Article

Volume 11, Number 9

23 September 2010

Q09008, doi:10.1029/2010GC003168

ISSN: 1525‐2027

Patterns in the recent 2007–2008 activity of Mount Etnavolcano investigated by integrated geophysical andgeochemical observations

A. AiuppaCFTA, Università di Palermo, I‐90123 Palermo, Italy (aiuppa@unipa.it)

Also at INGV, Sezione di Palermo, I‐90146 Palermo, Italy

A. Cannata, F. Cannavò, G. Di Grazia, and F. FerrariINGV, Sezione di Catania, I‐95125 Catania, Italy

G. Giudice, S. Gurrieri, and M. LiuzzoINGV, Sezione di Palermo, I‐90146 Palermo, Italy

M. Mattia, P. Montalto, D. Patanè, and G. PuglisiINGV, Sezione di Catania, I‐95125 Catania, Italy

[1] Seismic, deformation, and volcanic gas observations offer independent and complementary informationon the activity state and dynamics of quiescent and eruptive volcanoes and thus all contribute to volcanicrisk assessment. In spite of their wide use, there have been only a few efforts to systematically integrate andcompare the results of these different monitoring techniques. Here we combine seismic (volcanic tremorand long‐period seismicity), deformation (GPS), and geochemical (volcanic gas plume CO2/SO2 ratios)measurements in an attempt to interpret trends in the recent (2007–2008) activity of Etna volcano. Weshow that each eruptive episode occurring at the Southeast Crater (SEC) was preceded by a cyclic phaseof increase‐decrease of plume CO2/SO2 ratios and by inflation of the volcano’s summit captured by theGPS network. These observations are interpreted as reflecting the persistent supply of CO2‐rich gas bub-bles (and eventually more primitive magmas) to a shallow (depth of 1–2.8 km asl) magma storage zonebelow the volcano’s central craters (CCs). Overpressuring of the resident magma stored in the upperCCs’ conduit triggers further magma ascent and finally eruption at SEC, a process which we capture asan abrupt increase in tremor amplitude, an upward (>2800 m asl) and eastward migration of the sourcelocation of seismic tremor, and a rapid contraction of the volcano’s summit. Resumption of volcanic activ-ity at SEC was also systematically anticipated by declining plume CO2/SO2 ratios, consistent with magmadegassing being diverted from the central conduit area (toward SEC).

Components: 8000 words, 5 figures.

Keywords: volcano monitoring; Etna; geochemistry and geophysics.

Index Terms: 8419 Volcanology: Volcano monitoring (7280); 8499 Volcanology: General or miscellaneous; 8430Volcanology: Volcanic gases.

Received 8 April 2010; Revised 13 July 2010; Accepted 29 July 2010; Published 23 September 2010.

Aiuppa, A., et al. (2010), Patterns in the recent 2007–2008 activity of Mount Etna volcano investigated by integratedgeophysical and geochemical observations, Geochem. Geophys. Geosyst., 11, Q09008, doi:10.1029/2010GC003168.

Copyright 2010 by the American Geophysical Union 1 of 13

1. Introduction

[2] Volcanic eruptions, one of the most spectacularand dramatic demonstrations of nature’s power, arethe ultimate results of periodic events of magmasupply from the Earth’s mantle, storage in the uppercrust, and fast decompression (and degassing) uponsurface emplacement. These cycles of magma sup-ply accumulation‐eruption [Tilling and Dvorak,1993] produce measurable physical and chemicalsignals which, when captured by monitoring net-works, can allow mitigating the effects of volcaniceruptions [Scarpa and Tilling, 1996].

[3] It has long been recognized, for instance, thatrate of occurrence and mechanisms of basaltic erup-tions, punctuating the persistent degassing activityof Etna volcano (in southern Italy), follow somesystematic (periodic) long‐term trend [Imbó, 1928;Chester et al., 1985; Tanguy et al., 1997]. The“periodic” patterns of Etna’s post‐1865 activityrecently led Behncke and Neri [2003] to identify4 main eruptive cycles, each being characterizedby a shift of activity (over time scales of a fewdecades) from quiescence to summit eruptions (e.g.,eruptive activity confined at the summit craters), andfinally to flank eruptions. This sequence of eventswas thought to reflect the progressive magmaaccumulation within (and periodically tapping of) amain magma storage zone 3–5 km below sea level[Allard et al., 2006]; up to the final magma drainagefrom the reservoir during a large‐scale flank erup-tion (e.g., the 1991–33 eruption of Etna), leadingto termination of a cycle [Behncke and Neri, 2003].

[4] Etna’s activity is known to be “periodic” alsoon much smaller time scales: paroxysmal summiteruptions, consisting of violent and short‐lived epi-sodes of fire fountaining and lava emission, occurredwith unusually high periodicity at Etna’s SoutheastCrater (SEC) in 1998 to 2001 [Alparone et al., 2003],with repose periods between one event and the fol-lowing ranging from less than a few hours to severalmonths [Allard et al., 2006]. The mechanisms lead-ing to such recurrent SEC paroxysmal activity,which also persisted more recently in 2007 and 2008[e.g., Andronico et al., 2008], are still poorly under-stood [La Delfa et al., 2001; Allard et al., 2005].

