A multidisciplinary approach to detect active pathways for magma migration and eruption at Mt. Etna (Sicily, Italy) before the 2001 and 2002–2003 eruptions

Journal of Volcanology and Geothermal Research 136 (2004) 121 – 140 www.elsevier.com/locate/jvolgeores

A multidisciplinary approach to detect active pathways for magma migration and eruption at Mt. Etna (Sicily, Italy) before the 2001 and 2002–2003 eruptions S. Alparone a, D. Andronico a, S. Giammanco b,*, L. Lodato a b

a Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania-Piazza Roma 2, 95123 Catania, Italy Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Via Ugo La Malfa 153, 90146 Palermo, Italy

Abstract Two strong flank eruptions occurred in July – August 2001 and from late October 2002 to late January 2003 at Mt. Etna volcano. The two eruptions mainly involved the upper southern flank of the volcano, a particularly active area during the last 30 years, damaging several tourist facilities and threatening some villages. The composite eruptive activity on the upper southern flank of Mt. Etna during 2001 – 2003 has confirmed ‘‘a posteriori’’ the results of a multidisciplinary study, started well before its occurrence by combining geological, seismic and geochemical data gathered in this part of the volcano. We were able, in fact, to highlight fractured zones likely to be re-activated in the near future in this area, where the largest majority of eruptive fissures in the recent past opened along N120j to N180j ranging directions. The spatial distribution of earthquake epicentres during the period June 30th 2000 – June 30th 2001 showed the greatest frequency in a sector compatible with both the direction of the main fissures of the pre-2001 period and that of the 2001 and 2002 lateral eruptions. Soil CO2 and soil temperature surveys carried out in the studied area during the last 3 years have revealed anomalous release of magmatic fluids (mainly CO2 and water vapour) along some NNW – SSE-trending volcano-tectonic structures of the area even during inter-eruptive periods, indicating persistent convective hydrothermal systems at shallow depth connected with the main feeder conduits of Etna. The temporal changes in both seismic and geochemical data from June 30th, 2000 to June 30th, 2001 were compared with the evolution of volcanic activity. The comparison allowed to recognize at least two sequences of anomalous signals (August to December 2000 and April to June 2001), likely related to episodes of step-like magma ascent towards the surface, as indicated by the following eruptive episodes. The N120j to N180j structural directions are in accord with one of the main structural lines affecting eastern Sicily; they would be important pathways for magma uprise to the surface that will keep on feeding the eruptive activity of Etna in the near future. This study also pointed out the high instability of the southern slope of Etna, a sector where the potential hazard by lava flow invasion will remain high also in the near future. D 2004 Elsevier B.V. All rights reserved. Keywords: Mt. Etna; fault detection; soil gas; historical eruptive fissures; eruptive activity; local seismicity

* Corresponding author. Fax: +39-91-6809449. E-mail address: [email protected] (S. Giammanco). 0377-0273/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2004.05.014

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1. Introduction The 17 July –9 August 2001 and the 27 October 2002– 28 January 2003 flank eruptions of Mt. Etna damaged several tourist facilities and threatened some villages both on the southern and northern flanks of the volcano, raising once again the problem of forecasting both the moment of eruption onset and the location of an eruption site. Mt. Etna, although not characterised by highly explosive eruptions, has a very high frequency of eruptive episodes and its flank lava effusions, due to the considerable urbanization of its slopes, can easily reach inhabited areas. This makes volcanic hazard on Mt. Etna high, especially as regards the loss of property. The 2001 flank eruption, however, can probably be addressed as the first one for which location, eruptive mechanisms and possible moment of onset could have been identified some months before its occurrence. This paper describes data of a multidisciplinary study begun several months before the 2001 eruption and whose main results have been well confirmed by the occurrence of the eruption itself. We started by mapping the spatial distribution of ground fractures, both visible at the surface and buried by younger volcanic deposits, on a wide area of the upper southeastern flank of Mt. Etna. For this purpose, we used data from surface surveys, local seismicity, soil and fumarole CO2 emissions and soil and fumarole temperature. The aim of our study was first to highlight fractured zones likely to be re-activated in the near future, as they could be important pathways for magma uprise to the surface. We also analysed the temporal evolution both of the seismic and of the geochemical signals, in order to develop a model to describe magma migration before its eruption at the surface. In general, the study of the distribution of eruptive and dry fissures on active volcanoes, such as Mt. Etna, can be very useful in identifying zones of particular weakness in the volcanic edifice that are, therefore, more subject to opening due to magma intrusion at shallow depth. Recognition of such zones is of the utmost importance especially where they are close to human settlements. However, surface identification of volcano-tectonic structures on volcanoes characterised by persistent activity is not easy, because they can be buried under newer volcanic products (lava flows,

pyroclastic layers). Seismic data can be very useful in this respect, as they can help to detect active faulting and give information on the location of fault planes (Azzaro, 1999). A further tool may be derived from the geochemistry of soil gases. It is known that gas emissions may help to detect and monitor active faults in seismogenic and/or volcanic areas (e.g., Sugisaki et al., 1983; Klusman, 1993). Active tectonic structures are often high permeability zones that may act as preferential pathways for the rising of gases from the deep crust or from the mantle. If tectonic strain is almost continuously present, repeated fracturing of rocks along the fault planes prevents self-sealing caused by deposition of silica during steam loss. Carbon dioxide is the most popular gas tracer for this kind of investigation. It is the major constituent of the non-condensable component of magmatic gases and its release to the surface occurs also during periods of volcanic rest. The largest release of magmatic gas at Mt. Etna occurs through its summit vents (e.g., Allard et al., 1991; Caltabiano et al., 1994). However, minor but significant degassing occurs also through the flanks of the volcanic edifice (Allard et al., 1991; D’Alessandro et al., 1997). Flank emissions of CO2 at low altitude on Mt. Etna occur mainly in areas crossed by seismically active faults (Azzaro et al., 1998; Giammanco et al., 1997; 1998b). Anomalous soil degassing on Mt. Etna can also be attributed to buried tectonic structures (Azzaro et al., 1998; Giammanco et al., 1997, 1998b, 1999) as well as old eruptive fissures, only if associated with active tectonic stress (Giammanco et al., 1998b). In order to recognize the timing and mechanisms of magma emplacement at shallow depth in the recent activity of Mt. Etna, we examined the temporal trends of fracture developments in the upper southern flank of the volcano through visual observations, continuous monitoring of local seismicity and periodic monitoring of soil gas emissions during the period July 2000 – July 2001. Periodic field excursions at the summit area of a persistently active volcano allow to: (i) directly check variations of the morphology (e.g., due to emission of new volcanic products, collapse of crater rims, etc.); (ii) monitor the intensity of volcanic activity; (iii) relate them to variations of other geophysical and geochemical parameters. Seismic monitoring represents a powerful tool to forecast the potential resumption of volcanic activity,

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too. In fact, data on local seismicity can signal and trace the uprise of magma through the study of the spatial and temporal distribution of hypocentres (possible migrations associated with magma dynamics), kinematics, significant variations in the amplitude and frequency of the signal of the seismic tremor, presence of other phases associated with magmatic sources (long-period events). Lastly, soil CO2 efflux and ground temperature in steamy areas connected with deep faults can increase in time due to intrusion of fresh magma into shallow layers of the volcano.

