Geodetic and geological evidences of activetectonics in western Sicily (Italy): insights onthe seismogenic source of the 1968 Beliceearthquake sequence.

The night between the January 14th and 15th 1968, a wide area of western Sicily (Italy) was shaken by a strong earthquake, the main shock of a seismic crisis that hit the region until the January 25th. The disastrous event, that is the strongest seismic event recorded in Western Sicily in historical times, caused about 370 deaths and the destruction of many villages facing the Belice river valley, which was the epicentral area for this unexpected natural disaster. The seismic sequence consisted of a main shock with M=5.9 and a series of pre-shock and aftershocks that were clustered along a NE-SW alignment. Focal planes solutions provided by many authors in the last decades show controversial interpretations about the possible geometrical and kinematic pattern of the seismogenetic source. Computed focal solutions provide in fact different hypothesis on trend and kinematics of possible faulting mechanism that range from pure thrusting on NW-SE steeply dipping planes to right-lateral slip of NNW striking sub-vertical plane. This ambiguity remains at present-day still unresolved due to the fact that the 1968 earthquake sequence did not produced a typical seismic landscape and that, as result of low magnitude of events, coseismic fault ruptures were never observed until now. However, the lacking of any morphological expression and fault breaks at surface makes the 1968 Belice earthquakes sequence a geologically unconstrained event. Starting from the analysis of SAR interferometry (DInSAR) for western Sicily, which displays a differential ground motion near the Castelvetrano and Campobello di Mazara villages, new field surveys have been performed along the Belice Valley with the aim to verify possible correspondence between geodetic data and geological-morphological ones. In this respect, available geological maps have been upgraded by original field surveys, that were supported by the morphometric analysis of a 2x2m grid resolution DEM and by the interpretation of 1:10,000 scale aerial photographs. The re- measurement of a 20 years old GPS network has allowed to constrain the current deformation pattern of the area. Moreover, in correspondence of the offshore extension of the lineament marked by the interferometric data, a shaded relief representation of sea-floor bathymetry has been elaborated and available seismic lines have been analyzed and interpreted. Finally, the discovery of dislocated archaeological and recent marker near the epicentral area of the 1968 seismic sequence enabled us to provide, for the first time, the likely evidence of surface expression for coseismic faulting.