Morphotectonic and fault-earthquake relationships along the northern Ligurian margin (western Mediterranean) based on high resolution, multibeam bathymetry and multichannel seismic-reflection profiles
Résumé
We present here new data from the MALISAR surveys that focus on two areas of the northern Ligurian margin where we reveal recent and active deformation. A set of N60°E scarps that are oblique to the margin is seen at the foot of the continental slope. These correspond to cumulated reverse-strike slip faulting that is consistent with the present-day kinematics of earthquakes. At the north-western margin, the Marcel Fault appears at the surface as a 10-km-long scarp trending N60°E. Several earthquakes of moderate magnitude (3.8 < ML < 4.6) have focal mechanisms consistent with the geometry and the kinematics of, and have been located around, the Marcel Fault. At the north-eastern margin, offshore of Imperia, the major feature is a large promontory that is bound by a network of N60°E faults on its southern side. The structures and the deflection of the drainage network are consistent with a recent uplift of the Imperia Promontory. No surface ruptures have been identified in the epicentre area of the 1887 Ligurian earthquake (the major historical regional event: MW ~6.5-6.7), although the Imperia Fault network has the necessary characteristics to account for this earthquake. Therefore, from the present study, we propose that the rupture, as a reverse-strike slip faulting of a few or all of the segments belonging to the Imperia Fault network, was the source of the 1887 event. These MALISAR data have allowed identification of an 80-km-long transpressive system of Plio-Quaternary faults. The question of the potential activation of all of these faults during a single event is now of major concern, which has led to reappraisal of the regional seismic hazard as moderate to high.