Patterns in the recent activity of Mount Etna volcano investigated by integrated geophysical and geochemical observations

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1 Article Volume 11, Number 9 23 September 2010 Q09008, doi: /2010gc ISSN: Patterns in the recent activity of Mount Etna volcano investigated by integrated geophysical and geochemical observations A. Aiuppa CFTA, Università di Palermo, I Palermo, Italy (aiuppa@unipa.it) Also at INGV, Sezione di Palermo, I Palermo, Italy A. Cannata, F. Cannavò, G. Di Grazia, and F. Ferrari INGV, Sezione di Catania, I Catania, Italy G. Giudice, S. Gurrieri, and M. Liuzzo INGV, Sezione di Palermo, I Palermo, Italy M. Mattia, P. Montalto, D. Patanè, and G. Puglisi INGV, Sezione di Catania, I Catania, Italy [1] Seismic, deformation, and volcanic gas observations offer independent and complementary information on the activity state and dynamics of quiescent and eruptive volcanoes and thus all contribute to volcanic risk assessment. In spite of their wide use, there have been only a few efforts to systematically integrate and compare the results of these different monitoring techniques. Here we combine seismic (volcanic tremor and long period seismicity), deformation (GPS), and geochemical (volcanic gas plume CO 2 /SO 2 ratios) measurements in an attempt to interpret trends in the recent ( ) activity of Etna volcano. We show that each eruptive episode occurring at the Southeast Crater (SEC) was preceded by a cyclic phase of increase decrease of plume CO 2 /SO 2 ratios and by inflation of the volcano s summit captured by the GPS network. These observations are interpreted as reflecting the persistent supply of CO 2 rich gas bubbles (and eventually more primitive magmas) to a shallow (depth of km asl) magma storage zone below the volcano s central craters (CCs). Overpressuring of the resident magma stored in the upper CCs conduit triggers further magma ascent and finally eruption at SEC, a process which we capture as an abrupt increase in tremor amplitude, an upward (>2800 m asl) and eastward migration of the source location of seismic tremor, and a rapid contraction of the volcano s summit. Resumption of volcanic activity at SEC was also systematically anticipated by declining plume CO 2 /SO 2 ratios, consistent with magma degassing 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; 8430 Volcanology: Volcanic gases. Received 8 April 2010; Revised 13 July 2010; Accepted 29 July 2010; Published 23 September Aiuppa, A., et al. (2010), Patterns in the recent activity of Mount Etna volcano investigated by integrated geophysical and geochemical observations, Geochem. Geophys. Geosyst., 11, Q09008, doi: /2010gc Copyright 2010 by the American Geophysical Union 1 of 13

2 1. Introduction [2] Volcanic eruptions, one of the most spectacular and dramatic demonstrations of nature s power, are the ultimate results of periodic events of magma supply from the Earth s mantle, storage in the upper crust, and fast decompression (and degassing) upon surface emplacement. These cycles of magma supply accumulation eruption [Tilling and Dvorak, 1993] produce measurable physical and chemical signals which, when captured by monitoring networks, can allow mitigating the effects of volcanic eruptions [Scarpa and Tilling, 1996]. [3] It has long been recognized, for instance, that rate of occurrence and mechanisms of basaltic eruptions, punctuating the persistent degassing activity of Etna volcano (in southern Italy), follow some systematic (periodic) long term trend [Imbó, 1928; Chester et al., 1985; Tanguy et al., 1997]. The periodic patterns of Etna s post 1865 activity recently led Behncke and Neri [2003] to identify 4 main eruptive cycles, each being characterized by a shift of activity (over time scales of a few decades) from quiescence to summit eruptions (e.g., eruptive activity confined at the summit craters), and finally to flank eruptions. This sequence of events was thought to reflect the progressive magma accumulation within (and periodically tapping of) a main magma storage zone 3 5 km below sea level [Allard et al., 2006]; up to the final magma drainage from the reservoir during a large scale flank eruption (e.g., the eruption of Etna), leading to termination of a cycle [Behncke and Neri, 2003]. [4] Etna s activity is known to be periodic also on much smaller time scales: paroxysmal summit eruptions, consisting of violent and short lived episodes of fire fountaining and lava emission, occurred with unusually high periodicity at Etna s Southeast Crater (SEC) in 1998 to 2001 [Alparone et al.,2003], with repose periods between one event and the following ranging from less than a few hours to several months [Allard et al., 2006]. The mechanisms leading to such recurrent SEC paroxysmal activity, which also persisted more recently in 2007 and 2008 [e.g., Andronico et al., 2008], are still poorly understood [La Delfa et al., 2001; Allard et al., 2005]. [5] This changeable and dynamic nature of Etna s volcanism represents a real challenge to volcanologists. However, the recent