Geometry and kinematics of the fault systems controlling the unstable flank of Etna volcano (Sicily)

Journal of Volcanology and Geothermal Research 251 (2013) 5–15

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Geometry and kinematics of the fault systems controlling the unstable flank of Etna volcano (Sicily) R. Azzaro ⁎, A. Bonforte, S. Branca, F. Guglielmino Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Piazza Roma 2, 95123, Catania, Italy

a r t i c l e

i n f o

Article history: Received 19 January 2012 Accepted 1 October 2012 Available online 9 October 2012 Keywords: Faults Seismotectonics Ground deformation Geodynamic model Flank instability Mt. Etna

a b s t r a c t An updated tectonic framework of Etna's unstable flank has been defined as a result of multidisciplinary analyses carried out by integrating geological and geophysical data. The different typologies of datasets have been analyzed and correlated in order to constrain the geometry and kinematics of the fault systems controlling the unstable flank of Etna volcano and to better understand their complex relationship with the offshore morphostructures of the continental margin. In particular, we have considered as the main structural elements the following four fault systems: Pernicana, Ragalna, Tremestieri–Trecastagni and Timpe. Slip-rates and kinematics have been estimated in both long- and short-terms, respectively, from geological and seismotectonic/geodetic data. Data integration has allowed defining five kinematic domains in the sliding flank of Etna: (1) the NE block, bordered by the Pernicana fault and characterised by the highest deformation velocities; ground velocity progressively diminishes toward South, with a clockwise rotation of the vectors defining (2) the block embracing the central part of the Timpe system; (3) the Giarre wedge; (4) the Medium-East block, bounded by the S. Tecla and Trecastagni faults; and (5) the SE block bordered, by the hidden Belpasso-Ognina tectonic lineament. The dynamics of these blocks takes place through discontinuous movements: sudden short-term accelerations related to the magma intrusion are superimposed to a fairly constant mid-term ESE sliding. The proposed comprehensive model of the unstable flank provides the basic input parameters for applying analytical models to flank dynamics of Etna volcano. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The geological evolution and volcano-tectonic features of volcanic edifices are strongly conditioned by the structural setting of their basement and by regional tectonics (see the special volume edited by Tibaldi and Lagmay, 2006, and papers therein). In particular, the relationships between volcano dynamics and regional stress regime play an important role in the magma transport from the mantle to the surface and flank instability of volcanic edifices, with important implications for their geologic hazard (Nakamura, 1977; McGuire et al., 1997; Pasquaré and Tibaldi, 2003; Norini and Lagmay, 2005; Tibaldi, 2005; Tibaldi et al., 2005; Acocella, 2006). Mount Etna is a large polygenetic basaltic volcano built up over the past 500 ka on the eastern coast of Sicily in a geodynamic setting generated during the Neogene convergence between the African and European plates (Lentini et al., 2006; Branca et al., 2011). The structural framework of the Etna edifice is the result of a complex interaction between regional tectonics, flank instability processes and basement geometry (McGuire and Pullen, 1989; Borgia et al., 1992; Lo Giudice and Rasà, 1992; McGuire et al., 1996; Montalto et al., 1996; Rasà et al., 1996; Rust and Neri, 1996; Monaco et al., 1997; Bousquet and Lanzafame, 2004; Neri et al., 2005; Rust et al., 2005; Norini and ⁎ Corresponding author. E-mail address: [email protected] (R. Azzaro). 0377-0273/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jvolgeores.2012.10.001

Acocella, 2011). In particular, recent data obtained from different geophysical techniques (Lundgren et al., 2004; Puglisi and Bonforte, 2004; Rust et al., 2005; Walter et al., 2005; Solaro et al., 2010; Bonforte et al., 2011) have highlighted that the unstable flank of Etna is dismembered into different blocks, characterised by homogeneous kinematics. An updated structural setting of Etna volcano has recently been defined in the new volcano-tectonic map, at 1:100.000 scale (Azzaro et al., 2012a) and reported by Barreca et al. (2012) in a GIS database. The main structural lineaments of the unstable flanks of Etna, are the following fault systems (Fig. 1): Pernicana, Ragalna, Tremestieri–Trecastagni, Timpe and Ripe della Naca–Piedimonte–Calatabiano. The Pernicana fault system (PF) is a transtensive structure with left strike-slip extending from Piano Provenzana in the upper northeast flank down to the Ionian coast. This complex transtensive structure is formed by a main E–W striking segment showing a south-facing scarp, which in the eastern portion is characterised by a purely left-lateral displacement rate of 2.8 cm/a in the last decade (Groppelli and Tibaldi, 1999; Azzaro et al., 2001) and about 2 cm/a reconstructed over the past 150 years by Rasà et al. (1996). The eastward propagation of the PF consists of a set of en-échelon right-stepping, ESE-striking segments extending down to the coastline where the fault segments strike toward NNE. This fault system also includes the Fiumefreddo fault, an E–W trending normal fault with left-lateral strike-slip localized close to the Ionian coast (Azzaro et al., 1998; Tibaldi and Groppelli, 2002). The offshore continuation of the PF is a set of small NNE–SSW scarps, with

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Fig. 1. Tectonic sketch map of Etna volcano defined by structures with morphological expression (modified by Branca et al., 2011). Fault abbreviations: PF, Pernicana; RF, Ragalna; CF, Calcerana; TMF, Tremestieri; TCF, Trecastagni; FF, Fiandaca; ARF, Acireale; STF, S. Tecla; MF, Moscarello; SLF, S. Leonardello; RF, Ragalna; RP, Ripe della Naca; PD, Piedimonte; CL, Calatabiano. The arrows indicate the strike-slip component, the contour of the rift zones is in red; inset map shows the location of the study area.

few meters of vertical displacement, whose morphological evidence is confined within 2 km from the coastline (Chiocci et al., 2011; Argnani et al., 2012). The Ragalna fault system (RF) is a dextral transtensive structure, located in the lower western flank of Etna (Fig. 1). This fault system, interpreted as the western boundary of the unstable south flank (Neri et al., 2007), is formed by a main segment, N–S striking, showing an east-facing scarp with slip-rates of 3.4 and 3.7 mm/a calculated for left-lateral and dip-slip components of movement (Neri et al., 2007). The Tremestieri (TMF)–Trecastagni (TCF) is a normal fault system (Fig. 1) formed by two faults with right-lateral component representing the southern boundary of the unstable east flank (Lo Giudice and Rasà, 1992; Solaro et al., 2010). TMF is the longer NNW–SSE oriented segment. Conversely, the TCF is a short segment, NW–SE oriented. According to Chiocci et al. (2011) the TMF–TCF system is linked with a main transcurrent fault located in the offshore of Acitrezza village dissecting the laccolith of Ciclopi Islands. The Timpe is a normal fault system with a right-lateral component dissecting the lower eastern flank (Fig. 1). This fault system is formed by several main master segments named Fiandaca, Acireale, S. Tecla, Moscarello and S. Leonardello faults, showing slip-rates ranging from 1.0 to 2.7 mm/a (Azzaro, 2004). In particular, the Fiandaca fault (FF) is

roughly N–S oriented and rotates toward SE near Acitrezza. The Acireale fault (ARF), roughly N–S trending, forms a scarp up to 120 m high along the coast while the S. Tecla fault (STF) is NW–SE oriented, forming a scarp up to 180 m high and in the south tip it is linked with ARF. The Moscarello fault (MF), NNW–SSE trending, is the most prominent scarp of the Timpe system; finally, the S. Leonardello fault (SLF) forms a NNW trending graben as a consequence of the eastward extension of this fault system. The offshore continuation of the Timpe system is characterised by well-developed morphological scarps, up to 60–80 m of vertical displacement, that gradually rotate towards SE (Fig. 1, cfr. Chiocci et al., 2011). The Ripe della Naca–Piedimonte–Calatabiano system is formed of normal structures located along the northeast flank formed by several main segments. The Ripe della Naca faults (RP) consist of a couple of WSW–ENE oriented scarps from 80 to 120 m high. The Piedimonte fault (PD), WSW–ENE oriented, and the Calatabiano fault (CL), NE–SW striking, are interpreted as regional tectonic lineaments (Finetti et al., 2005; Lentini et al., 2006). RP–PD are buried by Holocene lava flows and truncated by the Pernicana fault segments (Azzaro et al., 2012a). Short normal faults showing a similar NE–SW trend are present along the Ionian coast. The continuation in the offshore of these small fault segments is given by several well-developed morphological scarps, NE

