Micromechanics of Sheared Granular Layers Activated by Fluid Pressurization.

Fluid pressurization of critically stressed sheared zones can trigger slip mechanisms at work in many geological processes. Using discrete element modeling, we simulate pore‐pressure‐step creep test experiments on a sheared granular layer under a sub‐critical stress state to investigate the micromechanical processes at stake during fluid induced reactivation. The global response is consistent with available experiments. The progressive increase of pore pressure promotes slow steady creep at sub‐critical stress states, and fast accelerated dynamic slip once the critical strength is overcome. Our multi‐scale analyses show that these two emergent behaviors correlate to characteristic deformation modes: diffuse deformation during creep, and highly localized deformation during rupture. Creep corresponds to bulk deformation while rupture results from grain rotations initiating from overpressure induced unlocking of contacts located within the shear band which, consequently, acts as a roller bearing for the surrounding bulk. Plain Language Summary: Fluids can be at the origin of catastrophic disasters, e.g., earthquakes related to deep subsurface fluid injections or lanslides triggered by short‐term changes of hydrological conditions. It is now well assumed that these phenomena originate from mechanisms taking place in critically stressed shear zones found along tectonic faults, rock mass fractures or localized deformation bands. The increase of pore pressure promotes slip along these shear zones as confirmed by numerous experimental and numerical studies. In this work, we present computer simulations that reproduce the progressive reactivation of a granular shear zone as a result of fluid pressurization. Our simulations provide grain‐scale information that improves understanding of fluid induced slip behaviors and illuminate micromechanical details of phenomenological, macroscale observations. Key Points: Fluid induced reactivation can be either stable or unstable depending on the deformation modeSlow steady creep is accommodated through distributed bulk deformation at sub‐critical stress statesAccelerated dynamic slip results from intense grain rearrangements localized within the shear band [ABSTRACT FROM AUTHOR]