advances of traditional volcano monitoring methods, combined with the advent of new techniques, have significantly improved our ability to characterize the volcano s activity state [Bonaccorso et al., 2003]; to the point that a range of precursory signals for the recent Etna s eruptions have been reported, including variations in seismicity [Patanè et al., 2003, 2008; Di Grazia et al., 2009], deformation [Bonaccorso et al., 2002; Aloisi et al., 2009], and changes in the chemistry [Caracausi et al., 2003; Aiuppa et al., 2007, 2008] and fluxes [Caltabiano et al., 1994, 2004] of volcanic gases. While however precursory signals to large scale flank eruptions are relatively straightforward to detect [Patanè et al., 2003], the milder and less voluminous SEC eruptions are more problematic to interpret [e.g., Burton et al., 2005]. In addition, a systematic effort toward a quantitative comparison and integration of the results derived from the various monitoring techniques is still lacking, though highly desirable. [6] Here we report on the results of an integrated geophysical (seismicity, deformation) and geochemical (CO 2 /SO 2 ratios of the volcanic gas plume) characterization of the recent ( ) activity of Etna, a period during which at least 5 paroxysmal eruptions of the SEC were observed. We show that this multidisciplinary approach allows quantitatively tracking of the cycles of magma accumulation, degassing and preeruptive to syneruptive ascent leading to SEC eruptions, and identification of the trigger mechanisms of this periodic activity with better detail than ever before. 2. Etna s Eruptions in [7] Etna s activity was limited to the volcano s summit area (and particularly to its upper eastern flank) in 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 has been repeatedly interrupted by a sequence of explosive effusive eruptive episodes (Figure 2a). These were each characterized by some distinctive features (see internal reports at it for details), but also displayed some recurrent characteristics [Andronico et al., 2008]. Most episodes (episodes 6 (4 5 September 2007), 7 (23 24 November 2007), and 8 (10 May 2008); Figure 2a) started with mild discontinuous ash emissions (rich in lithic fragments) from a pit crater on the eastern SEC flank (Figure 1), which after a few days (hours) were replaced by mild Strombolian explosions. Acceleration in the rate of occurrence of Strombolian explosions was in most cases transitional to a climax in activity, which typically lasted a few hours and 2of13

3 Figure 1. Map of Mt. Etna volcano, showing the locations of seismic stations (light blue triangles), GPS stations and baselines (yellow circles and dashed lines), and the CO 2 /SO 2 ratio measurement site (green square) belonging to the permanent networks run by INGV. The labels EBEL, EPLU, ECPN, EPDN, EMAL, and EMGL mark the seismic and/or GPS permanent stations used for data processing in this work. The thick black curves represent elevation contours at 1000 m intervals. The brown thick line, located near the summit area, represents the eruptive fissure that opened on 13 May Yellow dots and the orange area show the central craters and the area covered by the lava flows from the eruption, respectively [Behncke et al., 2009]. The red lines represent the main eruptive fractures as reported by Acocella and Neri [2005]. The inset in the top left corner shows the distribution of the four summit craters (VOR, Voragine; BN, Bocca Nuova; SEC, Southeast Crater; NEC, Northeast Crater). consisted in violent fire fountaining episodes, systematically accompanied by copious emission of lava flows issuing from a fracture field at the base of SEC, and flowing within the desert like Valle del Bove depression (Figure 1). The most recent SEC fire fountaining episode on 10 May 2008 (episode 8 in Figure 2a) was shortly after followed (on 13 May) by dyke intrusion of the upper SE flank [Aloisi et al., 2009], leading to opening of a E W trending fracture field, and the onset of an effusive eruption which lasted until 6 July 2009 (see Figure 1). 3. Materials and Methods [8] The composition of the volcanic gas plume issuing from Etna s central craters (CCs, Voragine and Bocca Nuova 1; Figure 1) was investigated by a permanent fully automated MultiGAS, as described elsewhere [Aiuppa et al., 2007, 2008]. The Multi- GAS integrated a Gascard II infrared spectrometer and a SO2 S 100 (from Membrapor 1 ) electrochemical sensor, through which measurement of the in plume abundances of CO 2 and SO 2 (the two most abundant volatiles in Etna s plume, after H 2 O [Aiuppa et al., 2008]) was achieved. Because of power consumption requirements, measurements were not taken continuously, but in 4 daily cycles each lasting 30 min (GMT times , , , ). During each cycle, CO 2 and SO 2 plume concentrations were measured at 1 Hz rate (see Aiuppa et al. [2009] for further details). After data acquisition, the average CO 2 /SO 2 ratio 3of13

4 Geosystems 3 G AIUPPA ET AL.: INTEGRATED OBSERVATION OF ETNA ACTIVITY /2010GC Figure 2 4 of 13