2. Morpho-tectonic features of Mt. Etna and relationships with the eruptive activity 2.1. Regional geodynamic framework Two geodynamic domains affect eastern Sicily (Fig. 1): the first (in the northern sector) is represented by the Apennine –Maghrebian thrust belt; the second affects the southern sector and is represented by the Iblean Foreland, which is the northern part of the Pelagian block (Lo Giudice et al., 1982). Mt. Etna is located at the boundary between these two domains and is crossed by faults belonging to three active regional systems (Fig. 1): (1) the Malta Escarpment (ME), whose NNW– SSE inland prosecution is represented by the Timpe Fault System (TFS) on the eastern flank of the volcanic edifice, which is the surface expression of important crustal structures that cut Etna’s basement (Atzori et al., 1978; Ghisetti, 1979); (2) the NE – SW trending Giardini –Messina System (GMS) (Ghisetti and Vezzani, 1980; Lo Giudice et al., 1982); (3) the WSW –ESE Mt. Kumeta – Alcantara System (MKAS), that affects Etna’s northern flank. The tectonic settings of Etna’s flanks, due to the interaction between the regional tectonics and volcano-related processes, today produce a local stress field characterised by E –W tensile stresses superimposed on the compressive one acting on a N – S direction (Bousquet et al., 1988). Gravitational spreading due to the weight of the volcanic pile together with the regional stress field produces extensional structures in the summit area of Etna and compressive ones in the basement (Kieffer, 1983; Borgia et al., 1992).

Fig. 1. Structural sketch map of eastern Sicily, including the Mt. Etna area, showing the main geodynamic domains and tectonic features. 1 = main faults (ME = Malta Escarpment; TFS = Timpe Fault System; MKAS = Mt. Kumeta – Alcantara System; TLS = Tindari – Letojanni System; GMS = Giardini – Messina System), 2 = Southern margin of Apennine Maghrebian Chain.

2.2. Styles of volcanic activity and related tectonic structures in the Etna area Mount Etna has currently four active craters at its summit. Two of them are terminal: the Voragine (VOR, until 1968 named the Central Crater) and the Bocca Nuova (BN, formed in 1968 aside of the VOR), whereas the other two, located on the flanks of the summit cone, are named subterminal: the North – East Crater (NEC, formed in 1911), which is the highest point of the volcano (3300 m a.s.l.), and the South –East Crater (SEC, born in 1971), actually reaching slightly less than 3300 m a.s.l. Persistent activity in these craters produces changes in the morphology of the crateric area, usually burying older eruptive structures.

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During its activity (starting about 500,000 years ago), Mt. Etna has produced a large number of eruptions both from its summit craters and from its flanks. The prevailing character of eruptions is effusive; however, when lateral eruptions occur, pyroclastic cones may form, often aligned along feeding fissures. More than 250 lateral cones are today visible on the flanks of Etna (Villari et al., 1988), down to an altitude of about 100 m a.s.l. The distribution of such cones is not random or subject to a purely radial symmetry, but linked to the orientation of the regional tectonic structures that affect the Etna area (Chester et al., 1985; Rasa` et al., 1995). The volcano actually displays three different zones of structural weakness through which magma can find an easier path to the surface: the North –East Rift (NER), the South –East Rift (SER), and the West Rift (WR) (Fig. 1). They appear as elongated areas of eruptive fissures and vents that diverge from the summit craters (Kieffer, 1975). According to Lo Giudice et al. (1982), these structural trends may have formed following the main tectonic stress regime acting in the region, as they parallel the main regional structural systems above described. Most of the largest eruptions of Mt. Etna during the last 20 years took place on its south flank. Moreover, historical flank eruptions that occurred in this part of the volcano tended to have a longer duration than those in other flanks. Since 1500, 11 out of 16 flank eruptions with duration longer than 50 days, or 68.7% of the total, occurred on the south flank: 1610, 1763, 1766, 1792, 1811, 1819, 1983, 1985, 1991 –1993, 1999, 2002 –2003 (Chester et al., 1985; Azzaro and Neri, 1992; this work). The area studied in the present work includes a large part of the upper SER. In this area, SER is made of extensional structures oriented primarily NW –SE and secondarily N – S and E – W, often with right lateral components of motion. The most recent tectonic dislocations are located close to the western edge of the Valle del Bove (VDB) depression, which occupies most of the eastern flank of Etna. Along the western portion of the VDB, direct faults are present with directions in the range N130j – N170j and eastward down throws as high as slightly more than 1 m. The relative horizontal motion of the faulted blocks towards the east can reach several metres (Mc Guire et al., 1990). Some of the most recent volcano-tectonic structures, as well as those

located just outside the VDB, have secondary dislocations with en-echelon geometry and right-lateral components. Old volcanic dykes outcropping on the western and southern walls of the VDB are mainly directed NW – SE (about 50% of total) and secondarily N – S (about 19.7% of total), with a higher density along the direction N145j, which coincides with the preferential direction of the more recent fissures present on the same walls of the VDB (Ferrari et al., 1991). All eruptive fissures that affect the outer part of the VDB in the southern flank of Etna start from the summit area of the volcano and develop towards the south down to an altitude of 1000 m, whereas at lower altitude they divert towards south –east.

