  We studied the influence of stress state and fluid injection rate on the
reactivation of faults. We conducted experiments on a saw-cut Westerly granite
sample under triaxial stress conditions. Fault reactivation was triggered by
injecting fluids through a borehole directly connected to the fault. Our
results show that the peak fluid pressure at the borehole leading to
reactivation depends on injection rate. The higher the injection rate, the
higher the peak fluid pressure allowing fault reactivation. Elastic wave
velocity measurements along fault strike highlight that high injection rates
induce significant fluid pressure heterogeneities, which explains that the
onset of fault reactivation is not determined by a conventional Coulomb law and
effective stress principle, but rather by a nonlocal rupture initiation
criterion. Our results demonstrate that increasing the injection rate enhances
the transition from drained to undrained conditions, where local but intense
fluid pressures perturbations can reactivate large faults.