[5] This changeable and dynamic nature of Etna’svolcanism represents a real challenge to volca-nologists. However, the recent advances of tradi-tional volcano monitoring methods, combined withthe advent of new techniques, have significantlyimproved our ability to characterize the volcano’s

activity state [Bonaccorso et al., 2003]; to the pointthat a range of precursory signals for the recentEtna’s eruptions have been reported, includingvariations in seismicity [Patanè et al., 2003, 2008;Di Grazia et al., 2009], deformation [Bonaccorsoet al., 2002; Aloisi et al., 2009], and changes inthe chemistry [Caracausi et al., 2003; Aiuppa et al.,2007, 2008] and fluxes [Caltabiano et al., 1994,2004] of volcanic gases. While however precursorysignals to large‐scale flank eruptions are relativelystraightforward to detect [Patanè et al., 2003], themilder and less voluminous SEC eruptions aremore problematic to interpret [e.g., Burton et al.,2005]. In addition, a systematic effort toward aquantitative comparison and integration of the resultsderived from the various monitoring techniques isstill lacking, though highly desirable.

[6] Here we report on the results of an integratedgeophysical (seismicity, deformation) and geo-chemical (CO2/SO2 ratios of the volcanic gas plume)characterization of the recent (2007–2008) activityof Etna, a period during which at least 5 paroxysmaleruptions of the SEC were observed. We show thatthis multidisciplinary approach allows quantita-tively tracking of the cycles of magma accumulation,degassing and preeruptive to syneruptive ascentleading to SEC eruptions, and identification of thetrigger mechanisms of this “periodic” activity withbetter detail than ever before.

2. Etna’s Eruptions in 2007–2008

[7] Etna’s activity was limited to the volcano’ssummit area (and particularly to its upper easternflank) in 2007–2008. While three (Bocca Nuova,Voragine and Northeast) of the four summit craters(Figure 1) were quiescent during the entire period,the mild passive degassing activity of the SEC hasbeen repeatedly interrupted by a sequence ofexplosive‐effusive eruptive episodes (Figure 2a).These were each characterized by some distinctivefeatures (see internal reports at http://www.ct.ingv.it for details), but also displayed some recurrentcharacteristics [Andronico et al., 2008]. Most epi-sodes (episodes 6 (4–5 September 2007), 7 (23–24November 2007), and 8 (10May 2008); Figure 2a)started with mild discontinuous ash emissions (richin lithic fragments) from a pit crater on the easternSEC flank (Figure 1), which after a few days (hours)were replaced by mild Strombolian explosions.Acceleration in the rate of occurrence of Strombolianexplosions was in most cases transitional to a climaxin activity, which typically lasted a few hours and

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consisted in violent fire fountaining episodes, sys-tematically accompanied by copious emission oflava flows issuing from a fracture field at the baseof SEC, and flowing within the desert‐like Valledel Bove depression (Figure 1). The most recentSEC fire fountaining episode on 10 May 2008(episode 8 in Figure 2a) was shortly after followed(on 13May) by dyke intrusion of the upper SE flank[Aloisi et al., 2009], leading to opening of a E–Wtrending fracture field, and the onset of an effusiveeruptionwhich lasted until 6 July 2009 (see Figure 1).

3. Materials and Methods

[8] The composition of the volcanic gas plumeissuing from Etna’s central craters (CCs, Voragine

and Bocca Nuova 1; Figure 1) was investigated bya permanent fully automatedMultiGAS, as describedelsewhere [Aiuppa et al., 2007, 2008]. The Multi-GAS integrated a Gascard II infrared spectrometerand a SO2‐S‐100 (from Membrapor1) electro-chemical sensor, through which measurement ofthe in‐plume abundances of CO2 and SO2 (the twomost abundant volatiles in Etna’s plume, after H2O[Aiuppa et al., 2008]) was achieved. Because ofpower consumption requirements, measurementswere not taken continuously, but in 4 daily cycleseach lasting 30 min (GMT times 0–0.30, 6–6.30,12–12.30, 18–18.30). During each cycle, CO2 andSO2 plume concentrations were measured at ∼1 Hzrate (see Aiuppa et al. [2009] for further details).After data acquisition, the average CO2/SO2 ratio

Figure 1. Map of Mt. Etna volcano, showing the locations of seismic stations (light blue triangles), GPS stations andbaselines (yellow circles and dashed lines), and the CO2/SO2 ratio measurement site (green square) belonging to thepermanent networks run by INGV. The labels EBEL, EPLU, ECPN, EPDN, EMAL, and EMGL mark the seismicand/or GPS permanent stations used for data processing in this work. The thick black curves represent elevation con-tours at 1000 m intervals. The brown thick line, located near the summit area, represents the eruptive fissure thatopened on 13 May 2008. Yellow dots and the orange area show the central craters and the area covered by the lavaflows from the 2008–2009 eruption, respectively [Behncke et al., 2009]. The red lines represent the main eruptivefractures as reported by Acocella and Neri [2005]. The inset in the top left corner shows the distribution of the foursummit craters (VOR, Voragine; BN, Bocca Nuova; SEC, Southeast Crater; NEC, Northeast Crater).

