Geological boundary conditions of the 1909 Lambesc (Provence, France) earthquake : structure and evolution of the Trévaresse ridge anticline

Abstract

Abstract The 06/11/1909, Lambesc, M=6 earthquake is the strongest instrumental seismic event recorded in French history. A review of the geology of the epicentral area combined with detailed field mapping and structural analysis allows constraining the tectonic and geomorphic evolution of the Trévaresse ridge anticline and associated fault that ruptured during the 1909 event. The Trévaresse fold is a WNW-trending, southerly-verging forced ramp anticline developed above the Trévaresse reverse fault during late Miocene and Pliocene times, and possibly after the early Pleistocene. Over the last 11 Ma, the Trévaresse reverse fault records an average reverse slip rate of 0.03 ± 0.02 mm/yr consistent with a homogeneous N005 – N010 trending shortening direction. The fault is segmented into two major segments (the eastern one being linked to a possibly active frontal blind thrust fault) separated by a relay fault zone associated with an en-échelon fold pattern consistent with a slight left-lateral component on the easternmost segment. Our structural analysis combined with recent estimates of the source parameters of the Lambesc earthquake indicates that the fault has to reach at least the basement-cover interface, at a minimum depth of 6 km. The westward longitudinal decrease in morphological maturity of the relief of the Trévaresse ridge anticline and its associated fault scarp suggests a westward propagation of the fold during partly diachronous activation of the two segments of the fault.