GNGTS 2014 - Atti del 33° Convegno Nazionale

34 GNGTS 2014 S essione 1.1 local reference frame computed to isolate the Mt. Etna volcanic deformation from the background regional tectonic pattern; see Palano et al. 2010 for details) by minimizing, for the above mentioned 10 IGS stations, the difference between their estimated positions and those implied by their coordinates (position and velocity) in the local reference frame. The resulting velocity field, reported in Fig. 3, illustrates the seaward motion of the eastern flank of Mt. Etna. In detail, sites close to the central sector of the unstable flank are characterized by velocities up to 35 mm/yr, while moving southward or northward the velocity field decreases to values of ca. 1-2 mm/yr. Geologic data. ��� ���� �� � �������� Mt. Etna is a basaltic stratovolcano locatedon the east coast of Sicily, at theboundarybetween theexternal unitsof the Apenninic-Maghrebian Chain and the flexured margin of the Hyblean Foreland (Branca et al. , 2011). This region is characterised by intense geodynamic processes at the scale of the volcano, flank instability which affects the eastern flank representing the result of interaction among regional stress regime, magma intrusion and basement geometry (Azzaro et al. , 2013). A number of active faults accommodate the large dynamics of the unstable sector: the right-lateral, NNW-SSE-striking Tremestieri-Trecastagni fault system (Fig. 1) is generally acknowledged as the southern boundary (Rasà et al. , 1996; Bonforte and Puglisi, 2003; Palano et al. , 2008; Bonforte et al. , 2011), although some authors suggest as an alternative the N-S-striking Ragalna fault system (Fig. 1) in the south-western flank of Etna (Borgia et al. , 1992; Rust and Neri, 1996; Neri et al. , 2007). Conversely, there is a general agreement on identifying the Pernicana fault system and the rift zones (North-East and Southern ones) as the northern and western boundaries of the unstable sector, respectively (Fig. 1). Insights into behaviour of the Pernicana fault became evident through geodetic data collected since 1997 (Azzaro et al. , 2001, Palano et al. , 2006), which evidenced a fairly stable slip with an average rate of about 28 mm/yr, related to the continuous long-term sliding motion of the eastern flank of the volcano toward the sea. Abrupt, even transient, increase in the seismic and geodetic strain release of the Pernicana fault has been measured during some recent flank eruptions (e.g. 2001 and 2002-03) and interpreted as a result of additional stressed induced by the magma ascent in the feeding system (Palano et al. , 2008; Alparone et al. , 2012; Bonaccorso et al. , 2013). The central part of the eastern flank is crossed by the Timpe fault system, a 20 km long and 5 km wide belt of mainly extensional structures, striking from N to NW and consisting of well- developed morphological scarps and hidden fault segments (Azzaro et al. , 2012). The relevant strain affecting this area is expressed by an intense seismicity - the largest earthquakes occurred in the past are located here - as well as fault creep, whose cumulated evidence in the long-term produces geological slip-rates varying from 1.0 to 5.3 mm/yr. The present behaviour of this fault zone has been obtained by the analysis of ground deformation decennial time series (GPS, SAR), that allowed to recognise as the faults bound kinematic domains showing different velocity and displacements, although all are characterised by discontinuous dynamics. In practise, the fairly constant mid-term (decennial) ESE seaward sliding is interrupted by sudden short-term (months to year) accelerations related to flank eruptions (Palano et al. , 2008; Bonforte et al. , 2011). Fig. 3 – Mid-term (2005.00-2013.99) geodetic velocity field and 95% confidence ellipses. The velocity field is referred to the local Etn@ref reference frame [see Palano et al. (2010) for details].