The ML 5.3 Epagny (French Alps) earthquake of 1996 July 15: a long-awaited event on the Vuache Fault

International audienceThe M L 5.3 Epagny earthquake that occurred on 1996 July 15 in the vicinity of Annecy (French Alps) was the strongest event to shake southeastern France in the last 34 years. Moderate to serious damage in the Annecy area is consistent with MSK intensities of VII-VIII. This earthquake occurred on the Vuache Fault, a geologically well-known, morphologically clear, NW-SE-trending strike-slip fault that links the southern Jura Mountains with the northern Subalpine chains. The hypocentre was located in Mesozoic limestones at shallow depths (1-3 km). The focal mechanism indicates left-lateral strike-slip motion on a N136°E-striking plane dipping 70° to the NE. Abundant field evidence was gathered in the days following the main shock. Several hundred aftershocks were recorded thanks to the rapid installation of a 16-station seismic network. All aftershocks occurred along the southernmost segment of the Vuache Fault, defining a 5-km-long, 3.5-km-deep, N130°E-striking rupture zone dipping 73° to the NE. The fault plane solutions of 60 aftershocks were found to be consistent with left-lateral slip on NW-SE-striking planes. At the SE tip of the aftershock zone we found ground cracks parallel to the fault close to the Annecy-Meythet airport runway; at the NW tip, near Bromines, we observed left-lateral displacement of concrete walls in a building. We also noticed flow changes in two springs close to that locality. Geodetic levelling across the fault revealed about 1 cm of uplift for the region north of the fault. The recording of aftershocks with a six-station accelerometric network showed that lacustrine deposits locally amplified the ground motion up to eight times, which explains how this moderate-magnitude shock could cause such heavy damage. Historical records draw attention to the central segment of the Vuache Fault, which has been locked for at least 200 years. Situated NW of the 1996 aftershock zone, between the Mandallaz and Vuache mountains, this segment forms a 12-km-long potential seismic gap where other M5 events or one single M6 event might occur