Contribution to the seismic hazard assessment of a slow active fault, the Vuache Fault in the southern Molasse Basin (France)

The Vuache Fault is a prominent structure cutting the southernmost Swiss Molasse basin, from the Subalpine massifs to the Jura Range. It controls a superficial (0 to 3 km), moderate (moment magnitude < or = 5) and recurrent (a few events per century) seismicity. In order to address the seismic hazard associated to this fault, we compiled existing data, performed new field investigations and reprocessed existing seismic lines. The newly acquired data validate the hypothesis of an active structure. Its imprint in the landscape and its Quaternary long-term activity are demonstrated, especially by the offset of incised small valleys. Some sites also reveal the occurrence of Quaternary deposits deformed along the fault. Despite the alternative interpretation (glacitectonism) already published, we favor the hypothesis of a tectonic origin for some of them. Concerning the fault slip rate, dating problems preclude definitely addressing the issue, but regional correlations suggest that long-term slip rate ranges from 0.15 to 0.4 mm/a. In addition, as previously concluded by other authors, there is probably a basement fault beneath the surface structure. A connection between the two is not completely demonstrated because of the poor quality of the seismic line at the key point, but this hypothesis should nonetheless be considered in seismic hazard assessment. The relative weight of a deep-seated fault (up to 10 or 15 km into the brittle crust) hypothesis may be low because the well-established data fit more with a scenario of shallow fault producing moderate to low magnitude earthquake. This hypothesis--which would drastically increase the possible maximum magnitude--should not however be neglected in seismic hazard assessment, especially because the coseismic origin of deformations in La Petite Balme is still a possible alternative.