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Figure 2

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for each measurement cycle was calculated fromthe gradient of the best fit regression line in a CO2

versus SO2 scatterplot [Aiuppa et al., 2009], resultsbeing shown in Figure 2b.

[9] Volcanic tremor and long‐period (LP) eventsare seismic signals thought to be related to dynamicsof fluid (gas, magma plus gas, hydrothermal fluids)transport inside a volcanic edifice [Chouet, 1996],and were thus selected in this study as key para-meters to compare with the volcanic gas plume datadescribed above. Recordings of volcanic tremor andLP events were derived from the Etna broadbandseismic network, which is composed of 26 stations,operating with a denser distribution around thesummit craters (Figure 1), and equipped with Nano-metrics TRILLIUM seismometers, with flat responsewithin the 40–0.01 s period range. Seismic signalswere sampled at high sampling rate (100 Hz) and therelative RMS recorded by EBEL station (Figure 2b)was calculated by usingmoving 10min long seismicwindows.

[10] In addition, we attempted to locate the sourceof both volcanic tremor and LP events. Since theonset of these volcano‐seismic signals is usuallynot clearly identified on the seismograms, con-ventional approaches of event location by pickingfirst arrivals cannot be applied. Therefore, differenttechniques, generally based on grid searching pro-cedures, are used. The tremor locations are retrievedby following the approach described by Di Graziaet al. [2006] and Patanè et al. [2008], inverting thespatial distribution of tremor amplitude, calculatedat 17 stations belonging to the broadband seismicnetwork (Figure 1). We consider a 6 × 6 km gridin horizontal and vertical direction, respectively,with spacing between nodes of 250 m. On the otherhand, for the LP events the semblance method[Neidell and Tarner, 1971; Patanè et al., 2008] wasused taking into account the seismic signals acquired

by the six stations nearest to the summit. Foursecond long windows of seismic signal, whose onsetcoincided with onset of the LP events (low‐passfiltered below 1 Hz), were considered. The 3‐D gridwas centered on the volcanic edifice with horizontaldimensions of 6 × 6 km, vertical extent of 3.25 km(from 0 km asl to the top of the volcano) and gridspacing of 125 m.

[11] The 35 CGPS stations of Mount Etna’s net-work record (since 2000) the ground deformationpattern of the volcano related to various sources(inflation‐deflation, dikes, etc.). The remote sta-tions of the network are equipped with Leica 1200receivers and Leica AT504 Choke Ring antennasmounted on concrete pillars, and are located allaround the volcanic edifice and on its surroundings.The raw GPS data are transmitted to the masterstation of Catania via UHF and Wi‐Fi radios, wherethey are processed. Eighteen stations of the net-work are processed in real time at low latency (1 s)and high frequency (1 Hz) by using GeodeticsRTD1 software [Bock et al., 2001]. This enablesfollowing the general trend of deformation of thevolcano (days, months), and the fast processesrelated to highly dynamic volcanic phenomena (lavafountains, fast uprise of dikes). The obtained seriesare, then, down‐sampled considering themean valuein a 6 h window. Although in 2007–2008 the con-figuration of the GPS network in the summit areawas not optimal for inverting the parameters of apressure source, the lengthening‐shortening of GPSbaselines was suitable to track the evolution ofinflation‐deflation cyclic phases eventually occur-ring in the investigated period. For this, we considertwo baselines (EPLU‐ENIC and ECPN‐EPDN;Figure 1), whose data set through the two yearperiod was the most continuous and completeamong the baselines based on the GPS stationssurrounding the summit craters. EPLU, ECPN andEPDN are remote GPS stations close to the summit

Figure 2. (a) Scheme of the volcanic activity of Mt. Etna during 2007–2008 (LF, lava fountain; EF, effusive activity;ST, Strombolian activity). The top numbers indicate the main episodes of explosive activity. (b) Time variation ofRMS of volcanic tremor (10−5 m/s; red line) and plume CO2/SO2 ratio (black line). The filtered CO2/SO2 ratio timeseries (thick blue line) was obtained applying a fourth‐order Butterworth low‐pass zero‐phase digital filter (cut fre-quency Fc = 0.016/d) to the resampled raw data; gas separation pressure (thick yellow line, in MPa) was calculatedfrom the filtered CO2/SO2 ratio time series by using the model pressure dependence of the magmatic gas CO2/SO2

ratio at Etna [see Aiuppa et al., 2007, Figure 1]. The roman numerals and the black vertical dashed lines indicate thefour cycles of variation of plume CO2/SO2 ratio recognized within the investigated period (period V corresponds tothe 2008 effusive phase). (c) Time variation of distance (in m) along EPLU‐ENIC (green) and ECPN‐EPDN (purple)GPS baselines. The thick green line shows the filtered EPLU‐ENIC time series, obtained applying a fourth‐orderButterworth low‐pass zero‐phase digital filter (cut frequency Fc = 0.016/d) to the raw data. (d) Maps and sections ofMt. Etna with the location of the volcanic tremor centroid calculated during periods characterized by high values ofplume CO2/SO2 ratio (gray areas in Figure 2b and black circles in Figure 2d) and by low values of plume CO2/SO2

ratio (orange areas in Figure 2b and red circles in Figure 2d).

