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.