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striking, characterised by several tens of meters of vertical displacement (Chiocci et al., 2011). Recently, the relationships between the structural setting of Etna's unstable eastern flank and the offshore morpho-lineaments of the continental margin facing the volcano have been defined by Chiocci et al. (2011). In particular, the authors showed that the continental margin is affected by a wide semicircular fault system as the expression of a large-scale retrogressive instability phenomenon that also involves the volcano costal belt. In this new interpretation the on- and offshore portions of the Timpe fault system appear as a part of the flank instability affecting the continental margin rather than the northernmost extension of the Malta Escarpment. In addition, multichannel seismic reflection data performed by Argnani et al. (2012) have evidenced that the Timpe fault system is not linked with the regional structural lineament of the Malta Escarpment. In this paper, an updated tectonic framework of the Etna unstable flanks is presented as a result of a multidisciplinary analysis obtained by integrating (1) detailed geological and structural investigations performed for compiling the new geological and volcano-tectonic maps of Etna volcano (Branca et al., 2011; Azzaro et al., 2012a); (2) seismotectonic data from historical ground surface rupture mapping (Azzaro, 2004); and (3) ground deformation measurements (Puglisi et al., 2001, 2004) and Permanent Scatterer (PS) data (Bonforte et al., 2011). These different typologies of datasets have been analyzed and

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correlated in order to constrain age, geometry and kinematics of the fault systems controlling the unstable flanks of Etna volcano and to better understand their complex relationships on- and offshore. 2. Seismotectonics The structural framework given by exposed fault segments is integrated with seismotectonic data, which allow a more complete definition of geometry of the tectonic elements and information on their activity in the mid-term (since the mid 19th century to date). For the purposes of this paper, we refer to the results of the analyses carried out for the new volcano-tectonic map of Etna (Azzaro et al., 2012a), considering the following two major features: (1) the distribution of the main historical and recent seismicity; and (2) the pattern of the hidden faults, i.e. tectonic elements not recognizable through the standard approach of the geological survey. The location of the ML ≥3.2 earthquakes occurring at Etna in the last four centuries is shown in Fig. 2. Information provided from macroseismic data in the long period is fairly complete and representative of the major seismicity affecting the piedmont parts of the volcano, where earthquake damage in the urbanised areas is historically welldocumented (Azzaro et al., 2012b). The eastern sector of the volcano, crossed by the Timpe fault system (including FF, STF, MF SLF), appears very active from the seismic point of view, both for number of events

Fig. 2. Distribution of the historical seismicity in the Etna region from 1600 to 2010 (macroseismic data from CMTE Working Group, 2008; Azzaro et al., 2010); magnitudes are calculated from the epicentral intensities I0 according to the relationship by Azzaro et al. (2011). Inset map a) shows the location of the earthquakes with ML ≥3.2 and hypocentral depth H≤4 km, recorded from 1999 to 2011 (instrumental data from Gruppo Analisi Dati Sismici, 2012); the contour of the rift zones is in red. Fault pattern and abbreviations as in Fig. 1, C.C. indicates the central craters.

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Fig. 3. Active faults from seismotectonic data (modified from Azzaro et al., 2012a). The tectonic pattern of faults with morphological expression is completed by hidden features revealed by surface faulting (coseismic ruptures and creeping), and buried faults defined by alignments of earthquake epicentres; the contour of the rift zones is in grey. Major strike-slip components (arrows) are also shown; SVF=S. Venerina fault, other abbreviations as in Fig. 1. Stippled areas indicate the sedimentary basement.

and the maximum intensity reached at the epicentre. The other fault systems are characterised by moderate to strong earthquakes mainly along the upslope segments of the Pernicana and Tremestieri faults, and a more scattered seismicity in the south-western sector. On the contrary, while no significant macroseismic effects are available for the wide deserted zone around the active summit craters, recent instrumental data highlight a recurrent seismic activity along the Pernicana fault (Alparone et al., 2012b) and the rift zones, the latest mainly related to the seismic swarms accompanying the 2001, 2002–03 and 2008–09 flank eruptions (Patanè et al., 2003; Barberi et al., 2004; Alparone et al., 2012a). Regarding the hidden faults, this term is intended in a morphological sense (see Yeats et al., 1997, and references therein), and indicates surface breaking faults that do not exhibit a clear and permanent associated geomorphology. In such a case the evidence of surface faulting is produced by coseismic displacement or creeping but it is generally a short-lived feature, being visible only if the fault trace crosses manmade features (Azzaro, 1999). In practice, hidden faults are not isolated but represent segments of major, continuous fault zones, appearing as extensions of exposed/buried faults or as connections between faults that can be considered distinct from each other from a purely geological point of view. Finally, in the map of Fig. 3 we also included the blind faults, i.e. deeper structures lacking any surface evidence but revealed by the alignment of strong historical earthquakes. In the south-western flank, the Ragalna fault system appears as an isolated element in the structural framework of the volcano, without an apparent direct connection to the other volcano-tectonic features. The system is formed by two structures that connect at their upslope termination with a cusp-shaped junction (Rust et al., 2005; Neri et al., 2007). The present activity is indicated by both a remarkable aseismic creeping (Ragalna fault) with dextral oblique-slip displacements, and a moderate seismicity, with shallow events not exceeding I0 VII EMS (or ML 3.7) located along the downhill extension of the Calcerana fault (countryside

north-east of Adrano). Other earthquakes are known to have occurred along an alignment, striking NE–SW, between the localities of Biancavilla and Ragalna (Fig. 2), which marked the pattern of a blind fault. In the southern flank, the Tremestieri–Trecastagni faults define a tectonic system extending for about 10 km from the southern termination of the S rift downslope to a few kilometres west of the coast-line (Azzaro et al., 2012a). Although poorly defined from the morphological point of view – the uppermost segments are hidden and blind, respectively – they are characterised by important dynamics with very shallow seismicity of moderate energy (earthquakes do not exceed I0 VII EMS or ML 3.7) and above all fault creep (Azzaro, 2004). These structures have a relevant role in controlling, by dextral oblique movements, the seaward movement of the volcano's eastern flank, and constitute a structural limit of first order between the unstable sector to the north and the relatively steady area to the south (Solaro et al., 2010; Bonforte et al., 2011, 2012a; Gambino et al., 2011). The continuity of the Timpe system is well-evidenced over much of the eastern flank: near the coast, the NNW–SSE structural trend (Moscarello–Acireale–S. Leonardello faults) dissects the base of the volcano's flank by prevailingly vertical movements, while in the interior a prominent feature is represented by the NW–SE striking system, which defines a right-lateral shear zone displacing the flank (Azzaro, 2004). In this framework, the Fiandaca–S. Tecla–S. Venerina faults appear to be important tectonic elements connecting the Timpe system with the central part of the volcano and the volcano-tectonic depression of the Valle del Bove (see the blind segment of the S. Tecla fault in Fig. 3). It should be stressed that all these faults are very active also from the seismic point of view, representing the sources of the strongest earthquakes reported in the seismic catalogue for the last centuries (CMTE Working Group, 2008). With a long-term behaviour (~200 years) characterised by a mean recurrence time of about 20 years for severe/destructive events (epicentral intensity VIII ≤ I0 ≤ IX–X EMS, corresponding to a magnitude range 4.3 ≤ ML ≤ 5.1 according to

