Development and maintenance of fluid overpressures in crustal fault zones by elastic compaction and implications for earthquake swarms

The ability of crustal faults to compact and to pressurize pore fluids is examined by combining geological observations, petrophysical measurements (permeability, P and S wave velocities, and porosity), and fully coupled hydromechanical modeling. A strike-slip fault located in the Argentera-Mercantour crystalline massif (southwestern French-Italian Alps) was analyzed in the field. This mature fault belongs to a large active fault system characterized by a recurrent seismic swarm activity (M-w<4) between 2 and 12km depth. The studied exposure corresponds to a 50m thick anastomosing fault composed of three types of rock: host-rock gneiss, damage-zone phyllonite, and core zone gouge. Laboratory measurements made at effective pressures ranging from 10 to 190MPa show that the studied fault differs from the classical model and has a high-porosity, high-permeability, and low-rigidity core zone surrounded by a low-porosity, low-permeability, and high-rigidity damage zone with respect to the host rock. The hydraulic and elastic properties are controlled by different microstructures such as foliation, microcracks, and pores developed during the exhumation history of the massif and the reactivation of inherited low-friction mylonitic foliation. Hydromechanical modeling is then used to investigate the spatio-temporal evolution of the fluid overpressures across the fault zone elements in response to elastic compaction. Models demonstrate that fluid pressure can be developed and maintained temporally in the studied fault zone. This study concludes on the key role played by the hydromechanical properties of faults during compaction and provides an explanation for seismic swarm triggering and maintenance.