Reactivation of a strike-slip fault by fluid overpressuring in the southwestern French-Italian Alps

A crustal-scale N130 degrees E strike-slip fault in the UbayeArgentera area, southwestern FrenchItalian Alps, was the locus of a seismic swarm in 2003-2004. Its reactivation is examined by a 2-D frictional fault analysis. The regional stress tensor in the vicinity of the fault is determined by inversion of focal mechanisms of the 38 events with largest magnitudes of the 2003-2004 swarm. Inversion shows that the axis of the maximum principal stress s1 is oriented nearly horizontal and at 63 degrees from the fault plane and that the intermediate principal stress s2 is almost parallel to the fault plane. A 2-D analysis with a static coefficient of friction of 0.4 (consistent with the presence of phyllosilicate-rich gouges at depth) shows that the N130 degrees E fault is unfavourably oriented and that its reactivation is possible only with pore fluid pressure excess in the hypocentral region (67 km below surface). A calculation based on a pore fluid factordifferential stress failure mode diagram shows that the required excess pressure is comprised between 7 and 26 MPa. The presence of thermal springs in the Argentera area indicates that the pore fluid is water. The pore water overpressure is likely achieved by fault zone compaction and hydraulic barriers. The hydraulic barriers can be provided by hydrothermal sealing of the fault damage zone and by the 12-km-thick sedimentary lid (marl-rich autochtonous sedimentary cover and EmbrunaisUbaye sedimentary nappes) whose permeability is lower than that of the underlying crystalline rocks.