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Azzaro et al., 2011), the Timpe fault system is highly relevant in terms of seismic hazard, especially when short exposure times (5–30 years) are considered (Azzaro et al., 2008; 2012b). The Ripe della Naca–Piedimonte–Calatabiano faults in the northeastern flank, despite their outstanding morphological evidence, do not appear active in historical times given their lack of seismicity or any surface faulting (Azzaro, 2004). For this reason they are not represented in the active fault map of Fig. 3. Finally, to the North the Pernicana fault system decouples the entire eastern flank of the volcano from the NE rift as far as the sea (Tibaldi and Groppelli, 2002; Neri et al., 2004; Bonforte et al., 2007a, 2007b; Guglielmino et al., 2011b). It extends for nearly 20 km and consists of two main segments, the western one expressed by an evident sinistral oblique fault, the eastern one appearing as a strongly segmented left-lateral fault zone devoid of morphological expression (Acocella and Neri, 2005). While the primary mechanism triggering the Pernicana fault is still debated – active for dyke-induced rifting or passive for gravity-induced sliding (Acocella et al., 2003; Walter et al., 2005; Bonforte et al., 2007b; Currenti et al., 2008, 2010) – it is widely recognised as the most active structure in the volcano. Its important dynamics is demonstrated by the very frequent seismic activity occurring in the western and central sections of the fault (Azzaro, 1997; Alparone et al., 2012b), with strong earthquakes reaching ML 4.7 and I0 VIII EMS, and by aseismic creep in its eastern sector, accommodating a large part of the overall displacement of the unstable flank (Obrizzo et al., 2001; Bonforte et al., 2007a). 3. Ground deformation Ground deformation measurements on Etna have been carried out by EDM techniques since 1970s and by GPS since 1988 (Puglisi et al., 2001, 2004). Geodetic studies highlighted that the deformation of the eastern and south-eastern flanks of Etna is quite different from the radial pattern that one would expect in a central basaltic volcano (Bonforte and Puglisi, 2003, 2006; Bonaccorso et al., 2006; Houlie et al., 2006; Bonforte et al., 2011; Guglielmino et al., 2011a). GPS data showed that the deformation rate of the eastern side is always higher than that expected by assuming a simple magmatic source. The first analytical model obtained before the 2001 eruption (Bonforte and Puglisi, 2003) highlighted a planar surface beneath the mobile flank at a depth of about 2 km b.s.l., gently dipping eastwards. Further improvements in the network during the late 1990s (Puglisi and Bonforte, 2004; Bonforte and Puglisi, 2006), revealed a first-order rotational sliding of the eastern flank, detecting for the first time the different blocks making up the unstable sector. The Pernicana fault and its extension as far as the coastline (Acocella and Neri, 2005) was confirmed to represent the northern boundary (Bonforte and Puglisi, 2006), and it was possible to investigate its relationship with flank dynamics (Bonforte et al., 2007a) and eruptive activity, especially along the NE rift (Bonforte et al., 2007b,c). On the southern flank, GPS measurements detected a gradual decrease and rotation of the displacements moving southwards, suggesting that the southern limit of the sliding sector was less sharp than the northern one. Another detachment level located at roughly sea level, was modelled by Bonaccorso et al. (2006) and Bonforte et al. (2008) following the 2001 and 2002–2003 flank eruptions. Due to the necessity of simplifying the models, all geodetic data inversions resulted in only one plane at a time and the different results highlighted the complexity of the detachment, probably consisting of a family of non-planar surfaces inherited from the lithological features of the basement (Bonforte et al., 2007a, 2009; Alparone et al., 2011). In the late 1990s, the spatial sampling of the ground deformation through Differential SAR Interferometry (DInSAR) analysis, allowed detecting unexpected dynamics especially on the lower south-eastern side of the volcano (Borgia et al., 2000; Froger et al., 2001; Solaro et al., 2010; Bonforte et al., 2011, 2012a; Gambino et al., 2011) and, combined with a geochemical approach (Neri et al., 2007), recognising

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the uphill extension of the Ragalna fault as far as the summit part of the volcano. Further improvements were obtained from the Permanent Scatterer (PS) and Small BAselines Subset (SBAS) approaches (Ferretti, et al., 2001; Berardino et al., 2002), based on time series analyses of InSAR data aimed at removing the main noise sources affecting DInSAR images (Zebker and Villasenor, 1992; Massonnet and Feigl, 1995; Zebker, et al., 1997; Bonforte et al., 2001; Onn and Zebker, 2006). This technique enables identifying fault displacements even in urban areas where evidence is difficult to detect by traditional field surveys. Ground deformation maps resulting from time series analysis allowed Solaro et al. (2010) and Bonforte et al. (2011) to investigate kinematics of the sliding flank, and to recognise important features interpreted as discontinuities in the ground velocity field. Zoning of the sliding sector resulting from the previous GPS and SAR studies is shown in Fig. 4, with the main tectonic discontinuities, marking abrupt changes in the ground velocity field, that identify five main domains: (1) the NE block, bordered to the North by PF and characterised by the highest deformation velocities; ground velocity progressively diminishes toward South, with a clockwise rotation of the vectors defining a (2) central block between MF and STF; (3) the Giarre wedge, on the easternmost corner of the volcano east of the Timpe system; (4) the Medium-East block, bounded by STF and TCF and (5) a relatively less active SE block, moving southwards, bordered by a hidden tectonic lineament evident also from soil gas surveys (Bonforte et al., 2012b), developing throughout the southern flank from Belpasso to Ognina (BOL). GPS time series plots of some stations lying on the different domains, are also reported in Fig. 4. It is clear that each domain is characterised by different dynamics in terms of velocity and displacements but all exhibit a discontinuous movement: the fairly constant mid-term (decennial) seaward sliding is interrupted by sudden short-term (months to year) accelerations related to flank eruptions (Acocella et al., 2003; Bonforte et al., 2007a; Puglisi et al., 2008; Bonforte et al., 2009). 4. Kinematic model of the sliding flank Both the complexity of the kinematics of the unstable sector as well as the extent of the details of the several fault systems decoupling the entire sector in different kinematic blocks have increased since the first simple conceptual models (Fig. 5). After the first insights about flank instability on Etna from field evidences (see introduction) and from analogue models (Merle and Borgia, 1996), new hypotheses on the geometry of the sliding sector were proposed based on instrumental data as DInSAR (Froger et al., 2001) and GPS (Bonforte and Puglisi, 2003). Furthermore, the first analytical models were proposed when ground deformation instrumental data became available, albeit on only a few points lying on the unstable flank (Bonforte and Puglisi, 2003; Puglisi and Bonforte, 2004; Houlie et al., 2006). These models gave the first evidence of a detachment beneath the eastern side of the volcano, more recently confirmed by comparing seismic and geodetic data (Bonforte et al., 2009; Alparone et al., 2011) and provided the first estimations of sliding rates. Later on, these models were progressively refined following the improvements of the geodetic networks that also revealed a very first arrangement of this flank in different sliding blocks (Bonforte and Puglisi, 2006). Integration of GPS dense network and DInSAR data also gave evidence of possible multiple detachment surfaces at different levels (Bonaccorso et al., 2006), that can be triggered by intrusive events able to promote flank instability, such as the 2001 and 2002 ones (Acocella et al., 2003; Bonforte et al., 2007a, 2007c, 2009). Besides the overall sliding and the depth of the detachment, further details in the complex arrangement of several blocks making up the unstable flank have been continuously furnished by field evidences during particularly high rates periods (Rust et al., 2005). More sophisticated remote sensing data processing (Solaro et al., 2010; Bonforte et al., 2011) has highlighted hidden lineaments not revealed by field data, completing the information from

