AbstractWe 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 increases with injection rate. Elastic wave velocity measurements along‐fault strike highlight that high injection rates induce significant fluid pressure heterogeneities, which explains that in such cases, 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 locally undrained conditions, where local but intense fluid pressures perturbations can reactivate large faults, and contribute to continuing seismicity beyond the period of injection.