Source mechanism of the 11 june 1909, LAMBESC (southern France) earthquake from macroseismic, seismological and feodetic data: constraints for active tectonic deformation in the southeastern France.

E. Baroux (1), N. A. Pino (1), G. Valensise (1), O. Scotti (2) and M. Cushing (2) European Geophysical Society XXVII General Assembly - Nice, 21-26 avril 2002.

Although the return period of large earthquakes in France is in the order of thousands of years, such rare events still pose a sizable level of seismic hazard. Only a few destructive events occurred in historical times in Provence, southeastern France, the most important being the 11 June 1909 Lambesc earthquake. Fortunately, this event occurred recently enough to be recorded by 30 European observatories, and its associated crustal strains were measured with geodetic techniques at an accuracy level similar to that of recent surveys. We gathered 30 seismograms from 15 observatories, the results of a recent re-evaluation of the coseismic elevation changes and the distribution of earthquake intensities. These data allowed us to assess the location and magnitude (Ms and Mw) of the event and to constrain its focal mechanism within reasonable bounds. Following this multidisciplinary approach, we propose a model for the causative fault of the 1909 earthquake where all the relevant parameters are constrained by at least two independent datasets. Our reappraisal of the seismological data yielded an Mw 5.8-5.9 and an Ms 6.0. This result is consistent with an estimate obtained from analytical modeling of the intensity data, which sets an upper limit of 5.8, and dislocation modeling shows that it is also consistent with the observed coseismic elevation changes. All available datasets help constraining the geometry of faulting and suggest that the Lambesc earthquake was generated by oblique (reverseright lateral) slip on a roughly E-W fault located beneath the Trevaresse anticline between 1 and 7 km depth. The sense of slip obtained for the 1909 fault is in agreement with the regional stress field calculated from the inversion of recent earthquake focal mechanisms and microtectonic data. Several similarly oriented geologic structures exist between the Lubéron, the main fold of Provence, and the Mediterranean Sea. This suggests that deformation similar to that witnessed by the 1909 event may be accommodated by yet unknown active faults located in front of the Lubéron east of the Rhone Valley. (1) Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 1, Roma, Italy, (2) Institut de Radioprotection et de Sureté Nucléaire, BERSSIN, Fontenay-aux-Roses, France