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Fig. 4. Summary of ground deformation measurements at Etna. Brown lines indicate the main features detected by SAR data, associated with the considered fault systems (abbreviations as in Figs. 1 and 2). The main blocks composing the sliding flank, indicated by different colours in the central map, are redrawn from Bonforte et al. (2011). Arrows indicate the mean velocities of the GPS stations, whose time series are reported in the plots (coloured according to the relevant block): CRI, Mt. Crisimo; EGIA, Giarre; EMSC, Mascalucia; ESRV, Scorciavacca; ESVN, S. Venerina; ETAC, Torre Archirafi; FOPE, Fossa del Pero; PDAP, Piano d'Api; PISA, Pisano; SGBO, S. Giovanni Bosco; STAZ, Stazzo.

the detailed volcano-tectonic mapping (Azzaro et al., 2012a) and the recent analogue model by Norini and Acocella (2011). In the model we are proposing, we integrated the tectonic elements recognised in the field according to standard procedures (see the methodological approach in Azzaro et al., 2012a) with the lineaments inferred from geophysical data (see Bonforte et al., 2009, 2011; Chiocci et al., 2011), including information on recent and long-term slip-rates, i.e. from the late Pleistocene to Holocene (100–3.9 ka). This time interval can indeed be considered representative for defining the evolution of a complex, large-scale geologic process such as flank instability. These features are used to define the overall geometry of the model both onshore and offshore, and to fix the boundaries of the unstable area. Fig. 6 shows the main structures involved in the flank instability processes, mapped with simplified geometries (linear elements, no bending) as they are intended to represent first order elements at the scale of the entire volcanic edifice. The tectonic elements controlling the flank instability processes can be identified with the following groups of structures: (1) the Ragalna fault (RF) and the Belpasso-Ognina lineament (BOL) that, together with the Tremestieri (TMF) and Trecastagni

faults (TCF), bound to the south the unstable flank by extensional movements with a minor dextral component; (2) the Timpe fault system (formed by FF, ARF, STF, SVF, MF, SLF), which dissects the eastern side of the volcano and accommodates large strains induced by the extensional, seaward sliding of this sector by dip-slip and right-lateral oblique displacements; and finally (3) the Pernicana fault (PF), which represents the main shear zone of Etna decoupling to the north the mobile sector by a oblique, left-lateral strike-slip. In the scheme, we also considered the lineaments resulting from DInSAR data in the upper part of the volcano, which essentially follow the axes of the volcanic NE and S rift zones and close the unstable area to the west. They connect the summit craters with the upslope tips of the Ragalna–Tremestieri–Trecastagni– Pernicana faults, defining the unstable sector onshore. Concerning the morphostructural setting of the Etna offshore proposed by Chiocci et al. (2011), the presence of instability elements along the continental margin is evident between the Fiumefreddo valley to the north and the Catania canyon to the south (Fig. 6). Although these two relevant morphological features appear located near the termination of the Pernicana fault and Belpasso-Ognina lineament, no structural linking

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Fig. 5. Main models of flank instability at Etna proposed in the literature during the past 20 years: a) Lo Giudice and Rasà, 1992; b) Borgia et al., 1992; c) Rust and Neri, 1996; d) Rasà et al., 1996; e) Merle and Borgia, 1996; f) Bonforte and Puglisi, 2003, 2006; g) Rust et al., 2005; h) Bonaccorso et al., 2006.

has been found by Argnani et al. (2012). The semicircular fault system affecting the continental margin, while appearing linked with some fault segments of the Timpe system, represents very shallow second order structures that passively accommodate the movement of this large sector of the continental margin toward the abyssal plain.

Fig. 6 also shows two sketch sections crossing the central part of the volcano. The most outstanding features playing a role in flank instability, are: (1) the high velocity body (HVB) confined in the central-eastern part of the volcano at a depth between 3 and 9 km b.s.l. (Vp 5.6–6.4 km/s), which is interpreted as high-density cumulates fractionated during

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Fig. 6. Pattern of the main volcano-tectonic elements proposed for modelling dynamics of the unstable sector of Etna; offshore features are redrawn from Chiocci et al. (2011). Legend: 1, faults and major strike-slip components; 2, lineaments and other structures delimiting the study area; 3, main morphological scarps; 4, landslide scarp; 5, rift zones; 6, active detachment plane defined by ground deformation data (Bonaccorso et al., 2006; Bonforte and Puglisi, 2006). Geometric and kinematic parameters are reported in Table 1; coordinates and GIS metadata are provided in the electronic supplementary material. Cross-sections show the high velocity body (HVB) defined by seismic tomography (Chiarabba et al., 2004); fault abbreviations as in Fig. 1.

Table 1 Geometric and kinematic parameters of active faults proposed for modelling kinematics of the unstable sector of Etna in an elastic half-space medium (WGS84 coordinates are provided in the electronic supplementary material). Slip-rates represent long- and short-term estimations, calculated from geological and historical data, respectively (Azzaro et al., 2012a). Components of movement: (v) vertical, (d) dextral, (s) sinistral. Note that the most critical definition concerns the dip and width of the faults, which have been inferred from the inversion of ground deformation data; for this reason, in some cases they are not available. Faults

Length (km)

Strike

Dip

Width (km)

Dominant faulting style

Pernicana (PF)

19.4

50°–115°a 0° 60° 135° 150° 100° 140° 5° 175° 140° 135° 165° 165° 125°

60°–82°Sa

2.6–1.6a

60°NE–89°E

2.7–1.6b

60°–89°NE 60°–89°NE

Ragalna (RF)

4.9

Tremestieri (TMF)

7.2

Trecastagni (TCF)

7

Fiandaca (FF)

13.8

Acireale (ARF) S. Tecla (STF) S. Venerina (SVF) Moscarello (MF) S. Leonardello (SLF) Belpasso-Ogninac (BOL)

6.2 7.6 5.5 11.3 9 22

a b c

89°E 89°E 60°–89°NE

Long-term slip-rates (mm/yr)

Short-term slip-rates (mm/yr)

Oblique, sinistral strike-slip

3.3–5.2 (v)

10 (v)–28 (s)

Dextral, oblique slip

1.3–1.5 (v)

3.7 (v)–3.4 (d)

1.6b

Dextral, oblique slip

2.5 (v)

4.0 (v)

b

Dextral, oblique slip

2.5 (v)

4.0 (v)

Dextral, oblique slip

1.0 (v)

2.0 (v)

Normal, with right-lateral components Normal, with right-lateral components Normal, with right-lateral components Normal with minor dextral slip Normal with minor dextral slip Dextral, oblique slip

1.3–4.3 (v) 4.3 (v)

3.0–10 (v)

1.6

3.5 3.5 1.6b

End-member values varying from west to east (Bonforte et al., 2007b). Depth, from north to south, of the sliding surface modelled in Bonforte and Puglisi (2006). Tectonic lineament revealed by InSAR data (Bonforte et al., 2011) and CO2 soil degassing (Bonforte et al., 2012b).