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craters, while ENIC is located at about 800 m asl(Figure 1). The azimuth of these baselines isoptimal for detecting inflation‐deflation phasessince EPLU‐ENIC is almost radial with respect tothe centroid of the LP epicenters (see below), whileECPN‐EPDN is crossing the summit craters.Distance variations (baseline displacements, in m)along EPLU‐ENIC and ECPN‐EPDN baselines areshown in Figure 2c. Distance variations werecalculated over 30 min long windows, coincidingwith the volcanic gas plume sampling intervals.

4. Results

[12] RMS values of volcanic tremor for the period2007–2008 are shown as a red line in Figure 2b. As

observed in previous investigations at Mt. Etna[e.g., Cannata et al., 2008], this parameter is strictlyrelated to volcanic activity. The highest RMS valueswere reached during the episodes of Strombolianactivity and lava fountaining, as can be seen inFigures 2a and 2b.

[13] The source locations of volcanic tremor atMt. Etna are also related to the volcano dynamicsand activity, as shown in several papers [e.g.,Patanè et al., 2008; Di Grazia et al., 2009]. Thesource centroids of volcanic tremor recorded duringeight periods are reported in maps and sections ofFigure 2d. Such periods were chosen in order toinvestigate phases of contrasting volcanic activitystate, but also on the basis of the variations ofthe plume CO2/SO2 ratio. In particular, four timeintervals, highlighted by gray areas in Figure 2b,coincided with quiescent degassing phases, whenthe volcanic gas plume exhibited maxima of theCO2/SO2 ratio (see below): in these time intervals,source centroids were roughly located below thesummit area at depths of 1–3 km asl (black circles inFigure 2d). The remaining four periods, evidencedby orange areas in Figure 2b, were characteristicof SEC syneruptive phases, when minima of theCO2/SO2 ratio were observed (see below): thesewere generally characterized by tremor centroidseastward shifted (roughly below SEC) and withshallower depth (∼2.5–3.0 km asl; red circles inFigure 2d).

[14] About 3000 LP events were located during2007–2008 and characterized by high signal‐to‐noise ratio at all six stations nearest to the summitarea (Figure 3). The retrieved LP sources werelocated at shallow depth (2.5–3.2 km asl) belowBocca Nuova Crater. These LP source locations areconsistent with those reported in previous works[Patanè et al., 2008;Cannata et al., 2009;Di Graziaet al., 2009].

[15] Plume CO2/SO2 ratios were characterized by asubstantial variability during the investigated period(Figure 2b), supporting further the idea of a dynamicnature of volcanic degassing processes at basalticvolcanoes in general [Edmonds, 2008], and Italianvolcanoes in specific (see data fromnearby Stromboli[Aiuppa et al., 2009]). CO2/SO2 ratios ranged fromas low as 1.5 (on 27–28 August 2007, just a fewdays prior to the 4–5 September 2007 lava fountainepisode), to as high as 26 (in mid‐January to mid‐February 2007), confirming the wide compositionalinterval observed in 2005 to 2006 [Aiuppa et al.,2007, 2008; Shinohara et al., 2008]. Closer inspec-tion of Figure 2b reveals however that time varia-

Figure 3. Map and cross section of Mt. Etna with thesource locations of about 3000 LP events recorded dur-ing 2007–2008. The radii of the circles are proportionalto the number of the locations in each grid node (seeblack circles and numbers in the bottom right cornerof the map).

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tions of CO2/SO2 ratios followed some systematicpatterns during the investigated period: low ratios(generally <7) were typically observed in the daysprior, during, and after the main paroxysmal epi-sodes of the SEC; while higher CO2/SO2 ratioscharacterized the intereruptive periods between thevarious SEC episodes (ratios typically averaged at>10, though ratios lower than 5 were episodicallyobserved, too). In order to better explore this long‐ tomedium‐term systematic behavior of the acquiredgas signal, we first resampled the original data setusing linear interpolation in order to fill gaps inacquisition and obtain a uniform time series of4 data per day; and then filtered the resampleddata set to remove the high‐frequency components.A fourth‐order Butterworth low‐pass zero‐phasedigital filter (cut frequency Fc = 0.016/d) wasapplied to confine the harmonic content to a rangeof frequencies of our interest. The residual (filtered)signal (shown in Figure 2b as a blue thick solidcurve) supports the existence of cyclic (quasi‐sinusoidal) variations (with typical periods of 3–4 months) of CO2/SO2 ratios; consisting of smoothprogressive increases of the ratios starting from theend of an eruptive episode, peaking of the ratio at>15 at the middle of the cycle, and then a decline ofthe ratio in the period (weeks) prior to the eruptiveepisode (Figures 2a and 2b). We thus recognized

four cycles of variation within the investigatedperiod (see roman numerals I–IV in Figure 2b).