5 for each measurement cycle was calculated from the gradient of the best fit regression line in a CO 2 versus SO 2 scatterplot [Aiuppa et al., 2009], results being shown in Figure 2b. [9] Volcanic tremor and long period (LP) events are seismic signals thought to be related to dynamics of fluid (gas, magma plus gas, hydrothermal fluids) transport inside a volcanic edifice [Chouet, 1996], and were thus selected in this study as key parameters to compare with the volcanic gas plume data described above. Recordings of volcanic tremor and LP events were derived from the Etna broadband seismic network, which is composed of 26 stations, operating with a denser distribution around the summit craters (Figure 1), and equipped with Nanometrics TRILLIUM seismometers, with flat response within the s period range. Seismic signals were sampled at high sampling rate (100 Hz) and the relative RMS recorded by EBEL station (Figure 2b) was calculated by using moving 10 min long seismic windows. [10] In addition, we attempted to locate the source of both volcanic tremor and LP events. Since the onset of these volcano seismic signals is usually not clearly identified on the seismograms, conventional approaches of event location by picking first arrivals cannot be applied. Therefore, different techniques, generally based on grid searching procedures, are used. The tremor locations are retrieved by following the approach described by Di Grazia et al. [2006] and Patanè et al. [2008], inverting the spatial distribution of tremor amplitude, calculated at 17 stations belonging to the broadband seismic network (Figure 1). We consider a 6 6 km grid in horizontal and vertical direction, respectively, with spacing between nodes of 250 m. On the other hand, for the LP events the semblance method [Neidell and Tarner, 1971; Patanè et al., 2008] was used taking into account the seismic signals acquired by the six stations nearest to the summit. Four second long windows of seismic signal, whose onset coincided with onset of the LP events (low pass filtered below 1 Hz), were considered. The 3 D grid was centered on the volcanic edifice with horizontal dimensions of 6 6 km, vertical extent of 3.25 km (from 0 km asl to the top of the volcano) and grid spacing of 125 m. [11] The 35 CGPS stations of Mount Etna s network record (since 2000) the ground deformation pattern of the volcano related to various sources (inflation deflation, dikes, etc.). The remote stations of the network are equipped with Leica 1200 receivers and Leica AT504 Choke Ring antennas mounted on concrete pillars, and are located all around the volcanic edifice and on its surroundings. The raw GPS data are transmitted to the master station of Catania via UHF and Wi Fi radios, where they are processed. Eighteen stations of the network are processed in real time at low latency (1 s) and high frequency (1 Hz) by using Geodetics RTD 1 software [Bock et al., 2001]. This enables following the general trend of deformation of the volcano (days, months), and the fast processes related to highly dynamic volcanic phenomena (lava fountains, fast uprise of dikes). The obtained series are, then, down sampled considering the mean value in a 6 h window. Although in the configuration of the GPS network in the summit area was not optimal for inverting the parameters of a pressure source, the lengthening shortening of GPS baselines was suitable to track the evolution of inflation deflation cyclic phases eventually occurring in the investigated period. For this, we consider two baselines (EPLU ENIC and ECPN EPDN; Figure 1), whose data set through the two year period was the most continuous and complete among the baselines based on the GPS stations surrounding the summit craters. EPLU, ECPN and EPDN are remote GPS stations close to the summit Figure 2. (a) Scheme of the volcanic activity of Mt. Etna during (LF, lava fountain; EF, effusive activity; ST, Strombolian activity). The top numbers indicate the main episodes of explosive activity. (b) Time variation of RMS of volcanic tremor (10 5 m/s; red line) and plume CO 2 /SO 2 ratio (black line). The filtered CO 2 /SO 2 ratio time series (thick blue line) was obtained applying a fourth order Butterworth low pass zero phase digital filter (cut frequency Fc = 0.016/d) to the resampled raw data; gas separation pressure (thick yellow line, in MPa) was calculated from the filtered CO 2 /SO 2 ratio time series by using the model pressure dependence of the magmatic gas CO 2 /SO 2 ratio at Etna [see Aiuppa et al., 2007, Figure 1]. The roman numerals and the black vertical dashed lines indicate the four cycles of variation of plume CO 2 /SO 2 ratio recognized within the investigated period (period V corresponds to the 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 order Butterworth low pass zero phase digital filter (cut frequency Fc = 0.016/d) to the raw data. (d) Maps and sections of Mt. Etna with the location of the volcanic tremor centroid calculated during periods characterized by high values of plume CO 2 /SO 2 ratio (gray areas in Figure 2b and black circles in Figure 2d) and by low values of plume CO 2 /SO 2 ratio (orange areas in Figure 2b and red circles in Figure 2d). 5of13

