Joint multidisciplinary study of the Saint-Sauveur-Donareo fault (lower Var valley, French Riviera): a contribution to seismic hazard assessment in the urban area of Nice
Résumé
The lower Var valley is the only large outcropping zone of Plio-Quaternary terrains throughout the southwestern Alps. In order to assess the seismic hazard for the Alps - Ligurian basin junction, we investigated this area to provide a record of earthquakes that have recently occurred near the city of Nice. Although no historical seismicity has been indicated for the lower Var valley, our main objective was to identify traces of recent faulting and to discuss the seismogenic potential of any active faults. We organized multidisciplinary observations as a microseismic investigation (the PASIS survey), with morphotectonic mapping and imagery, and subsurface geophysical investigations. The results of the PASIS dense recording survey were disappointing, as no present-day intense microseismic activity was recorded. From the morphotectonic investigation of the lower Var valley, we revealed several morphological anomalies, such as drainage perturbations and extended linear anomalies that are unrelated to the lithology. These anomalies strike mainly NE-SW, with the major Saint-Sauveur - Donareo lineament, clearly related to faulting of the Plio-Pleistocene sedimentary series. Sub-surface geophysical investigation (electrical resistivity tomography profiling) imaged these faults in the shallow crust, and together with the microtectonic data, allow us to propose the timing of recent faulting in this area. Normal and left-lateral strike-slip faulting occurred several times during the Pliocene. From fault-slip data, the last episode of faulting was left-lateral strike-slip and was related to a NNW-SSE direction of compression. This direction of compression is consistent with the present-day state of stress and the Saint-Sauveur-Donareo fault might have been reactivated several times as a left-lateral fault during the Quaternary. At a regional scale, in the Nice fold-and-thrust belt, these data lead to a reappraisal of the NE-SW structural trends as the major potentially active fault system. We propose that the Saint-Sauveur-Donareo fault belongs to a larger system of faults that runs from near Villeneuve-Loubet to the southwest to the Vésubie valley to the north-east. The question of a structural connection between the Vésubie - Mt Férion fault, the Saint-Sauveur-Donareo fault and its possible extension offshore through the northern Ligurian margin is discussed. The Saint-Sauveur-Donareo fault shows two en-échelon segments that extend for about 8 km. Taking into account the regional seismogenic depth (about 10 km), this fault could produce M ~6 earthquakes if activated entirely during one event. Although a moderate magnitude generally yields a moderate seismic hazard, we suggest that this contribution to the local seismic risk is high, taking into account the possible shallow focal depth and the high vulnerability of Nice and the surrounding urban areas.