tters 245 (2006) 616–629www.elsevier.com/locate/epsl

Earth and Planetary Science Le

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

Daniele Carbone a,*, Luciano Zuccarello a, Gilberto Saccorotti b, Filippo Greco a

a Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Piazza Roma 2, 95123 Catania, Italyb 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 2006Available 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 recordinggravity station during the 2002–2003 eruption of Etna volcano (Italy). Seismic signals recorded during the eruption are dominatedby volcanic tremor whose energy spans the 0.5–5 Hz frequency band. On three different occasions (12 November, 19–20November and 8–9 December 2002), we observed marked increases of the tremor amplitude (up to a factor of 4), which occurredsimultaneously with gravity decreases (up to 30 μGal). The three concurrent gravity/tremor anomalies last 6 to 12 hours andterminate with rapid (up to 2 hours) changes, after which the signals return back to their original levels. Based on volcanologicalobservations encompassing the simultaneous anomalies, we infer that the accumulation of a gas cloud at some level in the conduitplexus 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 volcanoand 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 techniquesaimed at monitoring active volcanoes and studying thedynamics of their plumbing systems have improvedsignificantly.

Among the other signals, volcanic tremor and micro-gravity changes are routinelymeasured and studied atmostvolcanoes. Volcanic tremor is a sustained seismic signalwhich is generally observed in association with magmatic

* Corresponding author.E-mail address: carbone@ct.ingv.it (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 beendocumented at many different volcanoes throughout theworld (see [1] for a comprehensive review of tremorstudies). Although different models have been proposed toexplain the source mechanism of tremor, all authorsconcord in attributing its origin to the complex interplayamong themagmatic hydrothermal fluids and their hostingrocks.

Microgravity studies represent a relatively new tech-nique to investigate and monitor active volcanoes, withrespect to some of the more established methods. Tem-poral changes of the gravity field in volcanic zones arerelated to sub-surface mass/volume/density changes orto elevation changes in response to magmatic processes

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and vary significantly in both space (wavelengths rang-ing from hundreds of meters to tens of kilometers) andtime (periods ranging from minutes to years) accordingto the size, depth and rate of evolution of the source.Thus, both discrete measurements (e.g. [2–8]) and con-tinuous observations [9,10] are accomplished. Discretemeasurements allow a good space resolution (dependingon the number of measurements in a single campaign)to be achieved, while the time resolution is limited bythe repetition time of the surveys (usually monthly toyearly); continuous measurements allow a good timeresolution to be reached but the space resolution isusually poor due to high cost of gravimeters that limitsdramatically the number of instruments available at asingle area.

Even though comparisons between multidisciplinarydata acquired at volcanic areas are now often performed(e.g. [11–17]), rigorous cross-analyses are not routinelyaccomplished, even at the most monitored sites, becauseof the different nature of the data sets themselves(especially as for sampling rate) and the relatively poor

Fig. 1. Schematic map showing the position of the four seismic broadband stlava flow fields from Etna's 2002–03 eruption are also reported. Contours a

interaction among research groups working on differentsubjects.

Mt. Etna is one of the best monitored volcanoes of theworld [18–23]. It is also highly active, with almost-perennial degassing from the summit craters and recur-rent summit and flank eruptions [24–26]. One of themost voluminous eruption of Etna's recent history beganon the night of October 26–27, 2002 with lava flowsissued from two different fissure systems at elevationsbetween 2850 and 2600 m on the southern flank of thevolcano, and between 2470 and 1900 m on thenortheastern flank (Fig. 1; [27]). The most importantfeature of the 2002–03 eruption was the extraordinaryexplosive activity from the southernmost fissure sys-tems, which led to a volume-of-pyroclastic-products/total-erupted-volume ratio of 0.5–0.6, the highest sincethe sixteenth century [27]. By comparison, the same ratiowas 0.22 for the 2001 Etna flank eruption (25.3×106 m3

of lava and 7.8×106 m3 of thephra [28]) and alwayswithin 0.1 for the flank eruptions throughout the twentiethcentury, with the only exception of the two 1974 eruptions

ations and the continuous gravity station used in the present study. There at 200 m intervals.

