Analysis of simultaneous gravity and tremor anomalies observed during the 2002–2003 Etna eruption

Earth and Planetary Science Letters 245 (2006) 616 – 629 www.elsevier.com/locate/epsl

Analysis of simultaneous gravity and tremor anomalies observed during the 2002–2003 Etna eruption Daniele Carbone a, * , Luciano Zuccarello a , Gilberto Saccorotti b , Filippo Greco a b

a Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Via Diocleziano 328, 80124 Napoli, Italy

Received 5 May 2005; received in revised form 17 January 2006; accepted 30 March 2006 Available online 6 May 2006 Editor: V. Courtillot

Abstract In this paper we discuss the data collected by a large aperture array of broadband seismometers and a continuously recording gravity station during the 2002–2003 eruption of Etna volcano (Italy). Seismic signals recorded during the eruption are dominated by volcanic tremor whose energy spans the 0.5–5 Hz frequency band. On three different occasions (12 November, 19–20 November and 8–9 December 2002), we observed marked increases of the tremor amplitude (up to a factor of 4), which occurred simultaneously with gravity decreases (up to 30 μGal). The three concurrent gravity/tremor anomalies last 6 to 12 hours and terminate with rapid (up to 2 hours) changes, after which the signals return back to their original levels. Based on volcanological observations encompassing the simultaneous anomalies, we infer that the accumulation of a gas cloud at some level in the conduit plexus feeding a new eruptive vent could have acted as a joint source. This study highlights the potential of joint gravity–seismological analyses to both investigate the internal dynamic of a volcano and to improve the confidence of volcanic hazard assessment. © 2006 Elsevier B.V. All rights reserved. Keywords: Etna; volcanic tremor; gravity changes; foam layer

1. Introduction Over the last few decades methods and techniques aimed at monitoring active volcanoes and studying the dynamics of their plumbing systems have improved significantly. Among the other signals, volcanic tremor and microgravity changes are routinely measured and studied at most volcanoes. Volcanic tremor is a sustained seismic signal which is generally observed in association with magmatic

* Corresponding author. E-mail address: [email protected] (D. Carbone). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.03.055

and hydrothermal activity, and its occurrence has been documented at many different volcanoes throughout the world (see [1] for a comprehensive review of tremor studies). Although different models have been proposed to explain the source mechanism of tremor, all authors concord in attributing its origin to the complex interplay among the magmatic hydrothermal fluids and their hosting rocks. Microgravity studies represent a relatively new technique to investigate and monitor active volcanoes, with respect to some of the more established methods. Temporal changes of the gravity field in volcanic zones are related to sub-surface mass/volume/density changes or to elevation changes in response to magmatic processes

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and vary significantly in both space (wavelengths ranging from hundreds of meters to tens of kilometers) and time (periods ranging from minutes to years) according to the size, depth and rate of evolution of the source. Thus, both discrete measurements (e.g. [2–8]) and continuous observations [9,10] are accomplished. Discrete measurements allow a good space resolution (depending on the number of measurements in a single campaign) to be achieved, while the time resolution is limited by the repetition time of the surveys (usually monthly to yearly); continuous measurements allow a good time resolution to be reached but the space resolution is usually poor due to high cost of gravimeters that limits dramatically the number of instruments available at a single area. Even though comparisons between multidisciplinary data acquired at volcanic areas are now often performed (e.g. [11–17]), rigorous cross-analyses are not routinely accomplished, even at the most monitored sites, because of the different nature of the data sets themselves (especially as for sampling rate) and the relatively poor

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interaction among research groups working on different subjects. Mt. Etna is one of the best monitored volcanoes of the world [18–23]. It is also highly active, with almostperennial degassing from the summit craters and recurrent summit and flank eruptions [24–26]. One of the most voluminous eruption of Etna's recent history began on the night of October 26–27, 2002 with lava flows issued from two different fissure systems at elevations between 2850 and 2600 m on the southern flank of the volcano, and between 2470 and 1900 m on the northeastern flank (Fig. 1; [27]). The most important feature of the 2002–03 eruption was the extraordinary explosive activity from the southernmost fissure systems, which led to a volume-of-pyroclastic-products/ total-erupted-volume ratio of 0.5–0.6, the highest since the sixteenth century [27]. By comparison, the same ratio was 0.22 for the 2001 Etna flank eruption (25.3 × 106 m3 of lava and 7.8 × 106 m3 of thephra [28]) and always within 0.1 for the flank eruptions throughout the twentieth century, with the only exception of the two 1974 eruptions

Fig. 1. Schematic map showing the position of the four seismic broadband stations and the continuous gravity station used in the present study. The lava flow fields from Etna's 2002–03 eruption are also reported. Contours are at 200 m intervals.

