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

The 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 Fran ce in the last 34 years. Moderate to serious damage in the Annecy area is c onsistent with MSK intensities of VII-VIII. This earthquake occurred on the Vuache Fault, a geologically well-known, morphologically clear, NW-SE-tren ding strike-slip fault that links the southern Jura Mountains with the nort hern Subalpine chains. The hypocentre was located its Mesozoic limestones a t shallow depths (1-3 km). The focal mechanism indicates left-lateral strik e-slip motion on a N136 degrees E-striking plane dipping 70 degrees to the NE. Abundant field evidence was gathered in the days following the main sho ck. Several hundred aftershocks were recorded thanks to the rapid installat ion of a 16-station seismic network. All aftershocks occurred along the sou thernmost segment of the Vuache Fault, defining a 5-km-long, 3.5-km-deep, N 130 degrees E-striking rupture zone dipping 73 degrees to the NE. The fault plane solutions of 60 aftershocks were found to be consistent with left-la teral slip on NW-SE-striking planes. At the SE tip of the aftershock zone w e found ground cracks parallel to the fault close to the Annecy-Meythet air port runway; at the NW tip, near Bromines, we observed left-lateral displac ement of concrete walls in a building. We also noticed Row changes in two s prings 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 a ftershocks with a six-station accelerometric network showed that lacustrine deposits locally amplified the ground motion up to eight times, which expl ains bow this moderate-magnitude shack could cause such heavy damage. Histo rical records draw attention to the central segment of the Vuache Fault, wh ich has been locked for at least 200 years. Situated NW of the 1996 aftersh ock 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 eve nt might occur.