Simulation of sheared granular layers activated by fluid pressurization

Fluid pressurization of critically stressed sheared zones can trigger slip mechanisms at play in many geological rupture processes, including earthquakes or landslides. The increasing fluid pressure reduces the effective stress, giving possibility to the shear zone to reactivate. Nonetheless, the mechanisms that dictate the mode of slip, from aseismic steady creep to seismic dynamic rupture, remain poorly understood. By using discrete element modeling, we simulate pore-pressure-step creep test experiments on a sheared granular layer under a sub-critical stress state. The goal is to investigate the micromechanical processes at stake during fluid induced reactivation. The global simulated response is consistent with both laboratory and in situ experiments. In particular, the progressive increase of pore pressure promotes slow steady slip at sub-critical stress states (creep), and fast accelerated dynamic slip once the critical strength is overcome (rupture). The analyses of both global and local quantities show that these two emergent slip behaviors correlate to characteristic deformation modes: diffuse deformation during creep, and highly localized deformation during rupture. Our results suggest that the fabric of pressurized shear zones controls their emergent slip behavior. In particular, rupture results from grain rotations initiating from overpressure induced unlocking of interparticle contacts mostly located within the shear band, which, as a consequence, acts as a roller bearing for the surrounding bulk.