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Fig. 2. Top four panels: overall spectral amplitudes of the vertical velocity component observed at EMTB, ERCB, EMFB and ETRB seismic broadband stations during the 7 November–10 December 2002 period. The overall spectral amplitude of each tremor sequence was calculated within the 0.02–10 Hz frequency range (0.5–50 s). A 4096-sample sliding window (about 10% overlap) was used for the FFT calculation. Lower panel: gravity, after removal of (i) the best linear fit, (ii) the theoretical Earth Tide and (iii) longer wavelength components (cut off frequency of the high-pass filter equal to 2.8 * 10− 6 Hz, corresponding to a period of 100 h) observed at PDN station during the same time interval. Bottom: timeline of the main volcanic events (after [17]) during the period under study. Numbers under each marker refer to the first column of Table 1.

(0.34 and 0.46 [28]). The activity on the southern flank lasted 93 days (27 October 2002–28 January 2003) and depicted a variable eruptive style. The explosive activity concentrated at a vent located at 2750 m a.s.l., where a cinder cone formed (Fig. 1). It mainly consisted of intense

column-forming fire fountains. From the second half of December, 2002, fire-fountaining activity became pulsating and alternated with mild strombolian activity. Effusive activity was discontinuous and occurred from different vents which opened mostly at the base of the 2750 m

Fig. 3. Velocity seismograms (vertical component, ERCB station), overall spectral amplitude and gravity (ERCB and PDN stations respectively; see caption to Fig. 2 for details) relative to three 60-h periods beginning respectively at 00:00, November 12, 2002 (a, b and c), 00:00, November 19, 2002 (f, g and h) and 12:00, December 7, 2002. Low-pass filters (cut off frequency equal to 6 * 10− 4 Hz, corresponding to a period of about 28 min) of the sub-sequences evidenced with grey strips in b–c, g–h and l–m are reported in d–e, i–j and n–o.

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cinder cone. They produced a fan-shaped lava-flow field with maximum length of 4 km (Fig. 1) until 28 January 2003, when the effusive activity ceased. A total volume of

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74.5 × 106 m3 (29.5× 106 m3 of lava and 45 × 106 m3 of thephra [28]) was erupted from the fissure system on the southern flank. The activity on the northern flank lasted

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9 days (27 October–05 November 2002) and was characterized by fire fountaining, strombolian and effusive activity from the vents which opened along the fracture system as it propagated downslope along the eastern border of Etna's Northeast Rift (Fig. 1). The explosive activity was less intense than that occurring on the southern flank and began to decrease slowly since 29 October. The effusive activity produced two flows with maximum length of 6.5 km (Fig. 1) until 3 November, when it came to an end. A total volume of 11 × 106 m3 (107 m3 of lava and 106 m3 of thephra [28]) was erupted from the fissure system on the northern flank. In order to study the dynamics of the volcanic processes occurring at Etna, some continuously running gravity stations were installed in 1998 and have worked intermittently since then [10]. During three separate periods on November and December, 2002, we noticed three contemporaneous anomalies in the gravity signal and the signal from some Broadband stations which were installed on the summit zone of Etna after the start of the 2002–03 eruption. The analysis performed on these joint anomalies, each spanning a time interval of a few hours, provides a unique insight into the operation of Etna's degassing system. In the following, after having presented the dataset (Section 2) and have discussed the implications of not having deformation data at a rate suitable to reduce our gravity data (Section 3), we propose a possible source mechanism to explain the joint tremor/gravity anomalies (Section 4). Then we perform a first approximation calculation to set quantitative constraints to the inferred mechanism (Section 5), before we conclude (Section 6).

