Source parameters of the 11 June 1909, Lambesc (Provence, southeastern France) earthquake: A reappraisal based on macroseismic, seismological, and geodetic observations

Destructive earthquakes are rare in France yet pose a sizable seismic hazard, especially when critical infrastructures are concerned. Only a few destructive events have occurred within the instrumental period, the most important being the 11 June 1909, Lambesc (Provence) earthquake. With a magnitude estimated at 6.2 [, 1942], the event was recorded by 30 observatories and produced intensity IX effects in the epicentral area, ∼30 km north of Marseille. We collected 30 seismograms, leveling data, and earthquake intensities to assess the magnitude and possibly the focal mechanism of this event. Following this multidisciplinary approach, we propose a source model where all relevant parameters are constrained by at least two of the input data sets. Our reappraisal of the seismological data yielded M_w 5.7-6.1 (6.0 preferred) and M_s 6.0, consistent with the magnitude from intensity data (M_e 5.8) and with constraints derived from modeling of coseismic elevation changes. Hence we found the Lambesc earthquake to have been somewhat smaller than previously reported. Our data sets also constrain the geometry and kinematics of faulting, suggesting that the earthquake was generated by reverse-right lateral slip on a WNW-striking, steeply north-dipping fault beneath the western part of the Trévaresse fold. This result suggests that the fold, located in front of the Lubéron thrust, plays a significant role in the region's recent tectonic evolution. The sense of slip obtained for the 1909 rupture also agrees with the regional stress field obtained from earthquake focal mechanisms and microtectonic data as well as recent GPS data.