ACTIVE THRUST TECTONICS IN WESTERN SICILY (SOUTHERN ITALY) - THE 1968BELICE EARTHQUAKE SEQUENCE

The 1968 Belice earthquake sequence, characterized by six main shocks with 5 < M < 5.4, represents the strongest seismic event recorded in w estern Sicily in historical times, The epicentral area is located in t he Belice Valley, a region lacking any topographic lineament likely to result from a fault with significant offsets of any kind. Instrumenta l data show that hypocentres of the major shucks are distributed along a roughly N-dipping plane extending from about 36 km to 1 km depth. F ault plane solutions show pure thrusting mechanisms on N-dipping, ENE- trending planes, or oblique slip with a right-lateral component of mot ion along steep WSW-dipping planes, both as a result of approximate N- S shortening. The observed destruction indicates that isoseismal areas are elongated in an ENE direction. Similarly, the epicentral distribu tion of events with M greater than or equal to 4 outlines a roughly el liptical ENE-elongated area located 20 km NW of the Sciacca-Rocca Ficu zza thrust front. This ENE-striking structure, representing the region al morphotectonic feature closest to the epicentral area, consists of two main imbricate fan systems. In the southernmost system, Quaternary deposits (tentatively dated as 1.0-0.7 Myr old) are involved In a lar ge ramp anticline uplifting them to a maximum altitude of 346 m. The o ccurrence of Holocene lacustrine piggy-back basins on the rear of this structure also indicates late Quaternary activity of the underlying t hrust. Seismological, structural and morphotectonic observations sugge st that multiple ruptures might have occurred during the 1968 sequence on a blind crustal thrust ramp located beneath the epicentral area. S lip propagated southwards along the shallow ramp-flat system character izing the thin-skinned foreland fold and thrust belt of southwestern S icily, being dispersed in flexural folding processes and diffuse strai n along this path.