6 Figure 3. Map and cross section of Mt. Etna with the source locations of about 3000 LP events recorded during The radii of the circles are proportional to the number of the locations in each grid node (see black circles and numbers in the bottom right corner of the map). craters, while ENIC is located at about 800 m asl (Figure 1). The azimuth of these baselines is optimal for detecting inflation deflation phases since EPLU ENIC is almost radial with respect to the centroid of the LP epicenters (see below), while ECPN EPDN is crossing the summit craters. Distance variations (baseline displacements, in m) along EPLU ENIC and ECPN EPDN baselines are shown in Figure 2c. Distance variations were calculated over 30 min long windows, coinciding with the volcanic gas plume sampling intervals. 4. Results [12] RMS values of volcanic tremor for the period 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 strictly related to volcanic activity. The highest RMS values were reached during the episodes of Strombolian activity and lava fountaining, as can be seen in Figures 2a and 2b. [13] The source locations of volcanic tremor at Mt. Etna are also related to the volcano dynamics and activity, as shown in several papers [e.g., Patanè et al., 2008; Di Grazia et al., 2009]. The source centroids of volcanic tremor recorded during eight periods are reported in maps and sections of Figure 2d. Such periods were chosen in order to investigate phases of contrasting volcanic activity state, but also on the basis of the variations of the plume CO 2 /SO 2 ratio. In particular, four time intervals, highlighted by gray areas in Figure 2b, coincided with quiescent degassing phases, when the volcanic gas plume exhibited maxima of the CO 2 /SO 2 ratio (see below): in these time intervals, source centroids were roughly located below the summit area at depths of 1 3 km asl (black circles in Figure 2d). The remaining four periods, evidenced by orange areas in Figure 2b, were characteristic of SEC syneruptive phases, when minima of the CO 2 /SO 2 ratio were observed (see below): these were generally characterized by tremor centroids eastward shifted (roughly below SEC) and with shallower depth ( km asl; red circles in Figure 2d). [14] About 3000 LP events were located during and characterized by high signal tonoise ratio at all six stations nearest to the summit area (Figure 3). The retrieved LP sources were located at shallow depth ( km asl) below Bocca Nuova Crater. These LP source locations are consistent with those reported in previous works [Patanè et al., 2008; Cannata et al., 2009; Di Grazia et al., 2009]. [15] Plume CO 2 /SO 2 ratios were characterized by a substantial variability during the investigated period (Figure 2b), supporting further the idea of a dynamic nature of volcanic degassing processes at basaltic volcanoes in general [Edmonds, 2008], and Italian volcanoes in specific (see data from nearby Stromboli [Aiuppa et al., 2009]). CO 2 /SO 2 ratios ranged from as low as 1.5 (on August 2007, just a few days prior to the 4 5 September 2007 lava fountain episode), to as high as 26 (in mid January to mid February 2007), confirming the wide compositional interval observed in 2005 to 2006 [Aiuppa et al., 2007, 2008; Shinohara et al., 2008]. Closer inspection of Figure 2b reveals however that time varia- 6of13

7 Figure 4. (a) The main features of volcanic activity and (b) time variation of distance variations (in m) along the EMGL EMAL GPS baseline. Elongations along this line are sensible to pressure variations in the deep plumbing system of Etna [Mattia et al., 2007]. The thick line shows the filtered EMGL EMAL time series, obtained applying a fourth order Butterworth low pass zero phase digital filter (cut frequency Fc = 0.016/d) to the raw data. tions of CO 2 /SO 2 ratios followed some systematic patterns during the investigated period: low ratios (generally <7) were typically observed in the days prior, during, and after the main paroxysmal episodes of the SEC; while higher CO 2 /SO 2 ratios characterized the intereruptive periods between the various SEC episodes (ratios typically averaged at >10, though ratios lower than 5 were episodically observed, too). In order to better explore this long to medium term systematic behavior of the acquired gas signal, we first resampled the original data set using linear interpolation in order to fill gaps in acquisition and obtain a uniform time series of 4 data per day; and then filtered the resampled data set to remove the high frequency components. A fourth order Butterworth low pass zero phase digital filter (cut frequency Fc = 0.016/d) was applied to confine the harmonic content to a range of frequencies of our interest. The residual (filtered) signal (shown in Figure 2b as a blue thick solid curve) supports the existence of cyclic (quasisinusoidal) variations (with typical periods of 3 4 months) of CO 2 /SO 2 ratios; consisting of smooth progressive increases of the ratios starting