GNGTS 2013 - Atti del 32° Convegno Nazionale

shear) between the two blocks, respectively. In addition, while the western foreland crustal block is limited by the front of the chain to the west and north and its seismic activity is mostly related to the Scicli-Ragusa Fault System, the eastern part moves bounded by two main regional tectonic structures, the Scicli-Ragusa Fault System and the Hybleo-Maltese Escarpment Fault System. The activity of the latter is well evidenced by the distribution of hypocenters along the Ionian coast, from the Gulf of Catania up to several kilometres south of Siracusa, with prevailing trastensive kinematics. The brittle deformation of the eastern block seems to be confined inside the Plateau, where most of the seismic events occur. Focal mechanisms computed for this area, are characterized by prevailing transtensive solutions with a nodal plane prevalently oriented WNW-ESE to NW-SE, fitting with the direction emphasized by the location of some event clusters (Fig. 1b) and in agreement with the main geological features ( e.g. Catalano et al. , 2008). Moreover, the transtensive nature of this area is well depicted also by GPS measurements (i.e. the horizontal strain-rate axes have comparable magnitude). Southward, the earthquakes are confined to the edge of the Plateau, where the tomography shows a discontinuity, down to 15 km of depth. From the morphological point of view, this area exhibits an escarpment (100-400 m) delimiting the Hyblean Plateau from the southern coastal plain, which several authors relate to a fault NE-SW oriented (i.e. the Avola Fault, see Catalano et al. , 2008). GPS data reveal also that a contractional belt is the overriding feature of the the Foredeep- Chain edge zone. In particular, an higher shortening, with respect the western side of the Scicli-Ragusa Fault System, was found along an area extending from the Scordia-Lentini Graben to the lower southern slope of Mt. Etna. There, several geological studies have shown that normal faults controlling the graben have been reactivated by reverse motion during the last 0.85 Myr (e.g . Bousquet and Lanzafame, 2004; Catalano et al. , 2008). We showed that the earthquakes deepen below the chain, down to 40 km, moving from the plateau towards NW; a clear signal of an active foredeep. On the other hand, however, very few events occurred in the Scordia-Lentini Graben area. This raises the question whether the lacking of seismic energy release, compared to the pronounced shallow deformation, can be correlated to a seismic gap, which would result in a very high seismic hazard for the area. Nevertheless, an hypothesis can be advanced to explain the different deformation pattern, between the western and eastern sectors of the foredeep-chain edge zone, discussed above. This hypothesis implies that the phenomenon can be connected with the thick and ductile layer of alluvial sediments that fill the graben and deform more rapidly than the western area with more strength and brittle crust. This hypothesis could better reconcile both the higher strain rate and the low occurrence of earthquakes. References Amato A., Azzara R., Basili A., Chiarabba C., Cocco M., Di Bona M. and Selvaggi G.; 1995: Main shock and aftershocks of the December 13, 1990, Eastern Sicily earthquake. Ann. Geofis., 38, 255-266. Boccaletti M., Cello G. and Tortorici L.; 1990: Strike-slip deformation as a fundamental process during the Neogene- Quaternary evolution of the Tunisian-Pelagian area. Ann. Tecton., 4, 104-119. Boschi E., Guidoboni E., Ferrari G., Mariotti D. and Valensise G.; 2000: Catalogue of strong Italian earthquakes. Ann. Geofis., 43, 268 pp. Bousquet J.C. and Lanzafame G.; 2004: Compression and Quaternary tectonic inversion on the Northern edge of the Hyblean Mountains, foreland of the Appenine-Maghrebian chain in Eastern Sicily (Italy): geodynamic implications for Mt. Etna. GeoActa, 3, 165-177. Butler R.W.H., Grasso M. and La Manna F.; 1992: Origin and deformation of the Neogene-Recent Maghrebian foredeep at the Gela Nappe, SE Sicily. J. Geol. Soc. London, 149, 547-556. Catalano S., De Guidi G., Romagnoli G., Torrisi S., Tortorici G. and Tortorici L.; 2008: The migration of plate boundaries in SE Sicily: in uence on the large-scale kinematic model of the African promontory in southern Italy. Tectonophysics, 449, 41-62, doi:10.1016/j.tecto.2007.12.003. Faccenna C., Becker T.W., Lucente F.P., Jolivet L. and Rossetti F.; 2001: History of subduction and back-arc extension in the central Mediterranean. Geophys. J. Int., 145, 809-820, doi:10.1046/j.0956-540X.2001.01435.x. 220 GNGTS 2013 S essione 1.2