GNGTS 2014 - Atti del 33° Convegno Nazionale

discrete segments of right lateral strike-slip faults that appear to control the path of major rivers and the clockwise rotations of previous structures. The left-stepping en-chelon configuration of some segments in the Mt. Judica area produced push-up structures such as thrust-related ridges (Monaco and De Guidi, 2006). Seismological data. In order to study the seismotectonic pattern of the studied region, we worked on a seismic dataset extracted from the Italian seismic database of Istituto Nazionale di Geofisica e Vulcanologia (available online at http://csi.rm.ingv.it/ and http://bollettinosismico. rm.ingv.it/index.php for the 1981–2001 and 2003–2012 periods, respectively). Selected earthquakes, with minimum 10 P- and S-wave readings and magnitude of 1.0, were used as data source for a simultaneous inversion of a 3-D velocity structure and the hypocentre parameters. This resulted in more accurate foci locations and into mapping the velocity anomalies, which, being strictly dependent on the crustal structure, is of help in the seismotectonic interpretation. The seismic velocity modelling of Sicily was carried out by applying the LOTOS algorithm by Koulakov (2009). This code, which allows to determine the VP, VS and VP/VS, has some important features, such as quasi-continuous parameterization, which makes the resulting model grid independent. In particular, the grid nodes are installed according to the distribution of rays in the 1D model. The spacing of the grid is kept considerably smaller than the expected size of the anomalies in order to reduce the bias of the resulting models due to the grid configuration. Moreover, in order to further decrease the influence of the parameterization on the results, the inversion is repeated using several grids configurations with different basic orientations (e.g., 0°, 22°, 45°, and 66°). The results obtained for these grids are combined into one model by simple averaging. The obtained velocity images and the foci distribution depict a relevant Fig. 1 – Shaded relief rapresentation of central eastern Sicily with draped the major Mio-Pliocene compressive structures (thrust and synclyne). These tectonic features show a hock-shape geometry at their NE termination. This geometric configuration has been related to clockwise structural rotations developed alongside bounding wrench fault. 130 GNGTS 2014 S essione 1.2