2. Data presentation On the 30th of October a seismic array of six Broadband stations was installed on the summit zone of Etna to monitor with a suitable azimuthal coverage the activity related to the ongoing eruption. Each seismic station was equipped with a Guralp CMG-40T Broadband (60–0.02 s), 3-component seismometer. Data were recorded at a rate of 62.5 samples per second through Lennartz Marslite digital systems (20 bit) which use a GPS time base. Out of the six available instruments, in this work we use only data from the four stations closer to the eruptive vents (sites EMTB, ERCB, EMFB and ETRB in Fig. 1). A continuously recording gravity station, located on the Northeastern slope of the volcano (PDN; 2920 m a.s.l.; see Fig. 1), was also operated throughout the period encompassing the 2002–03 eruption [10,29]. This station is equipped with a LaCoste and Romberg gravimeter (D-185), featuring an analog feedback system [30]. Data are recorded at 1 datum/min sampling rate (each datum is the average calculated over 60 measurements) through a CR10X Campbell Scientific datalogger. Tremor sequences are analyzed by calculating the integral of the spectrum over the 0.02–10 Hz frequency range. Spectral estimates are obtained via FFT over subsequent 4096-sample (65.5 s) windows of signal. By choosing a suitable overlap of the sliding window (about 10%), we obtain overall spectral amplitude sequences at a rate of 1 datum/min, i.e. the same sampling rate of the gravity signal. Before calculating the FFT, each data segment is demeaned and corrected for the linear trend.

Fig. 4. (a): time changes of the correlation coefficient between the gravity sequence from PDN (lower graph in Fig. 2) and the overall spectral amplitude sequences from EMTB, ERCB, EMFB and ETRB seismic broadband stations (upper four diagrams in Fig. 2). A sliding 24-h window (50% overlap) was used for the calculation. (b): coherence functions in the frequency domain calculated over three 24-h intervals (grey strips in (a), encompassing the joint tremor/gravity anomalies) between gravity (PDN station) and overall spectral amplitude (ERCB station).

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We separately applied these procedures to the three components of ground velocity. For all the three different cases, we observed similar time behavior of tremor amplitudes and therefore, for the sake of conciseness, throughout in the following we present and discuss only results concerning the vertical-component data. The gravity signal is reduced for the effect of Earth Tides (modeled through the Eterna 3.30 data processing package [31]) and instrumental drift (modeled as the best linear fit), and high-pass-filtered (cut off frequency of 2.8 * 10− 6 Hz, corresponding to a period of 100 h) to remove the longer wavelength components which are not of interest to the present study. The overall spectral amplitude sequences for the 7 November–9 December period are presented in Fig. 2 (from top to bottom stations EMTB, ERCB, ETRB, EMFB), together with the gravity sequence acquired at PDN during the same period (lower panel of Fig. 2). The most important features appearing in Fig. 2 (evidenced with the grey strips) are the marked increases in the amplitude of the tremor (by a factor ranging between 3 and 4) which occurred at all the stations (a) on 12 November, (b) between 19 and 20 November and (c) between 8 and 9 December. These increases in the amplitude of the tremor occur simultaneously with decreases in the gravity value having a maximum amplitude ranging between 10 and 30 μGal (1 μGal= 10 nm s− 2; see Fig. 3b, c, g, h, l and m for details). The three simultaneous tremor amplitude increases/gravity decreases last respectively 12 h (12 November), 6 h (19–20 November) and 7 h (8–9 December), and terminate with steep changes, lasting 0.5 to 2 h, after which the sequences return to the mean amplitude they had before the anomaly took place (Fig. 3). As a further step of our analysis, we calculate the correlation between the gravity sequence and the tremor spectral amplitudes at each station, using a 12-h-long time window sliding along the signal with 50% overlap (Fig. 4a). A marked anti-correlation (with amplitude up to 0.95) occurs in correspondence of the 12 November,

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19–20 November and 8–9 December anomalies but also over the 13th of November, when a slighter increase in the tremor amplitude took place after the sharp decrease terminating the 12 November anomaly (Figs. 2 and 3b). Over the rest of the considered time interval, the amplitude of the anti-correlation never reaches values higher than 0.4. To investigate possible correlations over frequency bands higher than those associated with the maximum energy (corresponding to the longest period of each anomaly; see Fig. 3b, c, g, h), correlation analyses in the frequency domain between the gravity sequence and the overall spectral amplitude sequence from ERCB station are also computed (Fig. 4b) over the three 24-h periods evidenced with the grey strips in Fig. 4a (I, II and III). The coherence function is calculated by the Matlab® signal processing tool, which is based upon Welch's averaged periodogram method [32]. The first diagram (Fig. 4b I), relative to the 12 November anomaly, shows a peak at about 5 * 10− 4 Hz (T equal to about 30 min) with coherence values becoming lower as frequency increases. The second and third coherence diagrams (19–20 November and 8–9 December anomalies; Fig. 4b II, III) lack a distinctive peak; coherence values remain within 0.4. Thus, during the first anomaly the two time series depict significant correlations also over periods which are shorter than the duration of the anomaly itself. In order to characterize the wavetypes associated with the tremor signal we perform a polarization analysis, applying the covariance method [33], to 1-s-long windows of signal sliding with 50% overlap along the 0.5–5 Hz 3component seismograms. For each step of the analysis, this procedure allows to retrieve the spatial setting (azimuth and incidence angles) of the ellipsoid which best fits, in a least-square sense, the particle motion trajectory. The degree of linearity of the polarization ellipsoid is then expressed through the rectilinearity coefficient, which takes extremes values of 0 and 1 for purely-spherical and linear motions, respectively. Under this convention, high values of rectilinearity are representative of body-wave

