Eathquake potential of southeastern france and ligurian margin: new insight from seismological, historical and morphotectonic data.

E. Baroux (1), N. A. Pino (1), G. Valensise (1), O. Scotti (2) and M. E. Cushing (2) (1)Instituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 1, Roma, Italy (2)Institut de Radioprotection et de Sureté Nucléaire, BERSSIN, Fontenay-aux-Roses, France Special session 3- XXVIII General Assembly of the European Seismological Commission - Gênes, Italie - 1-6 septembre 2002

The return period of individual earthquake sources in southern France and in the Ligurian margin is very long, probably a few thousand years. Only two strong earthquakes occurred in the region in the past century, respectively on 11-06-1909 near Lambesc, Provence, and 19-07-1963 offshore Liguria (M1~6.0). Very few smaller events occurred in the digital instrumental period, and consequently it is difficult to determine source mechanisms and to understand whether they are in keeping with the regional geodynamic framework.. Fortunately, there exists a good instrumental and macroseismic record of the Lambesc event. We gathered 30 seismograms, the results of a recent re-evaluation of coseismic elevation changes and the distribution of earthquake intensities. Based on this multidisciplinary effort we propose that the 1909 earthquake was caused by oblique (reverse-right lateral) slip on an E-W fault between 1 and 7 km depth beneath the Trevaresse anticline, and had a Mw=5.8-5.9 (Ms=6.0, Me=5.8). These results are in close agreement with recent data concerning the seismotectonics of the region: for example, with the stress field obtained from microtectonic data collected along the nearby Middle Durance Fault; with the recent kinematics of the same fault inferred from geologic and geomorphic evidence; with the stress field calculated for SE France from focal mechanisms; with preliminary GPS results showing active contraction across the region; and with a recently hypothesised mechanism for the 1887 Imperia earthquake (Intensity=IX, Me=6.4). In addition, finite element modeling of the Ligurian margin by Bethoux et al. (EGS meeting, 2002) suggests that there may be a tendency for earthquakes up to M~6.0 to concentrate in the northwestern part of the Ligurian Sea. We discuss how the joint interpretation of these new data contributes to the understanding of the active tectonics of this region and to the assessment of its earthquake potential.