1.4–2.7 (v) 2.6 (v)

3.0 (v) 5.0 (v) 4 (v)–5 (d)

R. Azzaro et al. / Journal of Volcanology and Geothermal Research 251 (2013) 5–15

repeated intrusions (Chiarabba et al., 2004) or fossil magma chambers of ancient Mt. Etna (Laigle et al., 2000); (2) the presence of both extensional and compressive domains at different crustal levels separated by (3) two E-dipping planar surfaces of detachment – most likely a family of discontinuities – located at a depth ranging from 2 to 4 km b.s.l (Bonforte and Puglisi, 2003). According to the authors, before an eruption (recharging period) flank instability is favoured by the pressurization of the deeper feeding system of the volcano, which activates a deep-rooted detachment at the top of the rigid body (see section W–E in Fig. 6). The seismic and geodetic strains (Alparone et al., 2011) related to the extensional dynamics abruptly disappear at a depth of 2–4 km b.s.l., i.e. the level of the deeper sliding surface. Beneath this level, the stress field is unequivocally driven by the pressurization of the feeding system, while above the detachment plane the inflation may induce extension, rarefaction and depressurization of the upper central area. These, in turn, facilitate the ascent of magma towards the summit craters (Carbone et al., 2009; Bonaccorso et al., 2011), triggering further sliding above the shallower detachment. The activation of this detachment plane during shallow intrusions leading to flank eruptions (i.e. the 2001 to 2004 case-histories in Bonaccorso et al., 2006 and Bonforte et al., 2009), is marked by acceleration periods that are evident in the GPS time series in Fig. 4. The boundaries of the sliding sector are shown in the section N–S of Fig. 6. These structures join with the detachment plane at a depth of 2–4 km b.s.l., corresponding also to the maximum depth of the earthquakes along the Pernicana fault (Alparone et al., 2012b) and to the detachment plane also detected by magnetotelluric surveys (Siniscalchi et al., 2012). The overall geometry of the unstable sector is therefore formed by a family of shallow listric faults connected with a sole fault. This setting also extends to the Etna offshore with the semicircular pattern of skin-deep structures. In conclusion, although the overall tectonic framework might appear conceptually easy to apply to an analytical model (Table 1), modelling flank dynamics is indeed complicated by the overall clockwise rotation of the north-eastern flank, by the presence of rotational components (westwards tilt) of the different sub-blocks making up the unstable area between the Acireale and Fiandaca faults (ARF and FF, see also Azzaro et al., 2012a for details) as well as by at least two detachment planes responsible for the instability processes. Rotational movements are also evident on the south-eastern part of the sliding flank from ground deformation data reported in Bonforte et al. (2011), revealing the westwards tilt of each block; also at a wider scale, the entire southern and south-eastern flanks show an overall rotational behaviour with an outstanding active uplift of the southern periphery of the volcano. Regarding the structures delimiting the unstable sector, the very low dislocation of the Holocene volcanic products (Branca et al., 2011; Azzaro et al., 2012a) compared with the ground deformation measurements, and the immature morphological development of the Pernicana and Tremestieri–Trecastagni fault segments toward the coast (Acocella and Neri, 2005; Azzaro et al., 2012a), suggest that their seaward migration occurred in more recent times. This evidence is similar to the formation of lateral ramps in a rotational landslide, whose vertical displacement is progressively reduced toward the front due to the topographic inversion produced by the general tilting. 5. Concluding remarks In this work, geological and geophysical information has been analyzed and correlated in order to define an updated and comprehensive kinematic model of the sliding flank of Etna volcano. Starting from the new geological and volcano-tectonic maps, we recognised the main tectonic structures dissecting the unstable sector of the volcano and defined their long-term slip rates (100 to 3.9 ka); geometric relationships with the active volcanic rift zones have also been taken into account. The most recent seismotectonic data have been used to identify the faults active in the mid-term (last 200 a), to characterise the fault behaviour

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(stick-slip vs stable-sliding) and the prevailing faulting style. The overall tectonic framework has been defined by considering also the hidden fault segments, which constitute discrete elements of more extended structures, as well as the morphostructures detected in the offshore. Ground deformation data from GPS and DInSAR measurements have been exploited to highlight fault zones showing significant strain in the short-term (last 20 a) but also dynamics along lineaments not detectable by other methods, and to define the main kinematic domains composing the sliding flank of the volcano. Geodetic data has also provided indications on the geometry and depth of the detachment planes, marking the upper and lower limits of a transition zone between the sliding portion and the stable one. The integration of all the information obtained by the different approaches led to a final comprehensive model of the unstable sector, which contains the basic input parameters for modelling flank dynamics at Etna. The first-order overall sliding kinematics are complicated by a general horizontal (clockwise) rotation of the entire north-eastern sector and a general vertical (westward tilt) rotation of the south-eastern one; furthermore, a second-order westward tilt of each block making up the south-eastern sector has been evidenced by ground deformation data, suggesting a listric geometry of the faults. In order to facilitate the application of analytical models, the 3D geometry of the structures is simplified as rectangular planes and each element provided with the basic input parameters to be used in the computations. The considered tectonic elements identify the boundaries of the whole sliding sector (Pernicana, Ragalna, Tremestieri–Trecastagni and Belpasso-Ognina systems) and the main structures of the Timpe systems dissecting it into blocks with different kinematics. The defined domains are characterised by discontinuous dynamics: the fairly constant seaward movement (i.e. the mid-term flank sliding) is interrupted by sudden, short-lasting accelerations related to flank eruptions. At depth, the activation of the detachment planes driving flank instability is favoured by the pressurization of the feeding system or by shallow intrusions leading to flank eruptions. The immature degree of development of the faults delimiting the unstable sector, suggests that their propagation occurred in more recent times with respect that of the Timpe fault system. Furthermore, this evidences a retrogressive upward propagation of the instability phenomena, which on the whole involve both the continental margin and the eastern sector of Etna volcano. Acknowledgment This work was funded by the Italian Dipartimento della Protezione Civile in the frame of the 2007–2009 agreement with Istituto Nazionale di Geofisica e Vulcanologia — INGV, project V4: “Hazard connected to the flank dynamics of Etna”. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.jvolgeores.2012.10.001. References Acocella, V., 2006. Regional and local tectonics at Erta Ale caldera, Afar (Ethiopia). Journal of Structural Geology 28, 1808–1820. Acocella, V., Neri, M., 2005. Structural features of an active strike-slip fault on the sliding flank of Mt. Etna (Italy). Journal of Structural Geology 27 (2), 343–355. Acocella, V., Behncke, B., Neri, M., D'Amico, S., 2003. Link between flank slip ad 2002–2003 eruption at Mt. Etna (Italy). Geophysical Research Letters 30 (24), 2286. http:// dx.doi.org/10.1029/2003GL018642. Alparone, S., Barberi, G., Bonforte, A., Maiolino, V., Ursino, A., 2011. Evidence of multiple strain fields beneath the eastern flank of Mt. Etna volcano (Sicily, Italy) deduced from seismic and geodetic data during 2003–2004. Bulletin of Volcanology 73 (7), 869–885. Alparone, S., Barberi, G., Cocina, O., Giampiccolo, E., Musumeci, C., Patanè, D., 2012a. Intrusive mechanism of the 2008–2009 Mt. Etna eruption: constraints by tomographic images and stress tensor analysis. Journal of Volcanology and Geothermal Research 229–230, 50–63.