from the end of an eruptive episode, peaking of the ratio at >15 at the middle of the cycle, and then a decline of the ratio in the period (weeks) prior to the eruptive episode (Figures 2a and 2b). We thus recognized four cycles of variation within the investigated period (see roman numerals I IV in Figure 2b). [16] Figure 2c shows the distance variations along GPS baselines EPLU ENIC and ECPN EPDN. This reveals that both baselines lengthened almost continuously in the quiescent periods between the paroxysmal episodes of SEC, while they shortened during and after each episode. This general pattern was most evident from May to September 2007 (cycle II) and from January to May 2008 (cycle IV), while in September December 2007 (cycle III) the climax of the lengthening was reached 1 month prior to the November 2007 SEC episode (when an increase in the tremor amplitude was concurrently observed). Another significant, but obvious, difference in the pattern was observed after the onset of the 13 May 2008 eruption, when the EPLU ENIC baseline experienced a marked shortening lasting for several months due to the deflation that accompanied the first months of the eruption. [17] We finally observe that the cyclic inflationdeflation trends shown by the summit GPS baselines also extended to GPS baselines located at mid altitude: as an example, we report in Figure 4 the case of the EMGL EMAL line, which is located on the western flank of Mt. Etna (Figure 1), the sector of the volcano most sensitive than the others to 7of13

8 volcanic cycles and deep intrusions (for sources located at about 0 3 km bsl [Mattia et al., 2007]). 5. Discussion [18] Mt. Etna volcano was characterized in 2007 to 2008 by several paroxysmal episodes at SEC, and by a more than 1 year long effusive eruption starting on 13 May 2008 at an eruptive fissure located near the summit area (Figure 1). By making use of our multidisciplinary observations, we propose here that such changes in volcanic activity state were paralleled (and often anticipated) by systematic (cyclic) trends in the monitored geophysical (seismicity, deformation) and geochemical (plume composition) parameters (Figure 2). Our measurements thus allow deriving new constraints on the poorly characterized geometry of Etna s shallow plumbing system, and on the trigger mechanisms of the periodic subterminal activity of SEC. [19] We notice that when the time trends of volcanic tremor, deformation, and plume chemistry are observed in concert (Figure 2), a significant contrast emerges between the intereruptive phases of quiescent degassing at the CCs and the SEC eruptive episodes. During quiescent periods, tremor RMS amplitude was typically low, and the source location of volcanic tremor was centered below the CCs at depths of km asl; while resumption of volcanic activity at SEC was in all cases associated with large increases of RMS amplitude, and upward and eastward migration of tremor source location toward the eruptive vent (Figures 2b and 2d). These changes in amplitude and source depth of volcanic tremor were also paralleled by systematic trends in the measured plume CO 2 /SO 2 ratios (Figure 2b) and GPS data (Figures 2c and 4): the former were typically high in the repose periods between SEC episodes (when tremor was low in amplitude, and deep), and typically decreased prior to and during resumption of volcanic activity at SEC; while GPS data consistently showed a mild inflation of the volcano s summit during the intereruptive periods, which typically terminated a few days/weeks before the eruptions, and were then followed by sharp deflations during eruptive episodes. This apparent correspondence between temporal variations of plume composition and geophysical signals suggests that some link must exist at Etna between the source mechanisms of degassing and (shallow) seismicity and deformation, as discussed below. [20] First, the source locations of volcanic tremor suggest that Etna s recent eruptive episodes in have been fed by magmas originally stored in the central conduit region (Figure 5). Such a relatively shallow (<100 MPa pressure [Collins et al., 2009]) preeruptive storage in the summit central conduits is indeed supported by petrographic, chemical and isotopic (Sr, Nd) features of erupted magmas (which are essentially the plagioclase bearing trachybasalts typically erupted at the summit vents [Corsaro and Miraglia, 2009]), and by their H 2 O poor volatile contents [Collins et al., 2009]. According to tremor location data (Figure 5), magmas are stored, in the repose periods in between SEC episodes, at depths of 1 to 2.8 km asl; and are thought to passively release gases via the CCs, thus becoming the source of our CO 2 rich gas emissions (Figure 5a). The accumulation of magma and gas bubbles in this storage zone is likely responsible for the gentle inflation of the volcano s summit captured by the GPS network. Notably, the seismically inferred magma depths (1 to 2.8 km asl) are consistent with the observed high plume CO 2 /SO 2 ratios, these being valuable indicators of the source