[16] Figure 2c shows the distance variations alongGPS baselines EPLU‐ENIC and ECPN‐EPDN.This reveals that both baselines lengthened almostcontinuously in the quiescent periods between theparoxysmal episodes of SEC, while they shortenedduring and after each episode. This general patternwas most evident from May to September 2007(cycle II) and from January to May 2008 (cycle IV),while in September–December 2007 (cycle III) theclimax of the lengthening was reached ∼1 monthprior to the 23–24 November 2007 SEC episode(when an increase in the tremor amplitude wasconcurrently observed). Another significant, butobvious, difference in the pattern was observed afterthe onset of the 13 May 2008 eruption, when theEPLU‐ENIC baseline experienced a marked short-ening lasting for several months due to the deflationthat accompanied the first months of the eruption.

[17] We finally observe that the cyclic inflation‐deflation trends shown by the summit GPS base-lines also extended to GPS baselines located at midaltitude: as an example, we report in Figure 4 thecase of the EMGL‐EMAL line, which is located onthe western flank of Mt. Etna (Figure 1), the sectorof the volcano most sensitive than the others to

Figure 4. (a) The main features of volcanic activity and (b) time variation of distance variations (in m) along theEMGL‐EMAL GPS baseline. Elongations along this line are sensible to pressure variations in the deep plumbing sys-tem of Etna [Mattia et al., 2007]. The thick line shows the filtered EMGL‐EMAL time series, obtained applying afourth‐order Butterworth low‐pass zero‐phase digital filter (cut frequency Fc = 0.016/d) to the raw data.

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volcanic cycles and deep intrusions (for sourceslocated at about 0–3 km bsl [Mattia et al., 2007]).

5. Discussion

[18] Mt. Etna volcano was characterized in 2007 to2008 by several paroxysmal episodes at SEC, andby a more than 1 year long effusive eruption startingon 13 May 2008 at an eruptive fissure located nearthe summit area (Figure 1). By making use of ourmultidisciplinary observations, we propose here thatsuch changes in volcanic activity state were paral-leled (and often anticipated) by systematic (cyclic)trends in the monitored geophysical (seismicity,deformation) and geochemical (plume composition)parameters (Figure 2). Our measurements thusallow deriving new constraints on the poorly char-acterized geometry of Etna’s shallow plumbingsystem, and on the trigger mechanisms of the peri-odic subterminal activity of SEC.

[19] We notice that when the time trends of vol-canic tremor, deformation, and plume chemistryare observed in concert (Figure 2), a significantcontrast emerges between the intereruptive phasesof quiescent degassing at the CCs and the SECeruptive episodes. During quiescent periods, tremorRMS amplitude was typically low, and the sourcelocation of volcanic tremor was centered below theCCs at depths of 1–2.8 km asl; while resumptionof volcanic activity at SEC was in all cases asso-ciated with large increases of RMS amplitude, andupward and eastward migration of tremor sourcelocation toward the eruptive vent (Figures 2band 2d). These changes in amplitude and sourcedepth of volcanic tremor were also paralleled bysystematic trends in the measured plume CO2/SO2

ratios (Figure 2b) and GPS data (Figures 2c and 4):the former were typically high in the repose periodsbetween SEC episodes (when tremor was low inamplitude, and deep), and typically decreased priorto and during resumption of volcanic activity atSEC; while GPS data consistently showed a mildinflation of the volcano’s summit during the inter-eruptive periods, which typically terminated a fewdays/weeks before the eruptions, and were then fol-lowed by sharp deflations during eruptive episodes.This apparent correspondence between temporalvariations of plume composition and geophysicalsignals suggests that some link must exist at Etnabetween the source mechanisms of degassing and(shallow) seismicity and deformation, as discussedbelow.