3. Volcanic activity in the last three centuries 3.1. Eruptive fissures formed before the 1991– 1993 eruption During the last three centuries, many eruptions took place on the upper south flank of the volcano (i.e., above 2500 m a.s.l.). Table 1 lists the eruptions since 1900, together with the main direction of eruptive fissures. The main eruptive events of the last three centuries occurred: in 1792, with formation of the ‘‘Cisternazza’’ pit-crater (Tanguy, 1981); in 1819 with formation of the homonymous crater on the western edge of the VDB (Gemellaro, 1819); in 1910 with opening of fissures that spread with a fan-shaped geometry in the western part of the investigated sector (Ricco`, 1910). Moreover, in our opinion, another eruption on the south flank which is important, although not energetically significant, is that of 1787. That eruption actually led to the formation of the ‘‘Vulcarolo’’ fumarole (Gioeni, 1787), which has remained active until 2000, when it was covered by lava flows emitted from the SEC. Other eruptions of variable intensity that occurred in the study area, but whose fissures developed towards the east in the VDB were: 1956, actually a minor event with a small and short-lived eruptive fissure that opened on the western edge of the VDB at an altitude of about 2750 m a.s.l. (Cucuzza Silvestri, 1957), 1950 –1951 (Cumin, 1954), 1964 (Cucuzza

S. Alparone et al. / Journal of Volcanology and Geothermal Research 136 (2004) 121–140 Table 1 List of the eruptions and the main directions of the relevant eruptive fissures (sorted by classes of orientation) occurring in the south – east sector of the volcano since 1900 Year

1908 1910 1918 1928 1942 1949 1951 1956 1964 1971 1978 1979 1983 1985 1986 1986 – 1987 1989 1991 2001 2002

Trend of fractures N–S

NNE – SSW

 





 

ENE – WSW

E–W

ESE – WNW

SSE – NNW





     

   

 

 

   



 

 

Silvestri, 1965; Sturiale, 1968), and 1968 (Cucuzza Silvestri, 1969). The April – June 1971 eruption marks an important moment in the recent evolution of Etna, because one of the eruptive vents later evolved into what is today known as SEC. The 1971 eruption started with the opening of several vents on the south and southeast foot of the terminal cone at altitudes between 2870 and 3080 m a.s.l., along fissures directed both N – S (near Mt. Frumento Supino) and NW – SE (Fig. 2). Some weeks later, new eruptive fissures developed from the April vents towards ENE, crossing the VDB. Then they extended 2 km outside of VDB and turned to the east giving way to conspicuous lava effusions from their easternmost end (Fig. 2). In May 1971, the SEC appeared for the first time as a small depression located at the SE base of the summit cone (altitude of about 3000 m a.s.l.) (Rittmann et al., 1971; Calvari et al., 1994). Near the end of the 1971 eruption, the SEC was the site of unusual phreato-magmatic activity, with violent but silent explosions triggered by vaporisation of snow trapped within the layers of volcanic

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ash around the crater. In the following years, activity at SEC was characterised by strombolian activity, alternated with more violent paroxysms (up to lava fountains), and emission of lava flows. The birth and development of the SEC had a great importance in the morpho-structural evolution of the studied area, where the eruptive fissures that opened during the last 20 years had directions coinciding with the most evident above-mentioned structural trends (NNW – SSE, NW – SE, NE – SW, ENE – WSW). Since 1978, all major eruptive fissures that opened in this sector (Fig. 2) started from the SEC: 1979, 1983, 1985, 1986 – 1987, 1989, 1991 – 1993 (Azzaro and Neri, 1992). In particular, the analysis of the development of the 1989 eruptive fissures gave interesting information on the eruptive mechanisms in the studied area. During the 1989 eruption, a conspicuous amount of lava (about 26  106 m3 of lava, according to Armienti et al., 1989; about 28  106 m3 of lava, according to Bertagnini et al., 1990) was emitted from a fissure system with a N50j direction. Almost at the same time, another fissure system opened with orientation N150j and, although being about 7 km long, it produced eruptive activity only in its uppermost part (Bertagnini et al., 1990). The N150j system formed on previous dislocations with right-lateral components and the two systems together had an inner angle whose bisector corresponded to the direction of maximum tensile strength of the whole eastern sector of Etna (Kieffer, 1983). Considering the components of horizontal motion in the two fracture systems formed in 1989 (left-lateral in the N50j system and right-lateral in the N150j system), the VDB falls in a sector that is subject to extension towards ESE. Therefore, this direction would be that of the tensor of minimum strain, whereas the tensor of maximum strain would be oriented N – S (Bousquet et al., 1988). 3.2. Eruptive patterns during 1993 –2000 After the large 1991 –1993 flank eruption, discontinuous strombolian activity resumed at BN and NEC in 1995, at SEC in 1996 and lastly at VOR in 1997. This kind of activity continued until September 1998, although interrupted by several paroxysmal events generally consisting of violent lava fountaining and tephra emission. In 1995 and 1996, 10 of such

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Fig. 2. Sketch map of the upper southern flank of Mt. Etna showing the main known eruptive fissures since 1900. The July – August 2001 fissures system is shown with dashed lines. PL = Piano del Lago.

paroxysmal events took place at the NEC. Further paroxysmal episodes occurred on November 25, 1997 at the BN and on March 27, 1998 again at the NEC (La Volpe et al., 1999; La Delfa et al., 2001). On July 22, 1998, one of the most energetic episodes of explosive activity in the last century occurred at the VOR: tephra were ejected to a maximum height of 10– 11 km and fell over a large part of eastern Sicily (Andronico et al., 1999; La Delfa et al., 2001). After

this episode, strombolian activity remained intense till the end of August 1998, and was marked by another event of fire-fountaining on August 6. Since September 15, 1998, the most intense volcanic activity involved the SEC, which started a long sequence of paroxysmal events (21 in total). During the last of them, on February 4, 1999, an eruptive fissure broke the southeast slope of the SEC cone. The fissure trended N160j and fed a subterminal effusive

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eruption that ended on November 1999. Two more fissures formed on August 27th and on September 4th, the latter following violent lava fountaining at VOR, and minor explosive activity at BN and SEC. Like the first one, they started from the summit of the SEC, but were located slightly further north of it and trended, respectively, N140j and N120j. Therefore, the whole system of the 1999 eruptive fissures drained out magma along directions between N160j and N140j. During the last period of this eruption, a violent effusive and explosive terminal eruption occurred at the BN (October 17 –November 4), preceded by frequent and violent strombolian activity during the months of September – October 1999. Eruptive activity resumed at the SEC on January 26, 2000, producing 64 lava fountain episodes until June 24th. Each paroxysmal episode was usually accompanied by emission of very fluid lava flows, and was interspersed with periods of total or partial rest (Quattrocchi et al., 2001; Alparone et al., 2003). This activity produced two systems of eruptive fissures, respectively, located on the northern and the southern flank of the cone, that were oriented, respectively, N40j and N140j. The two systems were continuously buried by coeval explosive products and re-opened during the following episode.

