The 1998 offshore NW Sicily earthquakes in the tectonic framework of the southern border of the Tyrrhenian Sea

Abstract

The seismic sequence which occurred in 1998 south of the Island of Ustica (offshore North Sicily) has been interpreted in the frame of historical recent seismicity and the area’s tectonics. This seismic sequence, characterised by shallow-depth and low-magnitude earthquakes (Md max. 4.3), took place in the thick and complex stack of the Sicilian-Maghrebian fold-and-thrust belt. The spatial distribution of the epicentres recorded during the January-August 1998 shows a cluster roughly NW-SE trending. The few shocks which occurred immediately after the Md 4.1 shock of the 14th September were located south-east of Ustica with a rough NE-SW trend. The focal mechanisms of major shocks are of a thrust type with horizontal compressive axes generally N-S trending. In the kinematic evolution of the study area, alternating extensional and contractional events have been recognised as having taken place during the Plio-Pleistocene. The present day seismic activity pointing out a new contractional episode is well framed in this evolutionary trend. The occurrence of pre-existing faults and the large number of earthquakes with low-magnitude support the hypothesis that this seismicity could be related to a frictional (re)activation of faults. Active compression in offshore North Sicily probably reflects the northwards motion of Africa relative to the Eurasian plate.