[20] First, the source locations of volcanic tremorsuggest that Etna’s recent eruptive episodes in2007–2008 have been fed by magmas originallystored in the central conduit region (Figure 5). Sucha relatively shallow (<100 MPa pressure [Collinset al., 2009]) preeruptive storage in the summitcentral conduits is indeed supported by petro-graphic, chemical and isotopic (Sr, Nd) features oferupted magmas (which are essentially the plagio-clase‐bearing trachybasalts typically erupted atthe summit vents [Corsaro and Miraglia, 2009]),and by their H2O‐poor volatile contents [Collinset al., 2009]. According to tremor location data(Figure 5), magmas are stored, in the repose periodsin between SEC episodes, at depths of 1 to 2.8 kmasl; and are thought to passively release gases viathe CCs, thus becoming the source of our CO2‐richgas emissions (Figure 5a). The accumulation ofmagma and gas bubbles in this storage zone islikely responsible for the gentle inflation of thevolcano’s summit captured by the GPS network.Notably, the seismically inferred magma depths(1 to 2.8 km asl) are consistent with the observedhigh plume CO2/SO2 ratios, these being valuableindicators of the source depth of magmatic gases[Aiuppa et al., 2007, 2009]. Indeed, because oftheir contrasting solubilities in basaltic silicatemelts,CO2 and SO2 experience a contrasting degassingpath upon magma decompression: the former isexsolved from Etnean basaltic melts starting fromas deep as ∼21–24 km (CO2‐dominated fluid inclu-sions hosted in Mg‐rich olivines imply exsolutionpressure ≥800 MPa [Kamenetsky and Clocchiatti,1996]); while the latter is released to the gas phasestarting from ∼4–6 km (e.g., for pressures lowerthan 100–140 MPa [Spilliaert et al., 2006]). Assuch, a pressure‐dependent evolution of the mag-matic gas CO2/SO2 ratio can be predicted, from>200 at P > 200 MPa to ≤5 for close‐to‐surfacegas‐melt separations [Aiuppa et al., 2007]. In thisview, the high CO2/SO2 ratios observed in betweenSEC eruptive episodes (Figure 2b) reflect the rel-atively “deep” degassing of an accumulated magmabatch: using the model‐calculated pressure depen-dence of the magmatic gas CO2/SO2 ratio at Etna[Aiuppa et al., 2007], we derive from measured gascompositions a range of equilibrium pressures of13–16 MPa, which for a bulk rock density of2700 kg/m3 would correspond to depths of 480–600 m below the summit vents (or 2.7–2.8 kmelevation asl). These inferred source depths for thedischarged gas phase should be viewed as depths ofgas separation from (and thus last equilibration with)

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the silicate melt (Figure 5a); thus, gas bubbles mustderive from at least 2.7–2.8 km elevation asl, butprobably deeper in the system: this is in agreementwith the relatively “deep” source area of seismictremor during intereruptive periods. In more detail,our derived 2.7–2.8 km asl gas source area is fairlyconsistent with the upper limit of the source area ofseismic tremor (Figure 5a; 2.7–3 km asl); and alsocoincides with the main cluster of LP hypocenters(Figures 3 and 5a; 2.5–3.2 km asl) below BoccaNuova Crater. This apparent agreement may sug-gest that Etna’s upper feeding conduit system,imaged by the 1–3 km asl deep cluster of seismictremor locations (Figure 5) [Patanè et al., 2008],roughly terminates at ∼2700–2800 m, with the gasbubbles separated over this depth range feedingsurface gas emission; and triggering the shallower(>2500 m asl) very long period (VLP) and LP[Patanè et al., 2008; Cannata et al., 2009] seis-

micity. In fact, according to prevailing theory, LPevents originate in the acoustic resonances of fluid‐filled cracks triggered by pressure transients [Chouet,1996], and VLP events are assumed to be linked tomass movements, and to represent inertial forcesresulting from perturbations in flow of magma andgas through conduits [Uhira and Takeo, 1994]. Inboth models, the gas bubbles, rising through themagma column, coalescing and forming slugs, andcollapsing, play a fundamental role in generatingLP and VLP seismicity [e.g., Chouet et al., 2003;James et al., 2004].

[21] Second, our observations strongly suggest thatat least during 2007–2008, magma was systemati-cally transferred from the central upper plumbingsystem toward SEC prior to (in the days or hours)each paroxysmal event (Figure 5b). This was cap-tured by our observations as a decrease of CO2/SO2

Figure 5. Interpretative cross section of Mt. Etna, summarizing the key processes controlling the cyclic eruptiveactivity in 2007–2008. (a) During quiescent periods in between SEC episodes, magma is stored in 1–3 km asl deepreservoir below the CCs (as imaged by location of seismic tremor sources; black circles). The reservoir is supplied bydeep rising CO2‐rich gas bubbles [Collins et al., 2009; this study] and eventually more primitive melts [Corsaro andMiraglia, 2009]. Rising gas bubbles reequilibrate at reservoir conditions and are finally separated from the melt at∼2700–2800 m elevation asl, as indicated by the relatively high CO2/SO2 ratios of quiescent gas emissions from CCs(from which gas‐melt separation pressures of 13–16 MPa, or depths of 480–600 m below the summit vents, can becalculated [Aiuppa et al., 2007]). On their farther ascent through the upper CCs’ conduits, coalescing gas bubbles arethe source of the shallow Etna’s LP and VLP seismicity, which is systematically recorded below Bocca Nuova Craterat 2.5–3.2 km elevation asl. The steadily high CO2/SO2 ratios during quiescent phases point to a persistent supply ofgas bubbles, eventually leading to development of a gas bubble layer [Allard et al., 2005], and finally triggeringpressurization of the reservoir (as captured by the mild but systematic inflation of the volcano’s summit duringquiescent periods). As pressurization of the reservoir proceeds, failure of the fractured upper SE Etna’s flank [Neriand Acocella, 2006] finally leads to ascent of a magma plus gas mixture toward SEC, leading to (b) an eruptiveepisode. Resumption of volcanic activity at SEC is systematically anticipated by declining plume CO2/SO2 ratios andby abrupt increases in tremor amplitude, upward (>2.8 km asl) and eastward migration of the source location ofseismic tremor (red circles), and fast contraction of the volcano’s summit.

