Source study and tectonic implications of the 1995 Ventimiglia (border of Italy and France) earthquake (ML = 4.7)

We analyse the source process and the aftershock distribution of the April 21, 1995, Ventimiglia, M-L = 4.7 earthquake using the records of permanent high dynamic broad-band seismic stations and a temporary network deployed on land and at sea few hours after the earthquake. This event occurred on the western Mediterranean coast, near the border between Italy and France, at a depth of 9 km, at a point where Alpine tectonic units and Late Oligocene extensional structure overlap and are currently undergoing compressional stress. The focal solutions of the mainshock and three afterhocks depict a dominant reverse faulting with an important strike-slip component, which underlines two nodal planes: a NW-SE-dipping north fault and a NE-SW-dipping south fault. We operate a careful re-location of the aftershocks using a master-event technique and data from the temporal network and obtain a predominant NW-SE alignment. Then, we analyse the rupture process using an empirical Green function approach. We find that the mainshock broke a 0.5 to 1 km fault length and that the rupture propagated during 0.1-0.2 s probably in a SE direction. Those two arguments, together with the recent fault trace that exists close to the epicentre, leads us to propose that this event expresses the reactivation of an old transverse NW-SE structure with a dextral movement. This study thus emphasizes the role of inherited, deep-rooted, transcurrent features in the tectonic reactivation of this passive margin. It also underlines the importance of combining short-period and broad-band seismology to better resolve and understand regional tectonic processes in areas of moderate seismic activity and complex geology. (C) 1998 Elsevier Science B.V. All rights reserved.