Fig. 2. Top four panels: overall spectral amplitudes of the vertical velocity component observed at EMTB, ERCB, EMFB and ETRB seismicbroadband stations during the 7 November–10 December 2002 period. The overall spectral amplitude of each tremor sequence was calculated withinthe 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-passfilter 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 mainvolcanic events (after [17]) during the period under study. Numbers under each marker refer to the first column of Table 1.

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

Fig. 3. Velocity seismograms (vertical component, ERCB station), overall specaption to Fig. 2 for details) relative to three 60-h periods beginning respective(f, g and h) and 12:00, December 7, 2002. Low-pass filters (cut off frequencysub-sequences evidenced with grey strips in b–c, g–h and l–m are reported

column-forming fire fountains. From the second half ofDecember, 2002, fire-fountaining activity became pulsat-ing and alternated with mild strombolian activity. Effusiveactivity was discontinuous and occurred from differentvents which opened mostly at the base of the 2750 m

ctral amplitude and gravity (ERCB and PDN stations respectively; seely at 00:00, November 12, 2002 (a, b and c), 00:00, November 19, 2002equal to 6*10−4 Hz, corresponding to a period of about 28 min) of thein d–e, i–j and n–o.

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cinder cone. They produced a fan-shaped lava-flow fieldwith maximum length of 4 km (Fig. 1) until 28 January2003, when the effusive activity ceased. A total volume of

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

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9 days (27 October–05 November 2002) and wascharacterized by fire fountaining, strombolian and effusiveactivity from the vents which opened along the fracturesystem as it propagated downslope along the easternborder of Etna's Northeast Rift (Fig. 1). The explosiveactivitywas less intense than that occurring on the southernflank and began to decrease slowly since 29 October. Theeffusive activity produced two flows with maximumlength of 6.5 km (Fig. 1) until 3 November, when it cameto an end. A total volume of 11×106 m3 (107 m3 of lavaand 106 m3 of thephra [28]) was erupted from the fissuresystem on the northern flank.

In order to study the dynamics of the volcanic pro-cesses occurring at Etna, some continuously runninggravity stations were installed in 1998 and have workedintermittently since then [10].

During three separate periods on November andDecember, 2002, we noticed three contemporaneous ano-malies in the gravity signal and the signal from someBroadband stations which were installed on the summitzone of Etna after the start of the 2002–03 eruption. Theanalysis performed on these joint anomalies, each span-ning a time interval of a few hours, provides a uniqueinsight into the operation of Etna's degassing system.

In the following, after having presented the dataset(Section 2) and have discussed the implications of nothaving deformation data at a rate suitable to reduce ourgravity data (Section 3), we propose a possible sourcemechanism to explain the joint tremor/gravity anomalies(Section 4). Then we perform a first approximationcalculation to set quantitative constraints to the inferredmechanism (Section 5), before we conclude (Section 6).

Fig. 4. (a): time changes of the correlation coefficient between the gravityamplitude sequences from EMTB, ERCB, EMFB and ETRB seismic broad(50% overlap) was used for the calculation. (b): coherence functions in the frencompassing the joint tremor/gravity anomalies) between gravity (PDN sta

2. Data presentation

On the 30th of October a seismic array of sixBroadband stations was installed on the summit zone ofEtna to monitor with a suitable azimuthal coverage theactivity related to the ongoing eruption. Each seismicstation was equipped with a Guralp CMG-40T Broad-band (60–0.02 s), 3-component seismometer. Data wererecorded at a rate of 62.5 samples per second throughLennartz Marslite digital systems (20 bit) which use aGPS time base. Out of the six available instruments, inthis work we use only data from the four stations closerto the eruptive vents (sites EMTB, ERCB, EMFB andETRB in Fig. 1).

A continuously recording gravity station, located ontheNortheastern slope of the volcano (PDN; 2920m a.s.l.;see Fig. 1), was also operated throughout the periodencompassing the 2002–03 eruption [10,29]. This stationis equipped with a LaCoste and Romberg gravimeter(D-185), featuring an analog feedback system [30].Data are recorded at 1 datum/min sampling rate (eachdatum is the average calculated over 60 measurements)through a CR10X Campbell Scientific datalogger.