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of strombolian activity just a few days before the end of the month. On June 7, 2001 a new short cycle of strong paroxysmal events started. Each episode of this cycle was more intense than the previous one and all of them were characterised by opening of a fissure on the mid-upper northern flank of the SEC, with strong lava spattering up to lowintensity lava fountains and emission of well-fed lava flows. Simultaneously, strong strombolian activity, often evolving to short lava fountaining, took place at the top of the cone. This cycle ended on July 17th after 15 eruptive episodes, followed right after by the complex sub-terminal and lateral July – August 2001 eruption. During the pre-2001 eruption period, we also observed a progressive increase in strombolian activity at the BN, mainly during September – October 2000 and more evidently in February –March 2001, when bursts of lava increased both in frequency and intensity. Strombolian activity then deepened at BN in April. At the end of February 2001, weak explosive activity resumed at NEC, too. Conversely, the VOR showed only quiet degassing activity throughout the whole period. Table 2 shows a summary of volcanic activity of the period August 2000– July 2001. 3.4. The July – August 2001 eruption

3.3. The activity between July 2000 and July 2001 Two more paroxysmal episodes occurred at SEC on August 28 and 29, 2000, with similar characteristics to those previously occurring during the same year. At the end of November, the fissure cutting the northern slope of SEC re-activated after about 10 days of weak summit strombolian activity at BN. For about 2 weeks, a slow and viscous lava flow was emitted producing a small lava flow field. Eruptive activity started again at SEC on January 20, 2001, once more along its northern fissure. Lava effusion lasted until July 2001 and was relatively larger than during the previous episode (output rate between 1 and 10 m3/s). At the beginning of May, effusive activity stopped for about 2 days; on the morning of May 9, a new phase of strombolian activity started at the summit of SEC, followed by lava fountaining mainly along the northern fissure. The last weeks of May were characterised by discontinuous effusive activity and a new resumption

On July 12 at 23:44 GMT, a seismic swarm on the southern slope of Mt. Etna marked the beginning of a new eruption. From the 13th to the 19th of July, a 6km-long system of dry fractures formed in the eastern part of the Piano del Lago as well as on the southern wall of the VDB, with main direction N – S (Fig. 2). The following evolution of this fracture system was the formation of a graben-like structure that developed between 3100 m a.s.l. on the southern slope of the SEC and 2100 m a.s.l at the southern base of the Montagnola. On July 17, an eruptive fracture, directed N160j, opened on the southern side of the SEC at about 3000 m a.s.l. Lava flows outpoured from this vent since the very beginning of the eruption. This fracture developed further towards the south down to 2700 m of altitude, forming another graben-like structure bounded by fractures directed N40j with en-echelon features and a series of steamy fumaroles inside. On July 18th, an eruptive fissure opened just south of the Montagnola (Fig. 2). Activity at this

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Table 2 Summary of the volcanic activity of Mt. Etna from August 2000 to August 2001 Date

Vent NEC

August 28, 2000 August 29, 2000 September – October 2000 November 20 – December 4, 2000 January 20 – May 9, 2001

VOR

BN

SEC Paroxysmal episode Paroxysmal episode

Strombolian activity Discontinuous mild strombolian activity Discontinuous mild strombolian activity

Weak strombolian activity

May 9, 2001 May 9 – June 7, 2001

Mild effusive activity Paroxysmal episode Mild strombolian and effusive activity 15 paroxysmal episodes

June 7 – July 17, 2001 July 17 – August 9, 2001 October 27, 2002 – January 28, 2003

Mild effusive activity

2001 flank and subterminal eruption 2002 flank eruption

fissure was only explosive during the first hours, followed soon by the emission of a lava flow towards the south. On July 24th, the lava flow had reached 1040 m a.s.l., that is only about 4 km distant from the village of Nicolosi. On July 19th at 17:00 GMT, about 40 h after the opening of the eruptive fissure at 2100 m a.s.l, two craters opened just north of the Montagnola, at an altitude of 2575 m. Finally, on the night of July 20, a new eruptive fracture opened in the Valle del Leone between 2600 and 2545 m a.s.l, feeding a 1-km-long lava flow (Fig. 2). The eruption ended on August 9, 2001. 3.5. The 2002 –2003 eruption After the 2001 eruption, Etna’s activity consisted of periodic ash emission and weak to strong strombolian activity at NEC and BN. Between the 26th and the 27th of October 2002, a new flank eruption started both along the NER (where the eruptive activity stopped on November 5th) and on the upper southern flank of the volcano. Here the eruption was more intense and lasted for three months. It formed two large cones at about 2750– 2800 m a.s.l. and a highly composed lava flow field. A 2 –3-km-high sustained eruptive column produced an abundant ash and lapilli fallout for most of the first 2 months of eruption, then the explosive and effusive activity gradually decreased and on January 28th, 2003 the eruption ended.

4. Local seismicity 4.1. General patterns Seismicity related to the volcanic activity on Mt. Etna generally occurs in swarms. Many of them appear to be linked to the NNW – SSE structural trend (Bonaccorso et al., 1996; Patane` and Privitera, 2001). Activation of this tectonic line has been interpreted as a possible response to the intrusion of magma at shallow depth (between 4 km b.s.l. to 2 km a.s.l.; Alparone et al., 1994), as in the case of seismicity that occurred before and during the onset of the 1991– 1993 eruption (Ferrucci and Patane`, 1993; Alparone et al., 1994; Bonaccorso et al., 1996; Bonaccorso and Gambino, 1997). On that occasion, the geometry of the epicentral area and the focal mechanisms of dipslip were coherent with a tensile motion along the fault plane (Patane` et al., 1994). Similar evidence was already observed by Ferrucci et al. (1993) in the seismic activity connected to the 1989 eruption. Also in that case, it was shown that the NNW – SSE system played an important role in driving magma intrusion. The seismic sequence that preceded the 1989 eruption consisted of about 600 earthquakes (0.8 < M < 3.0), with focal depth in the range 0 –2 km below the sea level. For these events, Ferrucci et al. (1993), on the basis of the calculated focal solutions, found nonhomogeneous kinematics, including extensional, strike-slip and purely compressive motions. Extension