Fig. 5. Hourly number of polarization measurements (ERCB station) for which the rectilinearity coefficient is greater than 0.9 (see text for details). Grey strips highlight the periods over which the joint tremor/gravity anomalies took place.

622 D. Carbone et al. / Earth and Planetary Science Letters 245 (2006) 616–629 Fig. 6. Tilt sequences from three stations of the Etna network (upper panels) and signals from the levels fitted to the gravimeter at PDN (lower panel) relative to the for the 7 November–10 December 2002 period. Grey strips highlight the periods over which the joint tremor/gravity anomalies took place.

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arrivals. Results from application of this procedure to the available tremor sequences depict a complicate picture. With the only exception of station EMTB, the ground vibration at all the network sites is dominated by horizontal motion oriented transversally with respect to the direction pointing to the active zone; such feature does not significantly change throughout the analysed tremor sequences. The only remarkable change we observed is associated with peak values in the hourly number of linearly-polarized waves (rectilinearity N 0.9) occurring in concomitance of all the three common tremor/gravity anomalies and, similarly to the results of the above correlation analysis, also over November 13th (Fig. 5). 3. Implications of the lack of elevation control on the gravity data In volcanic areas, measured gravity changes may reflect underground mass/density/volume changes, ground deformations, or a combination of both factors. Elevation changes imply a variation of the distance between the observation point and the Earth's center, thus leading to a gravity increase or decrease for ground deflation or inflation, respectively. Conversely, tilt changes, that move the meter away from the horizontal position (where it measures the full force of gravity), produce a negative gravity effect. Thus, ground deformation measurements are crucial to a proper interpretation of the observed gravity variations. The only technique able to furnish deformation data at a rate useful to reduce gravity anomalies lasting 6-to-12 h is continuous GPS. Unfortunately, due to power-saving reasons, GPS stations operating on the summit zone of Mt. Etna during the period of interest acquired data for only 3 h every day, i.e. between 10:00 and 13:00 GMT, a time interval which does not cover any of the three gravity anomalies under study. Tilt changes measured during the three joint anomalies, using the levels fitted to the gravimeter (resolution = 2.5 μrad), were negligible (within 10 μrad; Fig. 6, lower panel). Data coming from 3 tilt station on Etna, also show negligible variations (Fig. 6, upper three panels). However, although suggesting the occurrence of negligible deformation, the absence of tilt changes does not definitely rule out the possibility that significant elevation changes took place at PDN station over the periods encompassing the three joint anomalies. In fact, following Cayol and Cornet [34], who used a 3D model with a spherical pressurized source located beneath an asymmetrical volcano, significant vertical deformations with negligible tilt changes can occur if the slope of the flanks of the volcano is greater than 20° and the observation point is within 0.5 ÷ 1 * a of the projection