depth of magmatic gases [Aiuppa et al., 2007, 2009]. Indeed, because of their contrasting solubilities in basaltic silicate melts, CO 2 and SO 2 experience a contrasting degassing path upon magma decompression: the former is exsolved from Etnean basaltic melts starting from as deep as km (CO 2 dominated fluid inclusions hosted in Mg rich olivines imply exsolution pressure 800 MPa [Kamenetsky and Clocchiatti, 1996]); while the latter is released to the gas phase starting from 4 6 km (e.g., for pressures lower than MPa [Spilliaert et al., 2006]). As such, a pressure dependent evolution of the magmatic gas CO 2 /SO 2 ratio can be predicted, from >200 at P > 200 MPa to 5 for close to surface gas melt separations [Aiuppa et al., 2007]. In this view, the high CO 2 /SO 2 ratios observed in between SEC eruptive episodes (Figure 2b) reflect the relatively deep degassing of an accumulated magma batch: using the model calculated pressure dependence of the magmatic gas CO 2 /SO 2 ratio at Etna [Aiuppa et al., 2007], we derive from measured gas compositions a range of equilibrium pressures of MPa, which for a bulk rock density of 2700 kg/m 3 would correspond to depths of m below the summit vents (or km elevation asl). These inferred source depths for the discharged gas phase should be viewed as depths of gas separation from (and thus last equilibration with) 8of13

9 Figure 5. Interpretative cross section of Mt. Etna, summarizing the key processes controlling the cyclic eruptive activity in (a) During quiescent periods in between SEC episodes, magma is stored in 1 3 km asl deep reservoir below the CCs (as imaged by location of seismic tremor sources; black circles). The reservoir is supplied by deep rising CO 2 rich gas bubbles [Collins et al., 2009; this study] and eventually more primitive melts [Corsaro and Miraglia, 2009]. Rising gas bubbles reequilibrate at reservoir conditions and are finally separated from the melt at m elevation asl, as indicated by the relatively high CO 2 /SO 2 ratios of quiescent gas emissions from CCs (from which gas melt separation pressures of MPa, or depths of m below the summit vents, can be calculated [Aiuppa et al., 2007]). On their farther ascent through the upper CCs conduits, coalescing gas bubbles are the source of the shallow Etna s LP and VLP seismicity, which is systematically recorded below Bocca Nuova Crater at km elevation asl. The steadily high CO 2 /SO 2 ratios during quiescent phases point to a persistent supply of gas bubbles, eventually leading to development of a gas bubble layer [Allard et al., 2005], and finally triggering pressurization of the reservoir (as captured by the mild but systematic inflation of the volcano s summit during quiescent periods). As pressurization of the reservoir proceeds, failure of the fractured upper SE Etna s flank [Neri and Acocella, 2006] finally leads to ascent of a magma plus gas mixture toward SEC, leading to (b) an eruptive episode. Resumption of volcanic activity at SEC is systematically anticipated by declining plume CO 2 /SO 2 ratios and by abrupt increases in tremor amplitude, upward (>2.8 km asl) and eastward migration of the source location of seismic tremor (red circles), and fast contraction of the volcano s summit. the silicate melt (Figure 5a); thus, gas bubbles must derive from at least km elevation asl, but probably deeper in the system: this is in agreement with the relatively deep source area of seismic tremor during intereruptive periods. In more detail, our derived km asl gas source area is fairly consistent with the upper limit of the source area of seismic tremor (Figure 5a; km asl); and also coincides with the main cluster of LP hypocenters (Figures 3 and 5a; km asl) below Bocca Nuova Crater. This apparent agreement may suggest that Etna s upper feeding conduit system, imaged by the 1 3 km asl deep cluster of seismic tremor locations (Figure 5) [Patanè et al., 2008], roughly terminates at m, with the gas bubbles separated over this depth range feeding surface gas emission; and triggering the shallower (>2500 m asl) very long period (VLP) and LP [Patanè et al., 2008; Cannata et al., 2009] seismicity. In fact, according to prevailing theory, LP events originate in the acoustic resonances of fluidfilled cracks triggered by pressure transients [Chouet, 1996], and VLP events are assumed to be linked to mass movements, and to represent inertial forces resulting from perturbations in flow of magma and gas through conduits [Uhira and Takeo, 1994]. In both models, the gas bubbles, rising through the magma column, coalescing and forming slugs, and collapsing, play a fundamental role in generating LP and VLP seismicity [e.g., Chouet et al., 2003; James et al., 2004]. [21] Second, our observations strongly suggest that at least during , magma was systematically transferred from the central upper plumbing system toward SEC prior to (in the days or hours) each paroxysmal event (Figure 5b). This was captured by our observations as a decrease of CO 2 /SO 2 9of13