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Alparone, S., Cocina, O., Gambino, S., Mostaccio, A., Spampinato, S., Tuvè, T., Ursino, A., 2012b. Seismological features of the Pernicana–Provenzana fault system (Mt. Etna, Italy) and implications for the dynamics of northeastern flank of the volcano. Journal of Volcanology and Geothermal Research. http://dx.doi.org/10.1016/j.jvolgeores.2012.03.010. Argnani, A., Mazzarini, F., Bonazzi, C., Bisson, M., Isola, I., 2012. The deformation offshore of Mount Etna as imaged by multichannel seismic reflection profiles. Journal of Volcanology and Geothermal Research. http://dx.doi.org/10.1016/j.jvolgeores.2012.04.016. Azzaro, R., 1997. Seismicity and active tectonics along the Pernicana fault, Mt. Etna (Italy). Acta Vulcanologica 9 (1/2), 7–14. Azzaro, R., 1999. Earthquake surface faulting at Mount Etna volcano (Sicily) and implications for active tectonics. Journal of Geodynamics 28, 193–213. Azzaro, R., 2004. Seismicity and active tectonics in the Etna region: constraints for a seismotectonic model. In: Bonaccorso, A., Calvari, S., Coltelli, M., Del Negro, C., Falsaperla, S. (Eds.), Mt. Etna: volcano laboratory: American Geophysical Union, Geophysical Monograph, 143, pp. 205–220. http://dx.doi.org/10.1029/GM143. Azzaro, R., Ferreli, L., Michetti, A.L., Serva, L., Vittori, E., 1998. Environmental hazard of capable faults: the case of the Pernicana fault (Mt. Etna, Sicily). Natural Hazards 17 (2), 147–162. Azzaro, R., Mattia, M., Puglisi, G., 2001. Fault creep and kinematics of the eastern segment of the Pernicana Fault (Mt. Etna, Italy) derived from geodetic observation and their tectonic significance. Tectonophysics 333, 401–415. Azzaro, R., Castelli, V., D'Amico, S., 2010. Compiling a catalogue of Mount Etna earthquakes from 1600 to 1831. Proc. of 32nd ESC General Assembly, Montpellier (France), September 6–10 2010, p. 154. SD4/P9/ID69. Azzaro, R., D'Amico, S., Tuvè, T., 2011. Estimating the magnitude of historical earthquakes from macroseismic intensity data: new relationships for the volcanic region of Mount Etna (Italy). Seismological Research Letters 82 (4), 520–531. Azzaro, R., Branca, S., Gwinner, K., Coltelli, M., 2012a. The volcano-tectonic map of Etna volcano, 1:100.000 scale: morphotectonic analysis from high-resolution DEM integrated with geologic, active faulting and seismotectonic data. Italian Journal of Geosciences (Boll. Soc. Geol. It.) 131 (1), 153–170. Azzaro, R., D'Amico, S., Peruzza, L., Tuvè, T., 2012b. Probabilistic seismic hazard at Mt. Etna (Italy): the contribution of local fault activity in mid-term assessment. Journal of Volcanology and Geothermal Research. http://dx.doi.org/10.1016/j.jvolgeores.2012.06.005. Barberi, G., Cocina, O., Maiolino, V., Musumeci, C., Privitera, E., 2004. Insight into Mt. Etna (Italy) kinematics during the 2002–2003 eruption as inferred by seismic stress and strain tensors. Geophysical Research Letters 31, L21614. http://dx.doi.org/10.1029/ 2004GL020918. Barreca, G., Bonforte, A., Neri, M., 2012. A GIS tool for integrated hazard evaluation on the faults of Mt. Etna (Sicily). Journal of Volcanology and Geothermal Research. http:// dx.doi.org/10.1016/j.jvolgeores.2012.08.013. Berardino, P., Fornaro, G., Lanari, R., Sansosti, E., 2002. A new algorithm for surface deformation monitoring based on small baseline differential SAR interferograms. IEEE Transactions on Geoscience and Remote Sensing 40, 2375–2383. Bonaccorso, A., Bonforte, A., Guglielmino, F., Palano, M., Puglisi, G., 2006. Composite ground deformation pattern forerunning the 2004–2005 Mount Etna eruption. Journal of Geophysical Research 111 (B12207). http://dx.doi.org/10.1029/2005jb004206. Bonaccorso, A., Bonforte, A., Currenti, G., Del Negro, C., Di Stefano, A., Greco, F., 2011. Magma storage, eruptive activity and flank instability: inferences from ground deformation and gravity changes during the 1993–2000 recharging of Mt. Etna volcano. Journal of Volcanology and Geothermal Research 200, 245–254. Bonforte, A., Puglisi, G., 2003. Magma uprising and flank dynamics on Mount Etna volcano, studied using GPS data (1994–1995). Journal of Geophysical Research 108, 2153. http://dx.doi.org/10.1029/2002jb001845. Bonforte, A., Puglisi, G., 2006. Dynamics of the eastern flank of Mt. Etna volcano (Italy) investigated by a dense GPS network. Journal of Geophysical Research 153, 357–369. Bonforte, A., Ferretti, A., Prati, C., Puglisi, G., Rocca, F., 2001. Calibration of atmospheric effects on SAR interferograms by GPS and local atmosphere models: first results. Journal of Atmospheric and Solar-Terrestrial Physics 63, 1343–1357. Bonforte, A., Branca, S., Palano, M., 2007a. Geometric and kinematic variations along the active Pernicana fault: implication for the dynamics of Mount Etna NE flank (Italy). Journal of Volcanology and Geothermal Research 160, 210–222. Bonforte, A., Gambino, S., Guglielmino, F., Obrizzo, F., Palano, M., Puglisi, G., 2007b. Ground deformation modeling of flank dynamics prior to the 2002 eruption of Mt. Etna. Bulletin of Volcanology 69, 757–768. Bonforte, A., Carbone, D., Greco, F., Palano, M., 2007c. Intrusive mechanism of the 2002 NE-rift eruption at Mt Etna (Italy) modelled using GPS and gravity data. Geophysical Journal International 169, 339–347. Bonforte, A., Bonaccorso, A., Guglielmino, F., Palano, M., Puglisi, G., 2008. Feeding system and magma storage beneath Mt. Etna as revealed by recent inflation/deflation cycles. Journal of Geophysical Research 113, B05406. http://dx.doi.org/10.1029/2007JB005334. Bonforte, A., Gambino, S., Neri, M., 2009. Intrusion of eccentric dikes: the case of the 2001 eruption and its role in the dynamics of Mt. Etna volcano. Tectonophysics 471, 78–86. Bonforte, A., Guglielmino, F., Coltelli, M., Ferretti, A., Puglisi, G., 2011. Structural assessment of Mount Etna volcano from Permanent Scatterers analysis. Geochemistry, Geophysics, Geosystems 12, Q02002. http://dx.doi.org/10.1029/2010GC003213. Bonforte, A., Carnazzo, A., Gambino, S., Guglielmino, F., Obrizzo, F., Puglisi, G., 2012a. A multidisciplinary study of an active fault crossing urban areas: the Trecastagni Fault at Mt. Etna (Italy). Journal of Volcanology and Geothermal Research. http:// dx.doi.org/10.1016/j.jvolgeores.2012.05.001. Bonforte, A., Federico, C., Giammanco, S., Guglielmino, F., Liuzzo, M., Neri, M., 2012b. Soil gases and SAR data reveal hidden faults on the sliding flank of Mt. Etna (Italy). Journal of Volcanology and Geothermal Research. http://dx.doi.org/ 10.1016/j.jvolgeores.2012.08.010. Borgia, A., Ferrari, L., Pasquarè, G., 1992. Importance of gravitational spreading in the tectonic and volcanic evolution of Mount Etna. Nature 357, 231–235.