Tremor sequences are analyzed by calculating theintegral of the spectrum over the 0.02–10 Hz frequencyrange. Spectral estimates are obtained via FFT oversubsequent 4096-sample (65.5 s) windows of signal. Bychoosing a suitable overlap of the sliding window (about10%), we obtain overall spectral amplitude sequences ata rate of 1 datum/min, i.e. the same sampling rate of thegravity signal. Before calculating the FFT, each datasegment is demeaned and corrected for the linear trend.

sequence from PDN (lower graph in Fig. 2) and the overall spectralband stations (upper four diagrams in Fig. 2). A sliding 24-h windowequency domain calculated over three 24-h intervals (grey strips in (a),tion) and overall spectral amplitude (ERCB station).

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We separately applied these procedures to the threecomponents of ground velocity. For all the three dif-ferent cases, we observed similar time behavior of tre-mor amplitudes and therefore, for the sake of con-ciseness, throughout in the following we present anddiscuss only results concerning the vertical-componentdata.

The gravity signal is reduced for the effect of EarthTides (modeled through the Eterna 3.30 data processingpackage [31]) and instrumental drift (modeled as thebest linear fit), and high-pass-filtered (cut off frequencyof 2.8*10−6 Hz, corresponding to a period of 100 h) toremove the longer wavelength components which arenot of interest to the present study.

The overall spectral amplitude sequences for the 7November–9 December period are presented in Fig. 2(from top to bottom stations EMTB, ERCB, ETRB,EMFB), together with the gravity sequence acquired atPDN during the same period (lower panel of Fig. 2). Themost important features appearing in Fig. 2 (evidencedwith the grey strips) are the marked increases in theamplitude of the tremor (by a factor ranging between 3 and4) which occurred at all the stations (a) on 12 November,(b) between 19 and 20 November and (c) between 8 and 9December. These increases in the amplitude of the tremoroccur simultaneously with decreases in the gravity valuehaving a maximum amplitude ranging between 10 and30 μGal (1 μGal=10 nm s−2; see Fig. 3b, c, g, h, l and mfor details). The three simultaneous tremor amplitudeincreases/gravity decreases last respectively 12 h (12November), 6 h (19–20 November) and 7 h (8–9 Decem-ber), and terminate with steep changes, lasting 0.5 to 2 h,after which the sequences return to the mean amplitudethey had before the anomaly took place (Fig. 3).

As a further step of our analysis, we calculate thecorrelation between the gravity sequence and the tremorspectral amplitudes at each station, using a 12-h-longtime window sliding along the signal with 50% overlap(Fig. 4a). A marked anti-correlation (with amplitude upto 0.95) occurs in correspondence of the 12 November,

Fig. 5. Hourly number of polarization measurements (ERCB station) for whiGrey strips highlight the periods over which the joint tremor/gravity anomal

19–20 November and 8–9 December anomalies but alsoover the 13th of November, when a slighter increase inthe tremor amplitude took place after the sharp decreaseterminating the 12 November anomaly (Figs. 2 and 3b).Over the rest of the considered time interval, theamplitude of the anti-correlation never reaches valueshigher than 0.4.

To investigate possible correlations over frequencybands higher than those associated with the maximumenergy (corresponding to the longest period of each anom-aly; see Fig. 3b, c, g, h), correlation analyses in thefrequency domain between the gravity sequence and theoverall spectral amplitude sequence from ERCB stationare also computed (Fig. 4b) over the three 24-h periodsevidenced with the grey strips in Fig. 4a (I, II and III). Thecoherence function is calculated by the Matlab® signalprocessing tool, which is based upon Welch's averagedperiodogram method [32]. The first diagram (Fig. 4b I),relative to the 12 November anomaly, shows a peak atabout 5*10−4 Hz (T equal to about 30 min) with cohe-rence values becoming lower as frequency increases. Thesecond and third coherence diagrams (19–20 Novemberand 8–9 December anomalies; Fig. 4b II, III) lack a dis-tinctive peak; coherence values remain within 0.4. Thus,during the first anomaly the two time series depict signi-ficant correlations also over periods which are shorter thanthe duration of the anomaly itself.

