Seismicity and hydro-mechanical coupling along strike-slip faults in flat-and-ramp structures : the case of the Vuache Fault in the Annecy Molasse Basin (French western Alps)

Summary

This study aims to understand the recurrent seismicity that occurs in a limited volume along a major fault in the French western Alps, the Vuache Fault, which crosses the geological Jura in a flat-and-ramp zone. In 1996, an M5.3 earthquake occurred near Annecy (France), located at a depth of approximately 2 km. In this article, we analyze the seismicity that has occurred since then and calculate the seismic velocity variations at local permanent seismic stations. The magnitude and focal mechanism of the 1996 M5.3 earthquake indicate that the process was tectonic in origin. However, the duration of the Omori decay of its aftershocks, their migration, the variations in spring flow, the existence of repeated swarms, the seasonal variations in seismic velocity and their relationship to rainfall and seismicity show that the continuation of this seismicity is linked to the pressurization of fluids in a deep, fractured aquifer connected to the surface. This aquifer appears to be limited to the sedimentary formations. Earthquakes, especially during the M5.3 aftershock migration, reach the depth of the Triassic gypsum. The aftershock sequence occurred in two different phases: an initial phase lasting around ten days, during which the earthquakes did not show any migration but rather a random spatial distribution, and a second phase showing a clear migration, suggesting a fluid diffusion process. During this last phase, the hydraulic diffusivity of the aquifer was calculated and estimated at around 2 m²/s. The same order of magnitude was obtained by using the correlation between seismic velocity variations and rainfall. This is a high value, close to those found during man-made fluid injections. This aquifer forms a confined, pressurized reservoir between two thrusts in the regional flat-and-ramp structure. The scenario described in this article could be found, at various scales, in flat-and-ramp regions where tectonic stresses are sufficient to generate seismicity.