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onto the surface of the source center (a is the radius of the spherical source). Also, in the case of a flat topography, noticeable vertical deformations without tilt changes occur at observation points which lie on the vertical axis of the spherical source. Considering the free-air gradient (usually taken as −308.6 μGal/m [35]) or the Bouguer corrected (using the Mogi [36] elastic model) free-air gradient (ranging between −233 and −244 μGal/m [35]), elevation increases between 3 ÷ 4.3 and 10÷ 12.9 cm should have occurred to induce the observed 10 to 30 μGal gravity decreases. 4. A possible source mechanism of the joint tremor/ gravity anomalies Results of the correlation analysis presented in Fig. 4a, with amplitude of the anti-correlation up to 0.95 in correspondence of the evidenced periods and a negligible correlation elsewhere, strongly suggest that during the three tremor amplitude increases/gravity decreases a common source activated which, during the rest of the period under study, either was not active or produced a weak, negligible effect. Unfortunately, our tremor data do not allow to set constraints on the location of this common source. The large station spacing of our network (Fig. 1) hinders the possibility of analyzing the seismic recordings by means of multichannel techniques aimed at deriving the propagation direction and apparent velocity of the tremor waves [37–39]. Furthermore, results of the polarization analyses (Section 2) indicate a dominance of transverse motion at most recording sites. This observation may be interpreted as the effect of the radiation pattern from a non-isotropic source mechanism. For instance, the resonance of a fluid-filled, crack-like buried cavity has been demonstrated to be an efficient generator of large SH components [40]. The presence of soft pyroclastic layers in the shallowest portion of the propagation medium might also induce efficient amplification and trapping of shear energy, as postulated in a study of the volcanic tremor associated with the 1999 Etna's eruption [39]. A further factor severely conditioning waveform amplitudes and particle motion is represented by wave conversions, reflections and scattering at the free-surface. The roughness of volcanic topographies spans a wide range of wavelengths, therefore affecting the propagation of seismic waves over a broad frequency band [41–44]. Given all these concurring effects, the particle motion azimuths and incidence angles determined through polarization analyses do not provide any useful insight into the location of the source of the three joint anomalies.

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Table 1 Chronology of the volcanic activity during the period of interest (after [27]) Date

Description of activity

6–11 November 1

2 3

4 5 6 7 8

Intense fire fountaining activity (1st period) Between the afternoon of 12 Lava fountains replaced by November and the following strombolian activity day 13 November Effusive activity resumes (base of the 2750 cone) 14 November Fire fountaining activity resumes (start of 2nd period; not as continuous as in the first period) 17 November Two new explosive vents open Between 20 and 21 A new effusive vent opens (base of November the 2750 cone) 25 November Two new explosive vents open (base of the 2750 cone) 8 December Lava fountains replaced by strombolian activity for a few hours 10 December Fire fountaining activity resumes and two new effusive vents open (base of the 2750 cone)

Constraints on the position of the joint source cannot be set using gravity data from only one station either. The lack of strict geometrical constraints on the inferred joint source, together with the unavailability of elevation data, which could allow the observed gravity variations to be reduced (see previous section), implies that only a speculative interpretation of the source mechanism can be attempted in the following. Volcanological observations conducted during the period of interest to the present study [27] are schematically reported at the bottom of Fig. 2 (numbers below each marker refer to the first column of Table 1). Both the 12 November and the 8 December anomalies occurred during periods of temporary switch of the activity from intense lava fountaining to mild strombolian activity. Furthermore, after the end of all the three anomalies, new effusive vents opened. Within the limitations of the time resolution of the volcanological observations conducted at summit active vent, the temporal fit is tight, thus likely to reflect a link between the occurrence of (a) the above mentioned features of the volcanic activity and (b) the joint tremor/gravity anomalies. It is important to stress that the second tremor/gravity anomaly (19–20 November) developed during nightly hours (between 11 pm and 4 am GMT), and therefore a possible further contemporaneous lava fountaining to mild strombolian activity switch could have passed unobserved. According to Andronico et al. [27], a progressive decrease in the magma fragmentation within the upper level of the system feeding the 2750 m vent (Fig. 1) culminated,