In order to characterize the wavetypes associated withthe tremor signal we perform a polarization analysis, ap-plying the covariancemethod [33], to 1-s-longwindows ofsignal sliding with 50% overlap along the 0.5–5 Hz 3-component seismograms. For each step of the analysis, thisprocedure allows to retrieve the spatial setting (azimuthand incidence angles) of the ellipsoid which best fits, in aleast-square sense, the particle motion trajectory. Thedegree of linearity of the polarization ellipsoid is thenexpressed through the rectilinearity coefficient, whichtakes extremes values of 0 and 1 for purely-spherical andlinear motions, respectively. Under this convention, highvalues of rectilinearity are representative of body-wave

ch the rectilinearity coefficient is greater than 0.9 (see text for details).ies took place.

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 December2002 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 theavailable tremor sequences depict a complicate picture.With the only exception of station EMTB, the groundvibration at all the network sites is dominated by horizontalmotion oriented transversally with respect to the directionpointing to the active zone; such feature does not signi-ficantly change throughout the analysed tremor sequences.The only remarkable change we observed is associatedwith peak values in the hourly number of linearly-po-larized waves (rectilinearityN0.9) occurring in concomi-tance of all the three common tremor/gravity anomaliesand, similarly to the results of the above correlationanalysis, also over November 13th (Fig. 5).

3. Implications of the lack of elevation control on thegravity data

In volcanic areas, measured gravity changes mayreflect underground mass/density/volume changes,ground deformations, or a combination of both factors.Elevation changes imply a variation of the distancebetween the observation point and the Earth's center,thus leading to a gravity increase or decrease for grounddeflation or inflation, respectively. Conversely, tiltchanges, that move the meter away from the horizontalposition (where it measures the full force of gravity),produce a negative gravity effect. Thus, grounddeformation measurements are crucial to a properinterpretation of the observed gravity variations.

The only technique able to furnish deformation data ata rate useful to reduce gravity anomalies lasting 6-to-12 his continuous GPS. Unfortunately, due to power-savingreasons, GPS stations operating on the summit zone ofMt. Etna during the period of interest acquired data foronly 3 h every day, i.e. between 10:00 and 13:00 GMT, atime interval which does not cover any of the threegravity anomalies under study. Tilt changes measuredduring the three joint anomalies, using the levels fitted tothe gravimeter (resolution=2.5 μrad), were negligible(within 10 μrad; Fig. 6, lower panel). Data coming from 3tilt station on Etna, also show negligible variations (Fig.6, upper three panels). However, although suggesting theoccurrence of negligible deformation, the absence of tiltchanges does not definitely rule out the possibility thatsignificant elevation changes took place at PDN stationover the periods encompassing the three joint anomalies.In fact, following Cayol and Cornet [34], who used a 3Dmodel with a spherical pressurized source locatedbeneath an asymmetrical volcano, significant verticaldeformations with negligible tilt changes can occur if theslope of the flanks of the volcano is greater than 20° andthe observation point is within 0.5÷1*a of the projection

onto the surface of the source center (a is the radius of thespherical source). Also, in the case of a flat topography,noticeable vertical deformations without tilt changesoccur at observation points which lie on the vertical axisof the spherical source.

Considering the free-air gradient (usually taken as−308.6 μGal/m [35]) or the Bouguer corrected (using theMogi [36] elastic model) free-air gradient (rangingbetween −233 and −244 μGal/m [35]), elevation in-creases between 3÷4.3 and 10÷12.9 cm should haveoccurred to induce the observed 10 to 30 μGal gravitydecreases.

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 incorrespondence of the evidenced periods and a negligiblecorrelation elsewhere, strongly suggest that during thethree tremor amplitude increases/gravity decreases acommon source activated which, during the rest of theperiod under study, either was not active or produced aweak, negligible effect.