10 ratios in Etna s CCs prior to and during each SEC eruptive episode. A decline of CO 2 /SO 2 ratios to 5 requires an upward migration (up to nearly atmospheric pressure) of the source area of volcanic gases, while an even lower ratio (to as low as 1.5, typical of the residual degassing phases) also implies a drastic decline of gas feeding to the CCs [Aiuppa et al., 2007]: both observations are consistent with a magma diversion from below the CCs toward the SEC rift zone, as captured by our measurements as an upward and eastward of seismic tremor. A geometrical connection between the central and SE craters conduits is consistent with recent infrasonic evidence of branched conduit geometry on Etna [Marchetti et al., 2009]. We thus conclude that the low CO 2 /SO 2 ratios, when combined with increasing tremor amplitude and upward and eastward migration of its source area, document the phase to preeruptive and syneruptive ascent of the magma batches to give rise to SEC eruptive episodes. Ground deformation (GPS) data are not suitable to infer the possible migration of magma toward the SEC, due to the geometry of the network relative to the possible sources below the SEC (too shallow and far with respect to the three GPS stations surrounding the summit craters). In spite of this, the recorded GPS signals indeed showed that the volcano s inflation terminated a few days/weeks before each eruption, possibly indicating that pressure increase in the system (see below) was coming to an end (consistent with the migration of magma/ gas toward the SEC). [22] Which are the trigger factors controlling the transition from passive degassing at the CCs to eruption at SEC, as recurrently occurred in ? Our GPS measurements (Figure 2c) clearly indicate that each eruptive episode was preceded by mild but systematic inflation of the volcano s summit. We then consider the possibility that each inflation cycle reflected the pressurization of the central crater magma storage zone, up to its failure leading to an eruption; and verify the hypothesis of whether or not an elongated vertical pressure source, bounded upward at about 2800 m asl and downward at about 1000 m asl, is consistent with the observed deformation pattern. Using the closed pipe model proposed by Bonaccorso and Davis [1999], and considering the tradeoffs between the pressure change and the horizontal radius of the ellipsoid, we find that measured elongations between GPS stations can be accounted for by such a pressurization source; provided a strength parameter of about Pa m 2 is used (where the strength is the product of the pressure change and the radius, DP*a 2 ), and the source is fixed just below the ECPN station, as suggested by the tremor localizations. In particular, an elongation of about 0.75 and 0.73 cm for the EPLU ENIC and EPDN ECPN baselines, respectively, is predicted from the model, which well fit the observations (for instance, the measured elongation during period II is about 0.7 cm for both baselines). We conclude then that the periodic pressurization of the central conduit storage zone was a viable source of deformation, and thus a most likely trigger for the recurrent eruptive episodes at SEC in [23] Petrologic data [Corsaro and Miraglia, 2009] indicate that the arrival of deeply rising more primitive magma batches (Figure 5a) possibly played a major role in causing pressurization of the central conduit storage zone prior to March 2007 and May 2008 (inflation cycles I and IV in Figure 2); while more evolved magmas were erupted in other SEC episodes (cycles II and III), arguing against any preeruptive magma replenishment in the reservoir. In these latter cases, we speculate that the supply of gas bubbles (not magma) to the central conduit storage zone, possibly sourced by the 3 5 km bsl deep Etna s magma reservoir [Spilliaert et al., 2006], was the main cause of pressurization, and thus eruption (Figure 5a). The water poor composition of olivine hosted melt inclusions from 2007 SEC eruptions, in particular, suggests that magmas have experienced extensive gas flushing (and thus dehydratation) prior to their eruption [Collins et al., 2009]; a process which can easily occur when resident magma stored in the upper central conduits is flushed by deeply derived gas bubbles, which CO 2 rich signature is well represented by our persistent passive plume emissions (Figure 5a). [24] The involvement of Etna s deep plumbing system (as source of primitive melts and/or gas bubbles) in generating the pressurization of the central crater storage zone, and thus in eruption trigger, is also supported by geodetic data. The pressurization of the deepest parts of the plumbing system are conventionally detected by considering GPS baselines located at mid altitude; that usually used for this aim is the EMGL EMAL line, which is located on the western flank of Mt. Etna, the sector of the volcano most sensitive than the others to volcanic cycles and deep intrusions (for sources located at about 0 3 km bsl [Mattia et al., 2007]). The elongation of this baseline (Figure 4) shows that this flank underwent a continuous inflation interrupted by drastic decreases during the eruptions at SEC. This pattern confirms (1) that the 10 of 13