on November 12th, with the collapse of the magma/gas mixture. This could have triggered the above cited switch of the activity from intense lava fountaining to mild strombolian activity. Subsequently, the fragmentation level started to rise again, leading to resumption of fire fountaining (Fig. 2 and Table 1). The same mechanism is likely to have operated during the third joint anomaly (8th of December) which took place under the same outward circumstances (the above cited switch in the volcanic activity) as the first one (Fig. 2 and Table 1). Therefore, we can conclude that the joint anomalies occurred during temporary modifications of the plumbing system feeding the 2750 vent, i.e. when it was much less efficient then usual in discharging to the atmosphere the high quantity of gases coming from below. The inhibition of the gas flow through the shallower levels of the discharge system and to the atmosphere, determines the conditions under which a foam layer forms [45,46], since the gas bubbles, flowing from below and being unable to reach the surface, are forced to accumulate at some structural barrier along the conduit plexus. Occasional growths of a gas cloud at some level within the plumbing system of the volcano could have caused the common tremor/gravity anomalies we observed. In fact, if gas bubbles substitute a denser material (magma), a localized mass decrease occurs which induces a gravity decrease observable at the surface. Furthermore, the growing of a bubbly foam layer may also act as an efficient radiator of seismic energy. Since the foam layer has a much lower sound velocity than the pure (basaltic) liquid [47], a sharp impedance contrast occurs at the boundary between the bubble cloud and the surrounding magma. This boundary acts like a reflector which traps the acoustic energy radiated by the coupled oscillations of the bubbles inside the cloud [48]. A particular aspect of the cloud behavior is that the eigenmodes of the foam occur at much lower frequency than the natural frequency of oscillation of individual bubbles [47,49]. A critical point to the reliability of the above hypothesis concerns whether the attenuation in the foam layer is low enough to allow the sustained resonance of the system. The elastic properties of an oscillator are expressed through the quality factor, Q, which may be written as [50]: Q−1 ¼ Qi−1 þ Qr−1

ð1Þ

where Qi − 1 and Qr − 1 represent the energy losses due to intrinsic attenuation and radiation, respectively. Kumagai and Chouet [51] studied the acoustic properties of a crack-like reservoir for various types of fluids under different physical conditions. For basalt-gas bubbly mixtures (gas volume fraction b 10%), Kumagai

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Fig. 7. Schematic cross section along the AB profile in Fig. 1. The contour map superimposed on the cross section represent the radius a spherical source should have to give a 10 μGal effect at PDN station, if the density change is 2.7 g/cm3. The radius should be multiplied by a factor equal to 1.4 to obtain the size of the source giving a 30 μGal effect at PDN.

and Chouet [51] found Qi and Qr ranging over the 10– 1000 and 10–100 intervals, respectively. For such ranges, the condition Q N 10, indicated by Jousset et al. [44] as a lower limit for obtaining sustained wave trapping into the fluid, would be generally met. For basaltgas foamy mixtures (gas volume fraction N 10%), no reliable estimates of Qi are available; however, the large impedance contrast with the surrounding undegassed basalt or hosting rock, leads to Qr values up to 200 [52]. Under these conditions, wave trapping at the foam– liquid interface may occur even for very small (b10) Qi values. Although future studies should be specifically aimed at quantifying this point, it is not unlikely that the value of Qi, relative to foams, exceeds such critical lower limit, especially for small bubble size. The peak values in the occurrence of linearly-polarized waves during the three anomalies (Fig. 5) are likely due to an increase in the radiation of body waves. Experimental observations at different volcanoes (e.g., [38,39,47]) report tremor wavefields composed by both body and surface waves. This observation is confirmed by numerical modeling of long-period seismicity in terms of the resonance of a fluid-filled cavity embedded in an elastic, solid medium. Jousset et al. [15] showed that the seismograms associated with this resonance consisted in both body wave energy issued from the terminal parts of the resonating conduit (or any significant change of its geometry) and surface waves emitted at the top end of the

conduit. Regardless of the details of the tremor source mechanism, the efficiency of surface-wave generation is expected to be heavily dependent upon the depth of the source. Following this argument, the energetic body waves contributions mentioned above could be due to the activation of a deeper source, especially in light of the fact that significant body-wave arrivals are observed during the early stages of the November 12th anomaly, in correspondence of the cessation of the fire-fountaining activity. Accordingly, the inferred foam layer is likely to be a tremor source deeper than that associated with the summit explosive activity. It is remarkable that a significant background level of body-waves contributions is observed at ERCB (Fig. 5). This may be attributed to the action of several natural and/or artificial sources which contribute to the local generation of highly-rectilinear waves. Since about 18:00 GMT of the 12th the tremor and gravity signals showed, superimposed to the longer period trends, strong fluctuations with period of about 30 min which are closely anti-correlated with each other. They have been evidenced in Fig. 3d and e, through the application of a low-pass filter with cut-off frequency of 6 * 10− 4 Hz (corresponding to a period of about 28 min). The peak in the coherence diagram, centered on about 5 * 10− 4 Hz (Fig. 4b I; see previous section), reflects the occurrence of these anti-correlated fluctuations. These cycles may be interpreted in light of a feed-back

