GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.1 movements. The Fiandaca, S. Tecla and S. Venerina faults displace the middle-low part of the eastern flank with a NW-SE strike and show prevailing right-lateral features. Their intense tectonic activity is confirmed by high ����������� ������� slip-rates, varying from 1.0 to 4.3 mm/yr (see Azzaro et al. , 2013a for an overview), as well as complex pattern of ground deformation, decennial time series (GPS, SAR) showing inside TFS kinematic domains with different velocities and displacements (Bonforte et al. , 2011). Basically, the fairly constant mid-term (decennial) ESE seaward sliding is interrupted by sudden short-term (months to year) accelerations related to flank eruptions. These faults are highly seismogenic representing the sources of the strongest earthquakes reported in the local seismic catalogue for the last centuries (CMTE Working Group, 2014). With a long-term behavior (~200 years) characterised by a mean recurrence time of about 20 years for severe/destructive events (epicentral intensity I 0 ≥ VIII EMS, corresponding to magnitude M w ≥ 4.6), the seismic potential of the Timpe fault system is highly significant in terms of local seismic hazard (Azzaro et al. , 2013b). It has to be stressed that these shallow earthquakes are accompanied by extensive phenomena of surface faulting, with end-to-end ruptures up to 6.5 km long and vertical offsets up to 90 cm (Azzaro, 1999). The coseismic evidence provides reliable information on the geometry of the causative fault segment and associated kinematics. In a more general framework, these earthquakes are the strongest events of a seismicity located mainly within the first 7 km of crust (Patane et al. , 2004; Alparone et al. , 2011; Alparone et al. , 2013; Alparone et al. , 2015), while the western sector of Etna is characterized by higher focal depths (10-30 km) and lesser seismic rate (Sicali et al. , 2014). Coulomb stress changes modeling. It has long been recognized that while an earthquake produces a net reduction of regional stress, earthquakes also are responsible of stress increase, therefore resulting in i) a redistribution of the stress in the surrounding rock volume and ii) an alteration of the shear and normal stress on surrounding faults. Depending on the critical state of failure, sites of positive static stress changes (≥ 0.1 bars) may be foci of future events (Stein, 1999). Spatial and temporal relationships between stress changes and earthquakes are commonly explained through the Coulomb failure stress change defined as (���������� ��� Reasenberg and Simpson, 1992)�: ∆��� �� ��� �� �� ��� CFS �� ∆τ� (∆σ n ���∆P) where ∆ is the shear stress change computed in the direction of slip on the fault, ��∆σ n is the normal stress changes (positive for extension), μ is the coefficient of friction and ��∆�P is the pore pressure change (King et al. , 1994; Harris, 1998; King and Cocco, 2000). For simplicity, we Fig. 1 – Active fault map of Mt. Etna. The main seismogenic structures are in bold; abbreviations indicate: FF, Fiandaca fault; MF, Moscarello f.; SLF, S. Leonardello f.; STF, S. Tecla f.; SVF, S. Venerina f. The contour of the rift zones is in brown; arrows indicate the horizontal regional σ1 stress field (from Patanè and Privitera, 2001). Stars stand for the 1865 earthquakes case-history: red, July event; orange, August event. Inset map a) shows the simplified geological setting of eastern Sicily: AMC, Apenninic-Maghrebian Chain; HF, Hyblean Foreland.