Unfortunately, our tremor data do not allow to setconstraints on the location of this common source. Thelarge station spacing of our network (Fig. 1) hinders thepossibility of analyzing the seismic recordings by meansof multichannel techniques aimed at deriving the pro-pagation direction and apparent velocity of the tremorwaves [37–39]. Furthermore, results of the polarizationanalyses (Section 2) indicate a dominance of transversemotion at most recording sites. This observation may beinterpreted as the effect of the radiation pattern from anon-isotropic source mechanism. For instance, the reso-nance of a fluid-filled, crack-like buried cavity has beendemonstrated to be an efficient generator of large SHcomponents [40]. The presence of soft pyroclastic layersin the shallowest portion of the propagation mediummight also induce efficient amplification and trapping ofshear energy, as postulated in a study of the volcanic tremorassociated with the 1999 Etna's eruption [39]. A furtherfactor severely conditioning waveform amplitudes andparticle motion is represented by wave conversions, re-flections and scattering at the free-surface. The roughnessof volcanic topographies spans a wide range of wave-lengths, therefore affecting the propagation of seismicwaves over a broad frequency band [41–44]. Given allthese concurring effects, the particle motion azimuths andincidence angles determined through polarization analysesdo not provide any useful insight into the location of thesource of the three joint anomalies.

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

Date Description of activity

6–11 November Intense fire fountaining activity(1st period)

1 Between the afternoon of 12November and the followingday

Lava fountains replaced bystrombolian activity

2 13 November Effusive activity resumes (base ofthe 2750 cone)

3 14 November Fire fountaining activity resumes(start of 2nd period; not ascontinuous as in the first period)

4 17 November Two new explosive vents open5 Between 20 and 21

NovemberA new effusive vent opens (base ofthe 2750 cone)

6 25 November Two new explosive vents open(base of the 2750 cone)

7 8 December Lava fountains replaced bystrombolian activity for a few hours

8 10 December Fire fountaining activity resumesand two new effusive vents open(base of the 2750 cone)

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Constraints on the position of the joint source cannotbe set using gravity data from only one station either.

The lack of strict geometrical constraints on the in-ferred joint source, together with the unavailability ofelevation data, which could allow the observed gravityvariations to be reduced (see previous section), impliesthat only a speculative interpretation of the source mech-anism can be attempted in the following.

Volcanological observations conducted during theperiod of interest to the present study [27] are schemat-ically reported at the bottom of Fig. 2 (numbers beloweach marker refer to the first column of Table 1). Both the12 November and the 8 December anomalies occurredduring periods of temporary switch of the activity fromintense lava fountaining to mild strombolian activity.Furthermore, after the end of all the three anomalies, neweffusive vents opened. Within the limitations of the timeresolution of the volcanological observations conductedat summit active vent, the temporal fit is tight, thus likely toreflect a link between the occurrence of (a) the abovementioned features of the volcanic activity and (b) the jointtremor/gravity anomalies. It is important to stress that thesecond tremor/gravity anomaly (19–20 November) devel-oped during nightly hours (between 11 pm and 4 amGMT), and therefore a possible further contemporaneouslava fountaining to mild strombolian activity switch couldhave passed unobserved.

According to Andronico et al. [27], a progressive de-crease in the magma fragmentation within the upper levelof the system feeding the 2750 m vent (Fig. 1) culminated,

on November 12th, with the collapse of the magma/gasmixture. This could have triggered the above cited switchof the activity from intense lava fountaining to mild strom-bolian activity. Subsequently, the fragmentation levelstarted to rise again, leading to resumption of fire foun-taining (Fig. 2 and Table 1). The same mechanism is likelyto have operated during the third joint anomaly (8th ofDecember) which took place under the same outwardcircumstances (the above cited switch in the volcanicactivity) as the first one (Fig. 2 and Table 1).

Therefore, we can conclude that the joint anomaliesoccurred during temporary modifications of the plumb-ing system feeding the 2750 vent, i.e. when it was muchless efficient then usual in discharging to the atmospherethe high quantity of gases coming from below.

The inhibition of the gas flow through the shallowerlevels 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 andbeing unable to reach the surface, are forced to accumulateat some structural barrier along the conduit plexus. Occa-sional growths of a gas cloud at some level within theplumbing system of the volcano could have caused thecommon tremor/gravity anomalies we observed. In fact, ifgas bubbles substitute a denser material (magma), a loca-lized mass decrease occurs which induces a gravity de-crease observable at the surface.

Furthermore, the growing of a bubbly foam layer mayalso act as an efficient radiator of seismic energy. Since thefoam layer has a much lower sound velocity than the pure(basaltic) liquid [47], a sharp impedance contrast occurs atthe boundary between the bubble cloud and the surround-ing magma. This boundary acts like a reflector which trapsthe acoustic energy radiated by the coupled oscillations ofthe bubbles inside the cloud [48]. A particular aspect of thecloud behavior is that the eigenmodes of the foam occur atmuch lower frequency than the natural frequency of oscil-lation of individual bubbles [47,49].