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mechanism acting within the framework of the overall source mechanism proposed above. A few hours after the November 12th collapse of the eruptive column, a small opening could have formed along the obstructed summit conduit, allowing gases trapped at depth to escape only slowly to the atmosphere, and thus inducing a pressure drop within the gas reservoir. Such a drop provokes gas exolution in the gas-saturated magma below the foam layer. The volume of the new-formed bubbles compensates for the previous pressure decrease. Therefore, the bubble formation ceases whilst the gas loss continues, and thus, after a while, a new cycle starts. A similar model, termed the “Soda Bottle” regime, was previously proposed by Hellweg [52] to explain the harmonic tremor observed at Lascar volcano (Chile). Remarkably that model was applied by Hellweg [52] to phenomena occurring at time scales much shorter than those discussed in this study. Following the above view, the lack of the 30-min period fluctuations means that the conduit was always completely obstructed (no small opening did form) while the second (19–20 November) and third (8 December) anomalies were in progress. Since it was not limited in size by the occurrence of the charge/discharge cycles, during both the last two joint anomalies the foam layer could (a) grow to a bigger size, becoming more efficient as tremor and gravity source and (b) reach the critical stage sooner than during the first anomalous period. That would explain why the second and third anomalies are stronger (as for both magnitude of the gravity decrease and the overall spectral amplitude of the tremor; Fig. 3g, h, l, m) and shorter-lasting than the first one. 5. Gas volume implied by the proposed model As stated before (previous section), strict constraints on the position of the inferred joint source can be set through neither volcanic tremor, nor gravity data. However, the energetic body-wave arrivals resulting from the polarization data (see Section 2) point to the action of a source deeper than that associated with the fire-fountaining activity. The order of magnitude of the gravity changes which occurred simultaneously with the increases in the amplitude of the tremor can provide some hints about the volume of the inferred source. Fig. 7 shows a schematic cross section along the AB profile (Fig. 1) and, superimposed on it, a contour map of how the radius of a gravity source, assumed to be sphere-shaped, should vary at various horizontal and vertical distances from PDN to produce a 10 μGal change at that station (or 30 μGal change, if the values of the radius reported next to the gray scale in the figure are multiplied by a constant factor equal to 1.4). Under the assumption of gas bubbles

substituting magma, the amplitude of the density decrease within the source body was set to 2.7 g/cm3. Following the source mechanism we proposed in the previous section, the joint gravity/tremor anomalies occur during temporary occlusions of the upper plumbing system feeding the 2750 m vent, with the fire fountaining activity being temporarily replaced by mild strombolian activity and a foam layer accumulating at depth. This hypothesis implies that the volume of the growing gas foam, inferred to trigger the observed gravity decreases, roughly corresponds to the gas volume that the 2750 m vent would have expelled through fire fountains over the time interval spanned by each gravity decrease. Furthermore, at the end of each anomaly, the gravity signal returns back to its original level (Fig. 3), implying either the complete ejection of the gas foam or a combination of (i) the decrease of the volume of the partially erupted foam and (ii) its departure from the accumulation zone, dragged away by the recovered flux towards the 2750 m vent, a circumstance increasing its distance from PDN station, with the consequent net decrease of the observed gravity effect. As for the first anomaly (12 November), the simple foam accumulation/ejection scheme is complicated by the establishment of the feed-back mechanism discussed in the previous section (see Fig. 3e). Conversely, as for the last two joint anomalies (19–20 November and 8–9 December; see Fig. 3j, o), following the above hypothesis, the duration of (i) the gravity decrease (5 ÷ 6 h) and (ii) the return of the gravity level to the original value (about 2 h) can be considered end points of a range of time intervals that, if our conceptual model holds true, should include the time interval needed for the gas foam to be erupted. According to Vergniolle and Jaupart [46], the gas volume flux during fire fountains episodes can be calculated by multiplying the exit velocity by the vent cross section. Following the same authors, the exit velocity is given by: m¼

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2*g*zm

ð2Þ

where zm is the height of the fire fountain and g is the acceleration of gravity. From aerial observations of the fountaining activity at the 2750 m vent during the 2002– 03 eruption, likely values for the vent cross section and zm range over the 2000 ÷ 3000 m2 and 100 ÷ 500 m intervals, respectively. Consequently, the gas volume flux spans the 1 ÷ 3 * 105 m3 s− 1 range. Compared with the above range of time intervals (2 ÷ 6 h), this figure implies a volume of gas at the surface between 1 and 7 * 109 m3. To convert this figure into the volume at the depth of the joint tremor/