Borgia, A., Lanari, R., Sansosti, E., Tesauro, M., Berardino, P., Fornaro, G., Neri, M., Murray, J.B., 2000. Actively growing anticlines beneath Catania from the distal motion of Mount Etna's decollement measured by SAR interferometry and GPS. Geophysical Research Letters 27, 3409–3412. Bousquet, J.C., Lanzafame, G., 2004. The tectonics and geodynamics of Mt. Etna: synthesis and interpretation of geological and geophysical data. In: Bonaccorso, A., Calvari, S., Coltelli, M., Del Negro, C., Falsaperla, S. (Eds.), Mt. Etna: Volcano Laboratory: American Geophysical Union, Geophysical Monograph, 143, pp. 29–47. http://dx.doi.org/ 10.1029/GM143. Branca, S., Coltelli, M., Groppelli G., Lentini, F., 2011. Geological map of Etna volcano, 1:50,000 scale. Italian Journal of Geosciences (Boll. Soc. Geol. It.) 130 (3), 265–291. Carbone, D., D'Amico, S., Musumeci, C., Greco, F., 2009. Comparison between the 1994–2006 seismic and gravity data from Mt Etna: new insight into the long-term behaviour of a complex volcano. Earth and Planetary Science Letters 279 (282292). http://dx.doi.org/10.1016/j.epsl.2009.01.007. Chiarabba, C., De Gori, P., Patanè, D., 2004. The Mt. Etna plumbing system: the contribution of seismic tomography. In: Bonaccorso, A., Calvari, S., Coltelli, M., Del Negro, C., Falsaperla, S. (Eds.), Mt. Etna: Volcano Laboratory: American Geophysical Union, Geophysical Monograph, 143, pp. 191–204. Chiocci, L.F., Coltelli, M., Bosman, A., Cavallaro, D., 2011. Continental margin large-scale instability controlling the flank sliding of Etna volcano. Earth and Planetary Science Letters 305 (1–2), 57–64. CMTE Working Group, 2008. Catalogo Macrosismico dei Terremoti Etnei, 1832–2008. Istituto Nazionale di Geofisica e Vulcanologia, Catania http://www.ct.ingv.it/ufs/ macro/. Currenti, G., Del Negro, C., Ganci, G., Williams, A., 2008. Static stress changes induced by the magmatic intrusions during the 2002–2003 Etna eruption. Journal of Geophysical Research 113. http://dx.doi.org/10.1029/2007JB005301. Currenti, G., Bonaccorso, A., Del Negro, C., Guglielmino, F., Scandura, D., Boschi, E., 2010. FEM-based inversion for heterogeneous fault mechanisms: application at Etna volcano by DInSAR data. Geophysical Journal International. http://dx.doi.org/10.1111/ j.1365-246X.2010.04769.x. Ferretti, A., Prati, C., Rocca, F., 2001. Permanent scatterers in SAR interferometry. IEEE Transactions on Geoscience and Remote Sensing 39, 8–20. Finetti, I.R., Lentini, F., Carbone, S., Del Ben, A., Di Stefano, A., Forlin, E., Guarnieri, P., Pipan, M., Prizzon, A., 2005. Geological outline of Sicily and lithospheric tectono-dynamics of its Tyrrhenian margin from new CROP seismic data. In: Finetti, I.R. (Ed.), CROP, deep seismic exploration of the Mediterranean region. Elsevier, pp. 319–376. Froger, J.L., Merle, O., Briole, P., 2001. Active spreading and regional extension at Mount Etna imaged by SAR interferometry. Earth and Planetary Science Letters 187, 245–258. Gambino, S., Bonforte, A., Carnazzo, A., Falzone, G., Ferrari, F., Ferro, A., Guglielmino, F., Laudani, G., Maiolino, V., Puglisi, G., 2011. Displacement across the Trecastagni Fault (Mt. Etna) and induced seismicity: the October 2009 to January 2010 episode. Annals of Geophysics 54 (4), 414–423. Groppelli, G., Tibaldi, A., 1999. Control of rock rheology on deformation style and slip-rate along the active Pernicana fault, Mt. Etna, Italy. Tectonophysics 305 (4), 521–537. Gruppo Analisi Dati Sismici, 2012. Catalogo dei terremoti della Sicilia Orientale– Calabria Meridionale (1999–2011). Istituto Nazionale di Geofisica e Vulcanologia, Catania http://www.ct.ingv.it/ufs/analisti/catalogolist.php. Guglielmino, F., Nunnari, G., Puglisi, G., Spata, A., 2011a. Simultaneous and integrated strain tensor estimation from geodetic and satellite deformation measurements (SISTEM) to obtain three-dimensional displacements maps. IEEE Transactions on Geoscience and Remote Sensing 49, 1815–1826. Guglielmino, F., Bignami, C., Bonforte, A., Briole, P., Obrizzo, F., Puglisi, G., Stramondo, S., Wegmuller, U., 2011b. Analysis of satellite and in situ ground deformation data integrated by the SISTEM approach: the April 3, 2010 earthquake along the Pernicana fault (Mt. Etna — Italy) case study. Earth and Planetary Science Letters 312 (3–4), 327–336. Houlie, N., Briole, P., Bonforte, A., Puglisi, G., 2006. Large scale ground deformation of Etna observed by GPS between 1994 and 2001. Geophysical Research Letters 33. http:// dx.doi.org/10.1029/2005gl024414. Laigle, M., Hirn, A., Sapin, M., Lepine, J.C., Diaz, J., Gallart, J., Nicolich, R., 2000. Mount Etna dense array local earthquake P and S tomography and implications for volcanic plumbing. Journal of Geophysical Research 105 (B9), 21633–21646. Lentini, F., Carbone, S., Guarnieri, P., 2006. Collisional and postcollisional tectonics of the Apenninic–Maghrebian orogen (southern Italy). In: Dilek, Y., Pavlides, S. (Eds.), Postcollisional Tectonics and Magmatism in the Mediterranean Region and Asia: The Geological Society of America, Special Papers, 409, pp. 57–81. Lo Giudice, E., Rasà, R., 1992. Very shallow earthquakes and brittle deformation in active volcanic areas: the Etnean region as an example. Tectonophysics 202, 257–268. Lundgren, P., Casu, F., Manzo, M., Pepe, A., Berardino, P., Sansosti, E., Lanari, R., 2004. Gravity and magma induced spreading of Mount Etna volcano revealed by satellite radar interferometry. Geophysical Research Letters 31, L04602. http://dx.doi.org/ 10.1029/2003GL018736. Massonnet, D., Feigl, K.L., 1995. Discrimination of geophysical phenomena in satellite radar interferograms. Geophysical Research Letters 22, 1537–1540. McGuire, W.J., Pullen, A.D., 1989. Location and orientation of eruptive fissures and feeder-dykes at Mount Etna; influence of gravitational and regional tectonic stress regimes. Journal of Volcanology and Geothermal Research 38, 325–344. McGuire, W.J., Moss, J.L., Saunders, S.J., Stewart, I.S., 1996. Dyke-induced rifting and edifice instability at Mount Etna. In: Gravestock, P.J., McGuire, W.J. (Eds.), Etna: 15 Years On. ODA/Cheltenham & Gloucester College of Higher Education, pp. 20–24. McGuire, W.J., Saunders, S.J., Stewart, I.S., 1997. Intra-volcanic rifting at Mount Etna in the context of regional tectonics. Acta Vulcanologica 9 (1/2), 147–156. Merle, O., Borgia, A., 1996. Scaled experiments of volcanic spreading. Journal of Geophysical Research 101, 13805–13817.