11 deep volcanic plumbing system was continuously pressurized from 2007 to 2008 and (2) that the depressurization in the shallow plumbing system due to the eruptions rapidly propagated downward (i.e., the connection between the shallow and deep parts of the plumbing system is efficient). This evidence supports the idea that during the intereruptive periods the shallow part of the plumbing system underwent a continuous pressurization, which weakened the strength of the upper part of the conduit and/or the summit craters, inducing the explosions at SEC. 6. Conclusions [25] Our combined geophysical and geochemical measurements here provide first evidence for the systematic trends in volcanic gas plume chemistry, seismicity and deformation accompanying the cyclic activity of Etna in Four cycles of increase decrease of plume CO 2 /SO 2 ratios, paralleled by inflation of the volcano s summit (GPS), were observed during the volcano s quiescent periods in between the SEC paroxysmal activity; during these periods of quiescence, seismic tremor was typically characterized by low amplitude and relatively deeper (1 2.8 km asl) location. Resumption of volcanic activity at SEC was systematically anticipated by declining plume CO 2 /SO 2 ratios, and marked by abrupt increases in tremor amplitude, upward (>2.8 km asl) and eastward migration of the source location of seismic tremor, and fast contraction of the volcano s summit. [26] These observations are consistent with preeruptive magma storage in a km deep reservoir below the CCs. The episodic arrival of new magma batches [Corsaro and Miraglia, 2009], or the persistent supply of deeply derived gas bubbles [Shinohara et al., 2008; Collins et al., 2009], periodically triggered pressurization of this shallow reservoir (inflation), leading to further magma ascent and eruption at SEC. That paroxysmal SEC episodes mark the violent release of a bubble rich magma layer, with bubbles having accumulated in a relatively shallow reservoir, is consistent with the FTIR sensed composition of lava fountaining gas jets [Allard et al., 2005]. A novel aspect of our observations is however that the bubbly magma is not accumulated below the SEC, as previously thought [Allard et al., 2005]. Our observations indeed strongly support a geometrical connection between the central and SE craters conduits, an idea which is consistent with recent infrasonic evidence of branched conduit geometry on Etna [Marchetti et al., 2009]. We suggest that at least during , magma was systematically transferred from the central upper plumbing system toward the SEC prior to (in the days or hours) each paroxysmal event. This idea is also consistent with the SEC releasing a CO 2 poor (CO 2 /SO 2 ratios <1) during intereruptive periods, and contributing overall to a minor fraction (<3%) of the total volcano s gas budget [Aiuppa et al., 2008]. According to Marchetti et al. [2009], shifting of activity from the CCs to the SEC may occur in response to changes in flow dynamics in a branched system, whereby passive degassing below the central vents prevails at low magma gas flow rates, while the SEC activates only when some threshold in magmagas ascent rate is exceeded. While our measurements are fully consistent with this scenario (our decreasing CO 2 /SO 2 ratios and increasing tremor amplitude are clear evidences of the ascent of a degassing vesicular magma prior to each SEC episode), we speculate that a structural control on magma eastward migration might also have played a role. It is worth noting in this context that no eruptive activity was reported since 2001 at Etna s CCs, which were instead most active in the activity period [Allard et al., 2006]. This virtual cessation of activity of the CCs started in coincidence with the 2001 and lateral eruptions [Neri et al., 2005], which marked the onset of a phase of unusual acceleration of the seaward (eastward) motion of the unstable Etna s eastern flank [Bonforte et al., 2008]. Acceleration of the east flank slip movement is thought to have altered the stress field and deformation pattern on the volcano s summit, with the opening of N S to NW SE trending fracture field extending from the CCs toward SEC [Neri and Acocella, 2006]. We propose that as pressurization of the summit magma reservoir was periodically attained, this relatively shallow [Neri and Acocella, 2006] weakness zoon acted as a main pathway for the upward (and eastward) magma migration, confining eruptive activity to the SEC. [27] This report is among the first in which geochemical and geophysical observations are integrated and compared in a systematic way [see, e.g., McGonigle et al., 2009]. We conclude that multidisciplinary studies have more potential to shed light into the complex processes underlying the time changing nature of active volcanoes, and we suggest that such an integrated approach should be a priority of volcanological research in the years to come. 11 of 13

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