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ratios in Etna’s CCs prior to and during each SECeruptive episode. A decline of CO2/SO2 ratios to∼5 requires an upward migration (up to nearlyatmospheric pressure) of the source area of vol-canic gases, while an even lower ratio (to as lowas ∼1.5, typical of the residual degassing phases)also implies a drastic decline of gas feeding to theCCs [Aiuppa et al., 2007]: both observations areconsistent with a magma diversion from below theCCs toward the SEC rift zone, as captured by ourmeasurements as an upward and eastward of seismictremor. A geometrical connection between the cen-tral and SE craters’ conduits is consistent withrecent infrasonic evidence of branched conduitgeometry on Etna [Marchetti et al., 2009]. We thusconclude that the low CO2/SO2 ratios, when com-bined with increasing tremor amplitude and upwardand eastward migration of its source area, documentthe phase to preeruptive and syneruptive ascent ofthe magma batches to give rise to SEC eruptiveepisodes. Ground deformation (GPS) data are notsuitable to infer the possible migration of magmatoward the SEC, due to the geometry of the networkrelative to the possible sources below the SEC (tooshallow and far with respect to the three GPS sta-tions surrounding the summit craters). In spite ofthis, the recorded GPS signals indeed showed thatthe volcano’s inflation terminated a few days/weeksbefore each eruption, possibly indicating that pres-sure increase in the system (see below) was comingto an end (consistent with the migration of magma/gas toward the SEC).

[22] Which are the trigger factors controlling thetransition from passive degassing at the CCs toeruption at SEC, as recurrently occurred in 2007–2008? Our GPS measurements (Figure 2c) clearlyindicate that each eruptive episode was preceded bymild but systematic inflation of the volcano’ssummit. We then consider the possibility that eachinflation cycle reflected the pressurization of thecentral crater magma storage zone, up to its failureleading to an eruption; and verify the hypothesis ofwhether or not an elongated vertical pressuresource, bounded upward at about 2800 m asl anddownward at about 1000 m asl, is consistent withthe observed deformation pattern. Using the closedpipe model proposed by Bonaccorso and Davis[1999], and considering the tradeoffs between thepressure change and the horizontal radius of theellipsoid, we find that measured elongationsbetween GPS stations can be accounted for by sucha pressurization source; provided a “strength”parameter of about 10 · 1010 Pa m2 is used (wherethe “strength” is the product of the pressure change

and the radius, DP*a2), and the source is fixed justbelow the ECPN station, as suggested by thetremor localizations. In particular, an elongation ofabout 0.75 and 0.73 cm for the EPLU‐ENIC andEPDN‐ECPN baselines, respectively, is predictedfrom the model, which well fit the observations (forinstance, the measured elongation during period IIis about 0.7 cm for both baselines). We concludethen that the periodic pressurization of the centralconduit storage zone was a viable source ofdeformation, and thus a most likely trigger for therecurrent eruptive episodes at SEC in 2007–2008.

[23] Petrologic data [Corsaro and Miraglia, 2009]indicate that the arrival of deeply rising moreprimitive magma batches (Figure 5a) possiblyplayed a major role in causing pressurization of thecentral conduit storage zone prior to March 2007and May 2008 (inflation cycles I and IV in Figure2); while more evolved magmas were erupted inother SEC episodes (cycles II and III), arguingagainst any preeruptive magma replenishment inthe reservoir. In these latter cases, we speculate thatthe supply of gas bubbles (not magma) to thecentral conduit storage zone, possibly sourced bythe 3–5 km bsl deep Etna’s magma reservoir[Spilliaert et al., 2006], was the main cause ofpressurization, and thus eruption (Figure 5a). Thewater‐poor composition of olivine‐hosted melt in-clusions from 2007 SEC eruptions, in particular,suggests that magmas have experienced extensivegas flushing (and thus dehydratation) prior to theireruption [Collins et al., 2009]; a process whichcan easily occur when resident magma stored inthe upper central conduits is flushed by deeplyderived gas bubbles, which CO2–rich signature iswell represented by our persistent passive plumeemissions (Figure 5a).

