A contribution to a seismotectonic model of the hinge zone between northern Sicily and southern Tyrrhenian by some seismological and structural constraints

An integrated research on the hinge zone between northern Sicily and its Tyrrhenian off-shore has been carried out, by means of structural, seismological and statistical analyses. The main structural features of the area are represented by both NWSE to W–E striking right-lateral and N–S to NE-SW striking leftlateral fault systems, responsible of uplifting (restraining) and subsiding (releasing) zones. Within this hinge zone the seismicity is mainly located between 10 and 20 km of depth and clustered according to the orientation of the outcropping fault systems. The brittle strain partitioning related to a right-lateral simple shear zone has been reconstructed in the onshore sector, while in the offshore the structural setting has been inferred from morpho-bathymetric and geological maps of the southern Tyrrhenian basin. Moreover, in this latter zone, a new seismological study has been carried out on a part of 2002 seismic sequence (more than 600 events), using a database obtained by merging the CSI 1.1 catalogue of INGV and new data recorded by 7 three-components stations of a temporary network installed by INGV. These seismic events, occurred between September and December 2002, have been relocated using an eight layered velocity model optimized for the southern Tyrrhenian. The focal mechanisms of 40 events have been determined. The Coulomb stress changes related to the main event have been also calculated. The general geometric characters of the whole set of investigated parameters well fit the simple shear deformational style reconstructed of the onshore study zone. Moreover, both focal mechanisms and principal stress components of about 100 events of 1992-1993 Pollina seismic swarm (Eastern Madonie Mts.) has been also carried out. The main focal mechanisms typologies and stress axis trends show a good agreement with the seismogenic volume anisotropy required by the seismotectonic model. This model is also supported by the high resolution tomographic results of the more shallow structures in the Pollina area, and by the study of two recent minor seismic sequences affecting the northern Sicilian sector (2010-2012) in the Caprileone area (Messina) and in the Isola delle Femmine offshore (west of Palermo). In this last sector high resolution morphobatimetric (multibeam) data highlight the presence of very young morphological features related to the ENE-WSW and NNW-SSE trending structural systems, probably responsible of the recent events.