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mostly characterized the first phases of 1989 seismicity, whereas compression occurred during the final stages. Both tensile and compressive deformations resulted also from ground fracturation that was produced aseismically for a length of about 6 km, although without evidence of the presence of magma in the shallowest portions of the volcanic edifice (Frazzetta and Lanzafame, 1990). 4.2. Seismic activity on the upper SER after July 2000 Seismicity along SER during the period June 30th 2000 –June 30th 2001 revealed interesting peculiarities in the release of seismic energy, in the spatial distribution of epicenters and in their relevant kinematics, as described in this paragraph. Seismic activity of Mt. Etna was monitored by the permanent local seismic network of the Sistema Poseidon (now Catania Section of the Istituto Nazionale di Geofisica e Vulcanologia—INGV). This array is composed of 39 seismic stations, 35 of which equipped with vertical seismometers and four with three-component seismometers. The seismic signals are transmitted in real time, via telephone lines and/or radio, to the data acquisition center in Catania, where they are continuously recorded in digital (after A/D conversion, with sampling rate set at 160 Hz) and directly acquired on paper. In total, from June 30, 2000 to June 30, 2001, about 300 earthquakes were recorded with 1.0 V M V 3.6, 7% of which with M z 2.5. Fig. 3

Fig. 3. Daily frequency of earthquakes with M z 1.5 occurring in the whole area of the upper southern flank of Etna, and associated cumulative strain release, during the period June 30th 2000 to June 30th 2001. The dashed lines with roman numbers highlight significant changes in the gradient of cumulative strain release (see text).

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shows the daily frequency of earthquakes occurrence and the cumulative curve of seismic strain release during the considered period. The temporal distribution of earthquakes highlights time intervals marked by different frequencies of occurrence in the seismicity. The seismic swarm in the first days of July marked the end of a period characterised by poor to nil activity that started in early 2000. After the July events, seismicity was characterised by a low, almost constant background activity, with the exception of the modest activity in the first half of November. In February 2001, a strong increase in the number of earthquakes was recorded together with a significant increase in the gradient of strain release (trend I in Fig. 3). A further increase in the gradient of strain release was observed in the first half of April 2001 (trend II in Fig. 3). It is noteworthy that the strain release recorded during the period February – May 2001 represents about 60% of the total release during the whole period considered. Earthquakes location was performed with the standard routine HYPOELLIPSE (Lahr, 1991) and the one-dimensional crustal velocity model with seven layers given by Hirn et al. (1991), with some minor modifications (mainly, the extrapolation of the model down to a depth of 30 km; Fig. 4c). Although the epicentral distribution of the best located earthquakes (RMS-root mean square error of residual travel time < 0.25 s; ERZ-vertical error < 2 km; ERH-horizontal error < 1.5 km; Gap-maximum azimuthal gap of seismic network < 180j) during the considered period involves almost the entire area under study, some sectors showed a higher frequency of events (Fig. 4a). The most evident ones are located: in the summit zone, very close to the craters area (A in Fig. 4a); in the VDB (B in Fig. 4a); in a sector just SW of Mt. Frumento Supino (C in Fig. 4a) and on an elongated zone that starts from the SEC and develops towards SSE (D in Fig. 4a). In general, the distribution of seismic events at depth allows to define focal volumes, well constrained in space, that involve relatively shallow portions of the crust: almost all of the hypocenters were actually shallower than 9 km (Fig. 4b), with those falling east of the central axis of the volcano (sectors A, B and D, Fig. 4a) having a tendency to be the shallowest (1 V H V 5 km). Furthermore, hypocentral depths in sector D were found to be shallower than in the other sectors and almost con-

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Fig. 4. (a) Epicentral location of earthquakes recorded in the study area (bounded by dashed lines) during the period July 1st 2000 to June 30th 2001. Shaded areas highlight the zones with greater clustering of epicenters (A = summit; B = Valle del Bove; C = Mt. Frumento Supino; D = Piano del Lago). The figure also shows some representative focal mechanisms of the analysed sample; (b) projection of the hypocenters of the same earthquakes along a E – W cross-section of the study area (all depths are relative to sea level); (c) velocity model used for the interpretation of seismic data at Mt. Etna (modified from Hirn et al., 1991).

stantly in the range 2 –5 km. Since mid-August 2000, sector D was characterized by a high rate of earthquakes with low to moderate magnitude (1.0 V M V 2.9). About 100 earthquakes were recorded until mid-April 2001, but no significant swarms have been recognized. Actually, these events were temporally distributed in a very irregular manner: a first increase in the daily number of quakes in this sector occurred during October – December 2000. In particular, the greatest strain release was recorded in November 2000. In February 2001, a new and more evident increase in seismicity was recorded, which

remained at high level until mid-April 2001 (in about 2 months, the strain release was 74% of that of the whole period considered). The temporal sequence of the events showed an epicentral migration towards the summit area, from SSE to NNW (Fig. 5). Analysis of the kinematics associated with the above-described seismic activity, carried out through calculation of the focal mechanisms of the earthquakes with M z 2.0 that occurred along sector D, showed a complex behavior which agrees well with literature data (Ferrucci and Patane`, 1993; Ferrucci et al., 1993; Alparone et al., 1994). In fact, although trans-tensive type

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the southern wall of VDB. Hypocenters were located in the depth range between the surface and about 5 km below the sea level, with higher concentration between the surface and a depth of 3 km below the sea level.

5. Soil gas geochemistry 5.1. Spatial distribution of soil gas anomalies

Fig. 5. Temporal variation in (a) latitude and (b) longitude values of the epicenters of the earthquakes belonging to cluster D recorded during the period January 1st to June 30, 2001. The plot highlights the progressive shift of epicentral locations from SSE to NNW towards the summit area of Mt. Etna.

motions were predominant, dip-slip ones were also observed (Fig. 4a). Besides, no preferential orientation of the azimuthal distribution of P axes, inferred from all available focal mechanisms, could be recognized. This feature could be related to magma intrusion within the volcano’s feeding system, which produced a local and inhomogeneous stress field. The remarkable seismic activity that preceded and accompanied the opening of the July – August 2001 fractures systems on the southern flank of Etna (Fig. 2) consisted of huge seismic swarms. The large number of earthquakes and the energy released allow to define this activity as the most important in the last 20 years. Seismicity started on the night of July 12th and continued with very high intensity until July 17th. During the first 9 h after the onset of the seismic swarm about 800 shocks were recorded, while through the whole period about 2600 earthquakes with M z 1 (Mmax = 3.9) occurred. More than 13% of the events had a magnitude of 2.5 or higher. The epicentral distribution of the located shocks fell mainly in the area affected by the eruptive fissures and in