A critical point to the reliability of the above hypothesisconcerns whether the attenuation in the foam layer is lowenough to allow the sustained resonance of the system.The elastic properties of an oscillator are expressed thro-ugh the quality factor, Q, which may be written as [50]:

Q−1 ¼ Qi−1þ Qr−1 ð1Þ

whereQi−1 and Qr−1 represent the energy losses due tointrinsic attenuation and radiation, respectively.

Kumagai and Chouet [51] studied the acoustic prop-erties of a crack-like reservoir for various types of fluidsunder different physical conditions. For basalt-gasbubbly mixtures (gas volume fraction b10%), Kumagai

Fig. 7. Schematic cross section along the AB profile in Fig. 1. The contour map superimposed on the cross section represent the radius a sphericalsource 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 1.4to obtain the size of the source giving a 30 μGal effect at PDN.

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and Chouet [51] found Qi and Qr ranging over the 10–1000 and 10–100 intervals, respectively. For suchranges, the condition QN10, indicated by Jousset et al.[44] as a lower limit for obtaining sustained wave trap-ping into the fluid, would be generally met. For basalt-gas foamy mixtures (gas volume fractionN10%), noreliable estimates of Qi are available; however, the largeimpedance contrast with the surrounding undegassedbasalt 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) Qivalues. Although future studies should be specificallyaimed at quantifying this point, it is not unlikely that thevalue ofQi, relative to foams, exceeds such critical lowerlimit, especially for small bubble size.

The peak values in the occurrence of linearly-polarizedwaves during the three anomalies (Fig. 5) are likely due toan increase in the radiation of body waves. Experimentalobservations at different volcanoes (e.g., [38,39,47])report tremor wavefields composed by both body andsurface waves. This observation is confirmed by numer-ical modeling of long-period seismicity in terms of theresonance of a fluid-filled cavity embedded in an elastic,solid medium. Jousset et al. [15] showed that the seis-mograms associated with this resonance consisted in bothbody wave energy issued from the terminal parts of theresonating conduit (or any significant change of its geo-metry) and surface waves emitted at the top end of the

conduit. Regardless of the details of the tremor sourcemechanism, the efficiency of surface-wave generation isexpected to be heavily dependent upon the depth of thesource. Following this argument, the energetic bodywaves contributions mentioned above could be due to theactivation of a deeper source, especially in light of the factthat significant body-wave arrivals are observed duringthe early stages of the November 12th anomaly, in corres-pondence of the cessation of the fire-fountaining activity.

Accordingly, the inferred foam layer is likely to be atremor source deeper than that associated with thesummit explosive activity. It is remarkable that asignificant background level of body-waves contribu-tions is observed at ERCB (Fig. 5). This may beattributed to the action of several natural and/or artifi-cial sources which contribute to the local generation ofhighly-rectilinear waves.

Since about 18:00 GMT of the 12th the tremor andgravity signals showed, superimposed to the longer periodtrends, strong fluctuations with period of about 30 minwhich are closely anti-correlated with each other. Theyhave been evidenced in Fig. 3d and e, through theapplication of a low-pass filter with cut-off frequency of6*10−4 Hz (corresponding to a period of about 28 min).The peak in the coherence diagram, centered on about5*10−4 Hz (Fig. 4b I; see previous section), reflects theoccurrence of these anti-correlated fluctuations. Thesecycles may be interpreted in light of a feed-back

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mechanism acting within the framework of the overallsource mechanism proposed above. A few hours after theNovember 12th collapse of the eruptive column, a smallopening could have formed along the obstructed summitconduit, allowing gases trapped at depth to escape onlyslowly to the atmosphere, and thus inducing a pressuredrop within the gas reservoir. Such a drop provokes gasexolution in the gas-saturated magma below the foamlayer. The volume of the new-formed bubbles compensatesfor the previous pressure decrease. Therefore, the bubbleformation ceases whilst the gas loss continues, and thus,after a while, a new cycle starts. A similar model, termedthe “Soda Bottle” regime, was previously proposed byHellweg [52] to explain the harmonic tremor observed atLascar volcano (Chile). Remarkably that model wasapplied by Hellweg [52] to phenomena occurring at timescales much shorter than those discussed in this study.

