ferrari for useful technical support and Steve Conway for the English revision of the text. Salvo Gambino and Tommaso Caltabiano are kindly acknowledged for providing us unpublished data. We are indeed grateful to two anonymous reviewers for their useful comments and suggestions which helped us to significantly improve the quality of the paper. This research has been supported by funds INGV-DPC (2005–2006). References Almendros, J., Ibanez, J.M., Alguacil, G., Del Pezzo, E., Ortiz, R., 1997. Array tracking of the volcanic tremor source at Deception Island, Antarctica. Geophys. Res. Lett. 24, 3069–3072. Alparone, S., Privitera, E., 2001. Characteristics of the intermittent volcanic tremor at Mt. Etna, Italy, during the 15 September 1998–4 February 1999 eruptive episode. Proceedings of the Cities on Volcanoes 2 Conference, Auckland, New Zealand, 12–14 February 2001, Earthquake Commission, Inst. Geol. Nucl. Sci. Inf. Ser., vol. 49, Lower Hutt, New Zealand. Alparone, S., Andronico, D., Lodato, L., Sgroi, T., 2003. Relationship between tremor and volcanic activity during the Southeast Crater eruption on Mount Etna in early 2000. J. Geophys. Res. 108 B (5), 2241. doi:10.1029/2002JB001866.
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