Hydrological triggering of the seismicity around a salt diapir in Castellane, France

The subalpine belts in Provence (France) present a low level of background seismicity. However, Castellane and some surrounding villages in the middle of one of the most renown belts, the Castellane arc, have been damaged twice in a century by significant earthquakes, in 1855 and 1951. The macroseismic database acquired after these events suggests that the sources were moderate (ML ∼ 4.5) but very shallow (∼ 1 km). A recent instrumental microseismic catalogue further attests to the shallow seismicity in the area. A 58 year hydrological record of the Verdon river discharge allows us to test whether this seismicity is modulated by some transient local state-of-stress changes induced by large aquifer and artificial-lake loadings. We show that 41% of the extreme discharges (those with a probability of exceedance at 0.1%) are followed by at least one seismic event within a 7–28 day optimal interval and that this correlation, which represents 8% of the earthquakes, is not due to chance. We consider several natural and artificial hydrological mechanisms, including artificial reservoir and aquifer elastic loading and induced pore-pressure variations. We favour a model in which the local salt–gypsum domes respond to the aquifer forcing.