[24] The involvement of Etna’s deep plumbingsystem (as source of primitive melts and/or gasbubbles) in generating the pressurization of thecentral crater storage zone, and thus in eruptiontrigger, is also supported by geodetic data. Thepressurization of the deepest parts of the plumbingsystem are conventionally detected by consideringGPS baselines located at mid altitude; that usuallyused for this aim is the EMGL‐EMAL line, whichis located on the western flank of Mt. Etna, thesector of the volcano most sensitive than the othersto volcanic cycles and deep intrusions (for sourceslocated at about 0–3 km bsl [Mattia et al., 2007]).The elongation of this baseline (Figure 4) showsthat this flank underwent a continuous inflationinterrupted by drastic decreases during the erup-tions at SEC. This pattern confirms (1) that the

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deep volcanic plumbing system was continuouslypressurized from 2007 to 2008 and (2) that thedepressurization in the shallow plumbing systemdue to the eruptions rapidly propagated downward(i.e., the connection between the shallow and deepparts of the plumbing system is efficient). Thisevidence supports the idea that during the inter-eruptive periods the shallow part of the plumbingsystem underwent a continuous pressurization, whichweakened the strength of the upper part of the con-duit and/or the summit craters, inducing the explo-sions at SEC.

6. Conclusions

[25] Our combined geophysical and geochemicalmeasurements here provide first evidence for thesystematic trends in volcanic gas plume chemistry,seismicity and deformation accompanying thecyclic activity of Etna in 2007–2008. Four cyclesof increase‐decrease of plume CO2/SO2 ratios,paralleled by inflation of the volcano’s summit(GPS), were observed during the volcano’s quies-cent periods in between the SEC paroxysmal activ-ity; during these periods of quiescence, seismictremor was typically characterized by low amplitudeand relatively deeper (1–2.8 km asl) location.Resumption of volcanic activity at SEC was sys-tematically anticipated by declining plume CO2/SO2

ratios, and marked by abrupt increases in tremoramplitude, upward (>2.8 km asl) and eastwardmigration of the source location of seismic tremor,and fast contraction of the volcano’s summit.

[26] These observations are consistent with pre-eruptive magma storage in a 1–2.8 km deep reser-voir below the CCs. The episodic arrival of newmagma batches [Corsaro and Miraglia, 2009], orthe persistent supply of deeply derived gas bubbles[Shinohara et al., 2008; Collins et al., 2009], peri-odically triggered pressurization of this shallowreservoir (inflation), leading to further magmaascent and eruption at SEC. That paroxysmal SECepisodes mark the violent release of a bubble‐richmagma layer, with bubbles having accumulated ina relatively shallow reservoir, is consistent with theFTIR‐sensed composition of lava fountaining gasjets [Allard et al., 2005]. A novel aspect of ourobservations is however that the bubbly magma isnot accumulated below the SEC, as previouslythought [Allard et al., 2005]. Our observationsindeed strongly support a geometrical connectionbetween the central and SE craters’ conduits, anidea which is consistent with recent infrasonic

evidence of branched conduit geometry on Etna[Marchetti et al., 2009]. We suggest that at leastduring 2007–2008, magma was systematicallytransferred from the central upper plumbing systemtoward the SEC prior to (in the days or hours) eachparoxysmal event. This idea is also consistent withthe SEC releasing a CO2‐poor (CO2/SO2 ratios<1) during intereruptive periods, and contributingoverall to a minor fraction (<3%) of the totalvolcano’s gas budget [Aiuppa et al., 2008]. Accord-ing to Marchetti et al. [2009], shifting of activityfrom the CCs to the SEC may occur in response tochanges in flow dynamics in a branched system,whereby passive degassing below the central ventsprevails at low magma‐gas flow rates, while theSEC activates only when some threshold in magma‐gas ascent rate is exceeded. While our measure-ments are fully consistent with this scenario (ourdecreasing CO2/SO2 ratios and increasing tremoramplitude are clear evidences of the ascent of adegassing vesicular magma prior to each SECepisode), we speculate that a structural control onmagma eastward migration might also have playeda role. It is worth noting in this context that noeruptive activity was reported since 2001 at Etna’sCCs, which were instead most active in the 1993–2001 activity period [Allard et al., 2006]. Thisvirtual cessation of activity of the CCs started incoincidence with the 2001 and 2002–2003 lateraleruptions [Neri et al., 2005], which marked theonset of a phase of unusual acceleration of theseaward (eastward) motion of the unstable Etna’seastern flank [Bonforte et al., 2008]. Accelerationof the east flank slip movement is thought to havealtered the stress field and deformation pattern onthe volcano’s summit, with the opening of N–S toNW–SE trending fracture field extending from theCCs toward SEC [Neri and Acocella, 2006]. Wepropose that as pressurization of the summit magmareservoir was periodically attained, this relativelyshallow [Neri and Acocella, 2006] weakness zoonacted as a main pathway for the upward (and east-ward) magma migration, confining eruptive activityto the SEC.

[27] This report is among the first in which geo-chemical and geophysical observations are inte-grated and compared in a systematic way [see, e.g.,McGonigle et al., 2009]. We conclude that multi-disciplinary studies have more potential to shedlight into the complex processes underlying thetime changing nature of active volcanoes, and wesuggest that such an integrated approach should bea priority of volcanological research in the years tocome.

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