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gravity source, a reliable value for the depth itself is needed. As stated before (Section 4), the data at our disposal do not allow to assess this parameter. However, pieces of information from other works can be used to this purpose. In particular, using measurements from a Fourier Transform Infrared Spectrometer, collected during a lava fountain episode from the Southeast Crater of Etna (3250 m a.s.l.) in June 2000, Allard et al. [53] postulate that the fountaining episode was driven by the violent emptying of a large gas slug, previously accumulated at a depth of about 1.5 km. Under the hypothesis of ideal gas behavior and assuming a pressure of 35 MPa at the 1.5 km depth and temperatures of 1500 and 1000 K at the same depth and at the surface, respectively, the above inferred 1 ÷ 7 * 109 m3 range turns into a deep gas volume of 4 to 30 * 106 m3. To test whether this result is consistent with the amplitude of the observed gravity changes, within the framework of the hypothesized source mechanism, the scheme in Fig. 7 can be utilized. Under the hypothesis of a sphere-shaped source, the inferred deep volume implies a radius within the 100 ÷ 200 m range and thus, at the 1.5 km depth, the source results to lie between the vertical axes of the summit craters zone and the 2750 m vent (see Fig. 7). This result is surely reasonable and points towards a feeding dyke extending from the central plumbing system to the peripheral 2750 m vent. Therefore, we found a match between two independent figures: (i) the volume of gas consistent with the amplitude of the observed gravity changes and (ii) the volume of the gas likely to be erupted during a time interval corresponding to the duration of the anomalous periods. This match lands support to the source mechanism we propose to explain the joint tremor/gravity anomalies. However, it is worth stressing the speculative nature of the above calculation. On one hand we made assumptions that cannot be verified, i.e. that the joint tremor/gravity source is placed at 1.5 km depth (a figure coming from [53] but impossible to corroborate on the grounds of our data; see previous section) and that the observed gravity decreases during the common anomalies are due exclusively to subsurface mass change (as stated in Section 3, a part of the gravity signal, or the all of it, could be due to elevation changes). On the other hand, the gas flux rate at the surface is calculated through parameters that can not be evaluated precisely (height of the fire fountains and cross section of the vent). 6. Concluding remarks To our knowledge, this study represents the first crossanalysis ever performed between tremor and gravity se-

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quences recorded simultaneously. The signals under study were acquired during the 2002–03 Etna's eruption, one of the most explosive of the last centuries. We have shown that a strong anti-correlation can establish between the overall spectral amplitude of the volcanic tremor and changes of the gravity field over anomalies with period of the order of a few hours. This occurrence reflects the activation of a joint source, in our view a gas cloud which could form under particular conditions within the conduit system feeding a new eruptive vent. Unfortunately, the intrinsic limitations of our data set prevent a quantitative interpretation of the source phenomena from being carried out, the main uncertainties regarding the position of the source and its dimensions. Even within the above limits, the correlation we found among tremor and gravity changes confirms that the dynamics of the eruptive activity of basaltic volcanoes occurs through the complex interplay between systems which can act over very different time scales. This “broad-band” feature of the volcanic phenomena requires that suitable sampling and cross-analysis of the relevant geophysical and geochemical parameters are performed in order to attain a better understanding of the magmatic systems and a proper assessment of the associated hazards. Finally, it is worth stressing that a project is already in progress for the installment of more continuously recording gravity stations on Etna, each within a few meters from a receiver of the permanent GPS network [54]. Currently, the continuous GPS stations of the Etna network acquire data at a 1 Hz rate and thus even gravity anomalies with a short period (a few minutes to a few hours) can be reduced for the effect of elevation changes. Including one or more seismic arrays into the permanent monitoring system of the volcano would contribute to putting constraints on the location of any joint source, with the net effect of greatly improving our ability to successfully forecast the evolution of the eruptive activity. Acknowledgments The final form of this manuscript benefited from comments by Philippe Jousset and another anonymous reviewer. We thank Danilo Galluzzo and Mario La Rocca for their unflinching help during the deployment and maintenance of the broadband seismic network. A special thank is also due to both Salvatore Rapisarda, who did everything possible to keep the seismic array working properly, and Salvatore Gambino, who made the tilt data we presented available. This work was carried out with the financial support from both the EU 5th framework project ‘e-Ruption’ (contract n. EVRI-CT-2001-40021) and the VOLUME project (European Commission FPG2004_Global-3).

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