R. Azzaro et al. / Journal of Volcanology and Geothermal Research 251 (2013) 5–15 Monaco, C., Tapponnier, P., Tortorici, L., Gillot, P.Y., 1997. Late Quaternary slip rates on the Acireale–Piedimonte normal faults and tectonic origin of Mt. Etna (Sicily). Earth and Planetary Science Letters 147, 125–139. Montalto, A., Vinciguerra, S., Menza, S., Patanè, G., 1996. Recent Seismicity of Mount Etna: Implication for Flank Instability. In: McGuire, W.J., Jones, A.P., Neuberg, J. (Eds.), Volcano instability on the Earth and other planets: Geological Society of London, Special Publications, 110, pp. 169–177. Nakamura, K., 1977. Volcanoes as possible indicators of tectonic stress. Journal of Volcanology and Geothermal Research 2, 1–16. Neri, M., Acocella, V., Behncke, B., 2004. The role of the Pernicana fault system in the spreading of Mount Etna (Italy) during the 2002–2003 eruption. Bulletin of Volcanology 66, 417–430. Neri, M., Acocella, V., Behncke, B., Maiolino, V., Ursino, A., Velardita, R., 2005. Contrasting triggering mechanisms of the 2001 and 2002–2003 eruptions of Mount Etna (Italy). Journal of Volcanology and Geothermal Research 144, 235–255. Neri, M., Guglielmino, F., Rust, D., 2007. Flank instability on Mount Etna: radon, radar interferometry and geodetic data from the southern boundary of the unstable sector. Journal of Geophysical Research 112. http://dx.doi.org/10.1029/2006JB004756. Norini, G., Acocella, V., 2011. Analogue modeling of flank instability at Mount Etna: understanding the driving factors. Journal of Geophysical Research 116, B07206. http:// dx.doi.org/10.1029/2011JB008216. Norini, G., Lagmay, A.M.F., 2005. Deformed symmetrical volcanoes. Geology 33, 605–608. Obrizzo, F., Pingue, F., Troise, C., De Natale, G., 2001. Coseismic displacements and creeping along the Pernicana fault (Etna, Italy) in the last 17 years: a detailed study of a tectonic structure on a volcano. Journal of Volcanology and Geothermal Research 109, 109–131. Onn, F., Zebker, H.A., 2006. Correction for interferometric synthetic aperture radar atmospheric phase artifacts using time series of zenith wet delay observations from a GPS network. Journal of Geophysical Research 111, B09102. http://dx.doi.org/10.1029/ 2005jb004012. Pasquaré, F., Tibaldi, A., 2003. Do transcurrent faults guide volcano growth? The case of NW Bicol Volcanic Arc, Luzon, Philippines. Terra Nova 15, 204–212. Patanè, D., Privitera, E., Gresta, S., Akinci, A., Alparone, S., Barberi, G., Chiaraluce, L., Cocina, O., D'Amico, S., De Gori, P., Di Grazia, G., Falsaperla, S., Ferrari, F., Gambino, S., Giampiccolo, E., Langer, H., Maiolino, V., Moretti, M., Mostaccio, A., Musumeci, C., Piccinini, D., Reitano, D., Scarfì, L., Spampinato, S., Ursino, A., Zuccarello, L., 2003. Seismological constraints for the dyke emplacement of the July–August 2001 lateral eruption at Mt. Etna volcano, Italy. Annals of Geophysics 46 (4), 599–608. Puglisi, G., Bonforte, A., 2004. Dynamics of Mount Etna volcano inferred from static and kinematic GPS measurements. Geophysical Research Letters 109, B11404. http:// dx.doi.org/10.1029/2003jb002878. Puglisi, G., Bonforte, A., Maugeri, S.R., 2001. Ground deformation patterns on Mount Etna, 1992 to 1994, inferred from GPS data. Bulletin of Volcanology 62, 371–384. Puglisi, G., Briole, P., Bonforte, A., 2004. Twelve years of ground deformation studies on Mt. Etna volcano based on GPS surveys. In: Bonaccorso, A., Calvari, S., Coltelli, M.,

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Del Negro, C., Falsaperla, S. (Eds.), Mt. Etna: Volcano Laboratory: American Geophysical Union, Geophysical Monograph, 143, pp. 321–341. http://dx.doi.org/ 10.1029/GM143. Puglisi, G., Bonforte, A., Ferretti, A., Guglielmino, F., Palano, M., Prati, C., 2008. Dynamics of Mount Etna before, during, and after the July–August 2001 eruption inferred from GPS and differential synthetic aperture radar interferometry data. Journal of Geophysical Research 113, B06405. http://dx.doi.org/10.1029/2006JB004811. Rasà, R., Azzaro, R., Leonardi, O., 1996. Aseismic creep on faults and flank instability at Mount Etna volcano, Sicily. In: McGuire, W.J., Jones, A.P., Neuberg, J. (Eds.), Volcano Instability on the Earth and Other Planets: Geological Society of London, Special Publications, 110, pp. 179–192. Rust, D., Neri, M., 1996. The boundaries of large-scale collapse on the flanks of Mount Etna, Sicily. In: McGuire, W.J., Jones, A.P., Neuberg, J. (Eds.), Volcano Instability on the Earth and Other Planets: Geological Society of London, Special Publications, 110, pp. 193–208. Rust, D., Behncke, B., Neri, M., Ciocanel, A., 2005. Nested zones of instability in the Mount Etna volcanic edifice, Sicily. Journal of Volcanology and Geothermal Research 155, 137–153. Siniscalchi, A., Tripaldi, S., Neri, M., Balasco, M., Romano, G., Ruch, J., Schiavone, D., 2012. Flank instability structure of Mt. Etna inferred by a magnetotelluric survey. Journal of Geophysical Research 117, B03216. http://dx.doi.org/10.1029/2011JB008657. Solaro, G., Acocella, V., Pepe, S., Ruch, J., Neri, M., Sansosti, E., 2010. Anatomy of an unstable volcano from InSAR: multiple processes affecting flank instability at Mt. Etna, 1994–2008. Journal of Geophysical Research 115, B10405. http://dx.doi.org/ 10.1029/2009JB000820. Tibaldi, A., 2005. Volcanism in compressional tectonic settings: is it possible? Geophysical Research Letters 32, L06309. http://dx.doi.org/10.1029/2004GL021798. Tibaldi, A., Groppelli, G., 2002. Volcano-tectonic activity along structures of the unstable NE flank of Mt. Etna (Italy) and their possible origin. Journal of Volcanology and Geothermal Research 115, 277–302. Tibaldi, A., Lagmay, A.M.F., 2006. Interaction between volcanoes and their basement. Journal of Volcanology and Geothermal Research 158, 1–5. Tibaldi, A., Lagmay, A.M.F., Ponomareva, V., 2005. Effects of basement structural and stratigraphic heritages on volcano behaviour and implications for human activities. Episodes 28 (3), 158–170. Walter, T.R., Acocella, V., Neri, M., Amelung, F., 2005. Feedback processes between magmatic events and flank movement at Mount Etna (Italy) during the 2002–2003 eruption. Journal of Geophysical Research 110, B10205. http://dx.doi.org/10.1029/2005JB003688. Yeats, R.S., Sieh, K., Allen, C.R., 1997. The Geology of Earthquakes. Oxford University Press, New York . 568 pp. Zebker, H.A., Villasenor, J., 1992. Decorrelation in interferometric radar echoes. IEEE Transactions on Geoscience and Remote Sensing 30, 950–959. Zebker, H.A., Rosen, P.A., Hensley, S., 1997. Atmospheric effects in interferometric synthetic aperture radar surface deformation and topographic maps. Journal of Geophysical Research 102, 7547–7563.