In our study area, visual evidence of diffuse gas leakage can be seen in the many fumaroles around the crater rim of the four summit cones, as well as in some steam emissions at their foot. Crater fumaroles usually change in size and location mainly in relation to the morphological changes of the summit craters due to the volcanic activity. Steaming areas, instead, tend to be more stable in their location. One of those areas, in particular (‘‘Vulcarolo’’, located at about 3000 m a.s.l. on the south flank), has been well known since the 1787 eruption (Fig. 6). The steam flux from the Vulcarolo was so high that in the early 20th century, a condenser was placed on its site to provide water (about 300 l day 1 of condensed steam, according to Ponte, 1927) to the nearby old volcanological observatory. The site of Vulcarolo has been covered by lavas and tephra erupted by the SEC since 1989. Another steaming area was visible a few hundred meters east of the Vulcarolo. It probably formed in 1978 when the SEC began its almost continuous activity and it has been described by Aubert et al. (1984) as a well-defined convective zone, probably related to one of the main feeding conduits of Etna. Other signs of anomalous soil degassing associated with weak soil temperature anomalies, although without steam emissions, were recognized near the Torre del Filosofo hut (henceforth indicated with TDF, now completely buried by the explosive products of the 2002– 2003 eruption) at an altitude of 2919 m a.s.l. (Fig. 6), where during winter, large areas of ground without snow could be seen just south and southeast of the building. As reported by local alpine guides, this phenomenon has been known since the building of the hut (1966 – 1970). Besides, the area around TDF has been studied by Aubert and Baubron (1988), Giammanco et al. (1998a) and Aubert (1999) as

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Fig. 6. Location map of the fumarolic areas and the sampling sites of soil gas (open circles with diamonds) in the upper southern flank of Mt. Etna. Shaded areas indicate the approximate extension of steamy ground near Torre del Filosofo (TDF) and Belvedere (see text for details). The Vulcarolo steam emission is also indicated, as well as the site of anomalous soil degassing along the 1949 eruptive fissure (P7). Segments with capital letters indicate sampling lines of soil CO2. Dashed lines across sampling profiles highlight the directions recognized from soil CO2 anomalies. SEC = South – East Crater; VDB = Valle del Bove. Altitudes in metres.

regards the geochemical characterisation of the emitted fluids and the mechanisms of gas and heat transfer through the shallow ground. All of these authors recognized the magmatic origin of CO2 emitted at this site. During the 1991 – 1993 eruption, another area of steam emissions formed at an altitude of about 2720 m a.s.l., where the 1991– 1993 eruptive fissure crossed the edge of the VDB depression (Belvedere area, Figs. 2 and 6). After the end of that eruption, preliminary investigations on such emissions (Baubron, 1996) explained their occurrence as due to residual degassing of the magma dyke that fed the 1991– 1993 lava flow. However, analyses periodically carried out since mid-2000 on the emitted gases at this site indicated their origin from degassing of fresh magma (Giammanco and Pecoraino, 2002). Another area of anomalous soil gas emission is located along the prolongation toward SSE of the 1949 eruptive fissure (site P7 in Fig. 6). Also in this case, both the chemical and the He and C(CO2) isotope compositions

of emitted gases clearly indicated their magmatic origin (Baubron, 1996; Giammanco et al., 1998a). Despite the soil gas studies already carried out on the area investigated in this work, no large-scale spatial correlation between anomalous degassing areas and tectonics has ever been attempted before. In order to reveal new zones of anomalous soil degassing, better define the surface extension of ground degassing and hence verify the association between ground degassing and known or hidden tectonic structures, several surveys of soil CO2 concentrations were carried out in the upper southeast flank of Etna on October 26 and 30, 1998. Surveys were performed from the foot of the SEC down to an altitude of about 2600 m a.s.l. (Fig. 6). Soil CO2 concentration values along the sampling lines were measured by inserting a teflon probe in the soil to a depth of 50 cm and taking a gas sample with a gas-tight syringe. Each sample was then analysed using a portable IR spectrophotometer with fixed wavelength (Mod. LFG20, Analytical Development, Co.,UK). The instrumental accuracy was within F 3% (2r). Such error was obtained from repeated measurements ( z 3) in each sampling site, and did not affect the results appreciably. Before each measurement, the instrument was set to zero in air, so that CO 2 concentrations are expressed as ppmv (STP) in excess of air concentration. This method limited the number of sampling points, because no measurements could be performed on unaltered, blocky lava flows, and this type of flow covered most of the surface of the studied area at the time when measurements were carried out. Fig. 7 shows the results of the soil CO2 measurements (sampling step of about 20 m) along the sampling lines shown in Fig. 6. Sampling transects were located at various altitudes and were kept as perpendicular as possible to the axis of the known steamy zones. The length of each transect ranged from 60 m (line D) to 240 m (line G). Each single transect was completed in less than 3 h, and during this time no substantial variation in the volcanic activity and/or atmospheric conditions occurred, as confirmed by the stable barometric pressure measured during the soil gas sampling. Sampling lines A, B and C were surveyed in an area of visible steam emissions associated with a soil thermal anomaly (soil temperature about 60 jC at 50-cm depth) located between the site of the old Vulcarolo and the fumarolic area studied by

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Fig. 7. Values of CO2 concentration in the soil measured along the sampling lines on the upper southern flank of Mt. Etna (letters refer to those indicated in Fig. 6). Values are expressed in ppm by volume (CO2 STP) above the atmospheric CO2 content (340 ppm vol), which is taken as zero. The vertical lines show the inferred axis of hidden active faults. The figure does not report sampling lines F and G because all soil CO2 concentrations along them were found equal to the atmospheric content.

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Aubert et al. (1984). Along these transects, soil CO2 concentration reached maximum values between 20,000 and 25,000 ppmv. These values are anomalous in that they exceed by far the average CO2 content in air (340 ppm vol). The maxima observed in lines A, B and C were located along a direction trending N160j. Further soil CO2 measurements were made along other four lines (D, E, F and G in Fig. 6) located some hundred metres south of the previous ones, in areas without visible steam emissions. Clear soil gas anomalies were detected in lines D and E, but no anomalous degassing was seen in lines F and G (all values were equal to CO2 concentration in air). The locations of the anomalies in lines D and E fit well with the main direction of elongation of the above described steaming area near TDF. 5.2. Temporal evolution of soil CO2 concentration and soil temperature before the 2001 eruption Since July 2000, periodic measurements of the concentration of CO2 in the soil were made at two sites located, respectively, a few tens of meters SE of TDF and at the Belvedere fumaroles, a few meters below the edge of VDB (Fig. 6). Measurements were carried out with the same method used for spatial surveys, but leaving a fixed probe in the ground, so as to avoid errors due to changes in the exact location of sampling points. Soil and air temperature were also measured at the same time. At Belvedere, measured soil CO 2 concentrations were always higher than at TDF (Fig. 8c). The highest value was recorded in early July 2000, followed by a decreasing trend that lasted until early 2001. High soil CO2 concentrations were again observed in early February 2001 and April –May 2001, the latter marking the beginning of an increasing trend. At site TDF, soil CO2 emission had a somewhat similar evolution, but, differently from site Belvedere, high values were

