Tracking fluid pressure buildup from focal mechanisms during the 2003–2004 Ubaye seismic swarm, France

<jats:p>Recent studies have shown that the Ubaye seismic swarm that occurred in the French southwestern Alps in 2003–2004 was triggered by fluid overpressures. This contribution provides additional constraints on the temporal and spatial changes in fluid overpressure during this swarm. The orientations of the double‐couple nodal planes of an extended set of 74 focal solutions, spanning the whole 2003–2004 episode, are compared with the regional stress field. Based on a Mohr‐Coulomb analysis, these comparisons provide estimates of fluid pressures along seismic fault planes. We show that the fluid overpressures required to reactivate the cohesionless fault planes vary through time, with values close to 35 MPa at the inception of the swarm. Overpressures then increase up to 55 MPa during the burst of seismic activity and lastly decrease down to 20 MPa at the end of the crisis. We also show that the fluid overpressures are developed as patches along two parallel faults bordering a releasing bend structure characterized by low to null overpressure. The development of moderate fluid overpressure at the swarm inception enables the reactivation of normal, transtensional, and strike‐slip faults while the development of larger fluid overpressures during the burst of seismic activity progressively enables the reactivation of further misoriented normal, transtensional, and transpressional faults. In order to reconcile the spatial and temporal evolution of the fluid overpressures and the seismic activity, we propose that creep compaction could be the process allowing the successive development of fluid overpressure and the migration of seismicity.</jats:p>