Following the above view, the lack of the 30-minperiod fluctuations means that the conduit was alwayscompletely obstructed (no small opening did form) whilethe second (19–20 November) and third (8 December)anomalies were in progress. Since it was not limited insize by the occurrence of the charge/discharge cycles,during both the last two joint anomalies the foam layercould (a) grow to a bigger size, becoming more efficientas tremor and gravity source and (b) reach the criticalstage sooner than during the first anomalous period. Thatwould explain why the second and third anomalies arestronger (as for both magnitude of the gravity decreaseand 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 onthe position of the inferred joint source can be set throughneither volcanic tremor, nor gravity data. However, theenergetic body-wave arrivals resulting from the polariza-tion data (see Section 2) point to the action of a source dee-per than that associated with the fire-fountaining activity.

The order of magnitude of the gravity changes whichoccurred simultaneously with the increases in the am-plitude of the tremor can provide some hints about thevolume of the inferred source. Fig. 7 shows a schematiccross section along the AB profile (Fig. 1) and,superimposed on it, a contour map of how the radiusof a gravity source, assumed to be sphere-shaped, shouldvary at various horizontal and vertical distances fromPDN to produce a 10 μGal change at that station (or30 μGal change, if the values of the radius reported nextto the gray scale in the figure are multiplied by a constantfactor equal to 1.4). Under the assumption of gas bubbles

substituting magma, the amplitude of the densitydecrease within the source body was set to 2.7 g/cm3.

Following the source mechanism we proposed in theprevious section, the joint gravity/tremor anomaliesoccur during temporary occlusions of the upper plumb-ing system feeding the 2750 m vent, with the firefountaining activity being temporarily replaced by mildstrombolian activity and a foam layer accumulating atdepth. This hypothesis implies that the volume of thegrowing gas foam, inferred to trigger the observed gra-vity decreases, roughly corresponds to the gas volumethat the 2750 m vent would have expelled through firefountains over the time interval spanned by each gravitydecrease. Furthermore, at the end of each anomaly, thegravity signal returns back to its original level (Fig. 3),implying either the complete ejection of the gas foam ora combination of (i) the decrease of the volume of thepartially erupted foam and (ii) its departure from theaccumulation zone, dragged away by the recovered fluxtowards the 2750 m vent, a circumstance increasing itsdistance from PDN station, with the consequent netdecrease of the observed gravity effect. As for the firstanomaly (12 November), the simple foam accumula-tion/ejection scheme is complicated by the establish-ment of the feed-back mechanism discussed in theprevious section (see Fig. 3e). Conversely, as for the lasttwo joint anomalies (19–20 November and 8–9December; see Fig. 3j, o), following the above hypo-thesis, the duration of (i) the gravity decrease (5÷6 h)and (ii) the return of the gravity level to the originalvalue (about 2 h) can be considered end points of a rangeof time intervals that, if our conceptual model holds true,should include the time interval needed for the gas foamto be erupted.

According to Vergniolle and Jaupart [46], the gasvolume flux during fire fountains episodes can be calcu-lated by multiplying the exit velocity by the vent crosssection. Following the same authors, the exit velocity isgiven by:

m ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2*g*zm

pð2Þ

where zm is the height of the fire fountain and g is theacceleration of gravity. From aerial observations of thefountaining activity at the 2750 m vent during the 2002–03 eruption, likely values for the vent cross section and zmrange over the 2000÷3000 m2 and 100÷500 m intervals,respectively. Consequently, the gas volume flux spans the1÷3*105 m3 s−1 range. Compared with the above rangeof time intervals (2÷6 h), this figure implies a volume ofgas at the surface between 1 and 7*109 m3. To convertthis figure into the volume at the depth of the joint tremor/

627D. Carbone et al. / Earth and Planetary Science Letters 245 (2006) 616–629

gravity source, a reliable value for the depth itself isneeded. As stated before (Section 4), the data at ourdisposal do not allow to assess this parameter. However,pieces of information from other works can be used to thispurpose. In particular, usingmeasurements from a FourierTransform Infrared Spectrometer, collected during a lavafountain episode from the Southeast Crater of Etna(3250 m a.s.l.) in June 2000, Allard et al. [53] postulatethat the fountaining episode was driven by the violentemptying of a large gas slug, previously accumulated at adepth of about 1.5 km.