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observed also in September 2000, and very low values (close to air concentration) were observed after May 4, 2001. Comparison between soil CO2 and air temperature, a parameter that in other areas of Etna was found to have some influence on the temporal variations of soil degassing (Giammanco et al., 1995), showed no significant correlation at both sites (values of the linear regression coefficient between soil CO2 and air temperature were 0.10 at TDF and 0.24 at Belvedere). Soil temperature values measured at the two sites (Fig. 8b) were poorly correlated to each other (R = 0.23). Temperature values at Belvedere were generally close to the boiling point of water at the altitude of sampling (about 2700 m a.s.l.; average temperature value = 87.0 jC). A low correlation was found between soil temperature at Belvedere and air temperature values (R = 0.19). Significant increases in fumarole temperature were recorded from mid-September to mid-December 2000 (maximum deviation above average = 2.8 jC) and in May 2001 (maximum deviation above average = 0.7 jC). A good correlation was instead found at site TDF between local soil temperature and air temperature (R = 0.86), suggesting a significant influence of seasonal air temperature variations on soil temperature at this site. Notwithstanding this, anomalies in soil temperature were also recorded at site TDF in mid-October 2000 and in early February 2001. The difference between soil temperature and the relevant air temperature values was about + 4.3 and + 5.8 jC, respectively.

6. Discussion of data The uppermost part of the southeastern flank of Mount Etna is an area of particular relevance for the development of its eruptive activity. It is located at the boundary of a highly unstable sector because of the

Fig. 8. (a) Time variation of the intensity of volcanic activity at Mt. Etna’s summit vents during the period June 30th 2000 to June 30th 2001 based on an empirical scale from I (no activity) to V (paroxysmal activity with strong explosions and lava fountaining); (b) time variations of soil temperature at TDF, fumarole temperature at Belvedere and air temperature. The seasonal influence of air temperature on soil temperature at TDF is evident; (c) time variations of soil CO2 concentrations (ppmv) at TDF and in the fumarole emissions at Belvedere sampling sites; (d) daily frequency of earthquakes with M z 1.5 occurring in the whole area of the upper southern flank of Etna, and associated cumulative strain release ( J1/2), during the studied period (the dashed grey lines indicate the two periods of significant increase in the gradient of cumulative strain release discussed in the text); (e) hypocentral depth of recorded earthquakes in the study area vs. time. Arrows with numbers in the plots indicate the onset of periods of anomalous changes in the observed parameters as a consequence of magma ascent and degassing (see text for explanation).

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combined effect of regional and local tectonics (Borgia et al., 1992). Two of the most active and important structural trends of Sicily (the Iblean –Maltese and the Comiso – Messina systems) cross each other just underneath this flank of the volcano. Already Imbo` (1928) had the intuition that some sequences of eruptions on the S and NNE sectors of Mt. Etna tended to occur along similar directions, that probably represented directions of greatest ‘‘fragility’’ of the volcano. Tanguy (1980), based on the work of Imbo` (1928) and on the sector distribution of eruptive fissures on Mt. Etna, remarks that the S and NNE sectors are ‘‘axes of maximum fragility of the volcanic edifice’’. Recent studies suggested that both of these systems are pathways for magma ascent toward the surface (e.g., Rasa` et al., 1995) and that portions of them are frequently used by magma as temporary high-level storage volumes (e.g., Murray, 1990; Bonaccorso, 1996; Murru et al., 1999). In addition to this, or because of this, a large instability of the eastern flank triggered the formation of the VDB morphological depression and favours the relatively shallow formation of faults and fissures (Borgia et al., 1992). According to our structural study, these structural features have directions mostly ranging from N40j to N70j in the north-eastern part of the study area, and from N100j to N160j in its eastern and southern parts. All of these volcano-tectonic structures originate either from the VOR or from the SEC, which act as axis of weakness for the whole structure of this part of the volcanic edifice (Ferrucci et al., 1993). The high frequency of eruptions along these structural directions suggests that the tectonic forces acting here allow for the formation of faults that cut the volcanic edifice to a depth where they interfere with its main feeder conduits. The study of seismic events occurring from June 30, 2000 to June 30, 2001 supports the above statements and allows to better define the role of the different families of tectonic structures of the upper SE flank of Mt. Etna. The deepest hypocenters occurred beneath the summit craters and were almost certainly related to the dynamics of magma within the main axial conduits of the volcano. Apart from these, other relatively deep seismic events occurred more frequently beneath the westernmost part of the study area. Shallower earthquakes occurred beneath the western part of VDB (zone B in Fig. 4a), and

may be related to strain release due to local accumulation of magma in a small reservoir that has probably been active during the last 10 years (Bonaccorso, 1996; La Volpe et al., 1999; Bruno et al., 2001). The shallowest seismic events occurred beneath a broad area in the central part of the study area (zone D in Fig. 4a). The evident elongation of such seismic zone (oriented about NNW – SSE) suggests that it is produced by shallow magma intrusion along tectonic structures with the same direction. The variability of associated kinematics by focal mechanisms is likely to reflect the heterogeneity of the shallow crust being fractured. Soil gas surveys allowed to recognize crustal discontinuities with a high permeability to gases up to the surface, an indication that tectonic strain is still active. The occurrence of anomalous soil CO2 emissions along roughly NNW – SSE directions suggests that crustal discontinuities with this orientation are more subject than others to active tectonic strain that makes them open pathways for the release of magmatic fluids, and possibly for the eruption of magma itself. The opening and location of the July – August 2001 eruptive fissures have later confirmed such indication: both the direction and the location of soil CO2 anomalies along sampling lines above 2950 m of altitude corresponded almost perfectly to those of the uppermost eruptive fissure opened during the first day of the July –August 2001 eruption (Fig. 2). In this case, a convective system had developed in a zone of high ground permeability likely to correspond to a hidden volcano-tectonic structure probably tapping the feeding system of the SEC. More generally, the good spatial correspondence between the sites of anomalous soil CO2 emissions, the location of recent eruptive fissures and the location of the epicenters of recent earthquakes underline the important role of the faults directed NNW – SSE, but also those directed N – S, in driving magma when it intrudes into the shallowest portions of the volcanic edifice. A better understanding of the role of the tectonic structures identified in the study area in driving magma toward the surface can be obtained by correlating the temporal evolution both of seismic and geochemical signals recorded during the studied period with that of the volcanic activity during the same period (Fig. 8).

