GNGTS 2013 - Atti del 32° Convegno Nazionale

a total of 6131 P and 3668 S absolute arrival times and 52980 P and 30138 S catalogue-derived differential times. We performed the data inversion with an horizontal grid of 7x7 km node spacing, covering an area of 110 x 110 km, and a vertical step varying between 3 and 6 km, from the surface to 28 km of depth, following the one-dimensional reference velocity model of Musumeci et al. (2003). This mesh configuration allows to achieve a better spatial resolu- tion with respect the other tomographic studies carried out in the area, also ensuring a good over-determination factor (i.e. the ratio between known and unknown parameters). The finely tuned velocity distribution was used to relocate by TomoDDPS the whole dataset of events re- corded in the studied region. This provided an improvement in the quality of the final locations and an higher clustering of the earthquakes, which emphasizes some lineaments (Fig. 1b). As we can observe, tomographic images (Fig. 1c) reveal several sharp lateral velocity perturbations pointing out meaningful discontinuities in the crust which can be related to major tectonic structures. In particular, a low velocity anomaly was detected in the north- western sector (profile A), about corresponding with the Gela-Catania Foredeep. Location and geometry of this feature suggest that it may be related with the Hyblean Foreland crust bending beneath the outermost edifice of the Maghrebian Chain. Coastal and offshore zones (from Catania to Siracusa) and the western sector are also accompanied by a considerable velocity contrasts. These anomalies likely mark the known fault systems, striking about NW- SE, such as the HMEFS and those bounding the Augusta basin and the Scicli-Ragusa Fault System, respectively. Current results, compared to the previous tomographic inversion of Scarfì et al. (2007), obtained with a similar calculation code (TomoADD: Zhang and Thurber, 2005), maintain the fundamental characteristics but at the same time show a greater detail, this thanks to the larger amount of available data, which results in an higher density of seismic rays sampling the spatial mesh. Looking at the 3D event locations, to the north and northwest, earthquakes are distributed along the edge between the foreland and the chain, with depth increasing towards NW, down to about 40 km, to outline the flexure of the Hyblean crust below the thin-skinned outermost chain. Towards south, in the western district, the epicentres finely overlap to the Scicli-Ragusa Fig. 2 – Computed focal mechanisms. Red, strike-slip fault; blue, normal fault; black, inverse fault. Insets show the stereonets of the P and T-axes (a) and the orientations (stars) of the principal stress axes (b), with the 95% confidence limits (grey areas). 217 GNGTS 2013 S essione 1.2