Under the hypothesis of ideal gas behavior andassuming a pressure of 35 MPa at the 1.5 km depth andtemperatures of 1500 and 1000 K at the same depth and atthe 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 theframework of the hypothesized source mechanism, thescheme in Fig. 7 can be utilized. Under the hypothesis ofa sphere-shaped source, the inferred deep volume im-plies a radius within the 100÷200 m range and thus, atthe 1.5 km depth, the source results to lie between thevertical axes of the summit craters zone and the 2750 mvent (see Fig. 7). This result is surely reasonable andpoints towards a feeding dyke extending from the centralplumbing system to the peripheral 2750 m vent.

Therefore, we found amatch between two independentfigures: (i) the volume of gas consistent with the ampli-tude of the observed gravity changes and (ii) the volumeof the gas likely to be erupted during a time intervalcorresponding to the duration of the anomalous periods.This match lands support to the source mechanism wepropose to explain the joint tremor/gravity anomalies.

However, it is worth stressing the speculative nature ofthe above calculation. On one hand we made assumptionsthat cannot be verified, i.e. that the joint tremor/gravitysource is placed at 1.5 km depth (a figure coming from[53] but impossible to corroborate on the grounds of ourdata; see previous section) and that the observed gravitydecreases during the common anomalies are due exclu-sively to subsurface mass change (as stated in Section 3, apart of the gravity signal, or the all of it, could be due toelevation changes). On the other hand, the gas flux rate atthe surface is calculated through parameters that can not beevaluated precisely (height of the fire fountains and crosssection of the vent).

6. Concluding remarks

To our knowledge, this study represents the first cross-analysis ever performed between tremor and gravity se-

quences recorded simultaneously. The signals under studywere acquired during the 2002–03 Etna's eruption, one ofthe most explosive of the last centuries. We have shown thata strong anti-correlation can establish between the overallspectral amplitude of the volcanic tremor and changes of thegravity field over anomalieswith period of the order of a fewhours. This occurrence reflects the activation of a jointsource, in our view a gas cloud which could form underparticular conditions within the conduit system feeding anew eruptive vent. Unfortunately, the intrinsic limitations ofour data set prevent a quantitative interpretation of the sourcephenomena from being carried out, the main uncertaintiesregarding the position of the source and its dimensions.

Even within the above limits, the correlation we foundamong tremor and gravity changes confirms that the dyna-mics of the eruptive activity of basaltic volcanoes occursthrough the complex interplay between systems which canact over very different time scales. This “broad-band”feature of the volcanic phenomena requires that suitablesampling and cross-analysis of the relevant geophysicaland geochemical parameters are performed in order toattain a better understanding of the magmatic systems anda proper assessment of the associated hazards.

Finally, it is worth stressing that a project is already inprogress for the installment of more continuously re-cording gravity stations on Etna, each within a fewmetersfrom a receiver of the permanent GPS network [54].Currently, the continuous GPS stations of the Etna net-work acquire data at a 1 Hz rate and thus even gravityanomalies with a short period (a few minutes to a fewhours) can be reduced for the effect of elevation changes.Including one or more seismic arrays into the permanentmonitoring system of the volcano would contribute toputting constraints on the location of any joint source,with the net effect of greatly improving our ability tosuccessfully forecast the evolution of the eruptive activity.

Acknowledgments

The final form of this manuscript benefited fromcomments by Philippe Jousset and another anonymousreviewer. We thank Danilo Galluzzo and Mario La Roccafor their unflinching help during the deployment andmaintenance of the broadband seismic network. A specialthank is also due to both Salvatore Rapisarda, who dideverything possible to keep the seismic array workingproperly, and Salvatore Gambino, who made the tilt datawe presented available. This work was carried out withthe financial support from both the EU 5th frameworkproject ‘e-Ruption’ (contract n. EVRI-CT-2001-40021)and the VOLUME project (European Commission FPG-2004_Global-3).

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