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Seismic activity in early July 2000 was relatively deep (arrow 1 in Fig. 8e) and was associated with very high values of soil CO2 concentration (arrow 1 in Fig. 8c), particularly at site Belvedere. Anomalous high values of CO2 emission from the summit fumaroles and/or in soil gas along faults would be the surface expression of high CO2 pressure at depth due to arrival of fresh magma. Because of its very low solubility in basaltic melts (Stolper and Holloway, 1988; Pan et al., 1991), CO2 is among the first volatile species that begins to exsolve from fresh magma when it ascends towards the surface (depth of first exsolution in Etna’s magma between 24 and 17 km; Frezzotti et al., 1991; Clocchiatti et al., 1992) and it is continuously released in large amounts from ascending magma up to relatively shallow levels. About 2 months later, in late August, an increase in steam temperature was recorded at the Belvedere site (arrow 1 in Fig. 8b). In early November, an anomalous increase in soil temperature was recorded also at site TDF. Increases in soil and fumarole temperature would indicate a greater flux of heat carried by highenthalpy magmatic fluids (mainly water). Water has a higher solubility in magma than CO2 (Armienti et al., 1997), and consequently, its depth of first exsolution is much shallower ( < 4 km below the surface in Etna’s magma; Me`trich et al., 1993) than that of CO2. Therefore, increased outputs of water vapour indicate the intrusion of magma at shallow levels within the volcano. The two-stage degassing from an ascending magma body, due to different solubility of carbon dioxide and water in Etna’s magma, would explain the time lag observed (up to 2 months) between the increases of soil CO2 (late June) and the increases of fumarole temperature (late August –early November). The time lag would be a function of the velocity of magma ascent, in this case relatively slow. The long-lasting trend of CO2 decrease from July 2000 to January 2001 (dashed line I in Fig. 8c) would indicate a progressive depletion of the intruded magma body in this gas because of its continuous degassing. The increase in steam temperature at Belvedere in late August 2000 was followed by an increase both in the frequency of shallow earthquakes and in the eruptive activity at the summit vents. The whole sequence of signals above described, therefore, would be reasonably produced by intrusion of magma at progressively

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shallower levels in Etna’s feeding system along NW – SE-trending faults beneath the southern sector of the volcano and its progressive degassing due to continuous drop in lithostatic pressure. In early January 2001, we recorded a modest seismic swarm with some deep events that preceded by a few days the renewal of effusive activity at SEC. About 1 month later, we observed the onset of a strong increase in seismicity (arrow 2 in Fig. 8d), mostly at shallow depth (Fig. 8e). In the latter case, we also observed a spike-like increase both of soil CO2 and of soil temperature (arrow 2 in Fig. 8b and c). This sequence of signals may be explained in two alternative ways: (i) intrusion of a relatively small batch of fresh magma at depth and its rapid ascent, which produced gas anomalies at surface with almost no time lag between CO2 and temperature peaks and a slight increase of eruptive activity in late February (arrow 2 in Fig. 8a); (ii) purely tectonic processes that caused a sudden and significant increase in ground permeability and hence increase in the release of residual magmatic gas, probably from the CO2- and water-depleted magma that was feeding the ongoing effusive activity. A new episode of fresh magma migration from depth to the surface seems to have occurred since April 2001. This is inferred from the long sequence of deep earthquakes (arrow 3 in Figs. 8d and e), the subsequent increase of soil CO2 concentration and then of ground temperature (arrows 3 in Figs. 8c and b, respectively) at both monitored sites and, lastly, the beginning of a period of intense paroxysmal activity at SEC (arrow 3 in Fig. 8a) that led up to the violent lateral eruption of July –August 2001. The short time lag between the occurrence of the above-described signals suggests a relatively rapid magma migration towards the surface. In particular, during the impulsive increase of geochemical signals in April –May 2001, the time lag between the onset of soil CO2 anomalies and that of the anomalies in fumarole temperature was less than 1 month. The absence of geochemical anomalies at TDF after early May could be due to a strong increase of local ground permeability which greatly reduced soil degassing. Such phenomenon may be produced by ground deformation and fracturing induced by shallow magma intrusion along the NNW – SSE-trending fractures of that area.

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7. Conclusions The multidisciplinary approach used in this study allowed us to define more clearly the relationships between active tectonism, magma migration and eruptive activity in the most active area of one of the most active volcanoes of the world. Also, the acquired data served as an important tool to highlight sites of potential lateral eruptions in this part of the volcano and to monitor the dynamics of magma motion within the shallowest portions of the volcanic pile with a greater resolution. The N120j and N180j structural directions have acted in the past centuries, and still act today, as preferential pathways for magma migration to the surface. They are expression of regional faults that probably tap the deep source of Etna’s magma. This is based not only on the information from the existing literature, but also on our findings that these volcano-tectonic structures are continuously ‘‘open’’ to the release of magmatic fluids (i.e., they are characterized by very high permeability down to remarkable depth) and eventually of magma itself. The recent eruptions of 2001 and 2002 – 2003 confirmed that this part of the volcano is particularly subject to opening of eruptive fissures. Furthermore, based on the temporal changes of the monitored parameters, we were able to trace the arrival of magma that was later erupted during the July – August 2001, and we observed that the timing of magma migration from depth to the surface along the faults of the studied sector of the volcano would be relatively short, in the order of some weeks to some months. Continuous upgrading of field data on tectonic structures, collection of all historical information on past eruptions, real-time analysis of seismicity, detailed explorative soil gas surveys coupled with an adequate number of sites for monitoring geochemical parameters, will allow to improve and hopefully complete the knowledge required to assess the potential eruptive state of active volcanoes such as Mt. Etna.

Acknowledgements We wish to thank the Ente Parco dell’Etna for kindly providing the permission to work in protected

areas. We also thank Giovannella Pecoraino, Giuseppe Riccobono, Alfio Amantia and Salvatore Carbonaro for their help during the geochemical surveys, and Salvatore Gambino for providing air temperature data from a lower altitude site that we used as a control for our data on ground temperature. This paper benefited from the reviews by A. Minissale and an anonymous reviewer.

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