GNGTS 2013 - Atti del 32° Convegno Nazionale

transpressional mechanisms, mainly oriented from ENE to NW, either generally high-angle dipping and only in few cases low-angle. This evidence well fit the geological-structural data collected in the on-shore areas. The structural setting sketched by the integration of above mentioned geological and seismological data underlines an intense mechanical heterogeneity of the rock volumes where the strain is accommodated by fault arrays also superimposed on inherited geometries such as flat-ramp, thrusts and/or low-angle normal faults. The geometric characters of the rocks volumes seems to confirm the occurrence of a simple- shear deformational style according with a neotectonic model related to NW-SE trending maximum compressional stress axis producing a non-coaxial strain, even if in particular areas different seismogenic conditions are possible, due to the accommodation of the deformation leading a marked mechanical heterogeneity (Neri et al. , 2003, 2005). Recently some seismological data of the main seismic sequences affecting the Northern Sicilian sector and its offshore has been checked with the aim to improve the above mentioned seismotectonic model. Particularly, the 1992-1993 Pollina, 1998 S. Vito high, 2002 Palermo, 2009-2012 Caprileone seismic sequences have been analyzed. More events of the above mentioned seismic sequences were relocated using a process based on the HYPODD program (Waldhauser, 2001) and an eight layers velocity model optimized relocating some thousands events of the Southern Tyrrhenian (Chironi et al. , 2000). Moreover, new focal mechanisms were calculated using the FPFIT code (Reasenberg and Oppenheimer, 1985). The hypocentral relocation allowed well defining the seismogenic volume geometries involved in the analyzed seismic sequences. The new focal mechanisms confirm a large structural heterogeneity of seismogenic volumes as observed in the structural analysis in the onshore sector of the chain (Fig. 3). These new analysis of seismological data seems to confirm the congruence of the seismo- tectonic model of the hinge zone, where a main NW-SE stress field is still active, which induce a non-coaxial deformation expressed by a complex strike slip system: NE-SW/N-S trending antithetic, and WNW-ESE/W-E trending synthetic, both transpressional or transtensional . Fig. 3 – Focal mechanisms of the main events of the seismic sequences calculated by the FPFIT code. 156 GNGTS 2013 S essione 1.2