Fault reactivation during fluid pressure oscillations: transition from stable to unstable slip

High‐pressure fluid injection in deep geo‐reservoirs can induce earthquakes. Recent observations suggest that cyclic injections might trigger less seismicity than monotonic injections. Here, we report triaxial laboratory experiments conducted on faulted quartz‐rich sandstone that provide new insight into the physics of fault‐fluid interactions subjected to cyclic fluid pressure variations. The experiments were performed at 30 and 45 MPa confining pressure, imposing constant or sinusoidal fluid pressure oscillations of amplitudes ranging from 0 to 8 MPa in addition to a far‐field constant loading rate (10‐4 and 10‐3 mm.s‐1). The results show that: (i) In agreement with the Mohr‐Coulomb theory, faults reactivate at the static friction criterion, which is generally reached at the maximum fluid pressure during oscillations. (ii) Oscillating fluid pressure perturbations promote seismic behaviour rather than aseismic slip, and (iii) increasing the oscillation's amplitude enhances the onset of seismic activity along the fault. We demonstrate that this behaviour is caused by slip rate variations resulting from the fluid pressure oscillations. Without fluid pressure oscillations, increasing the far‐field loading rate also promotes seismic activity. Our experiments demonstrate that the seismicity intensification due to cyclic fluid injections could be promoted at shallow depth, where confining pressure is relatively low, resulting in large strain rate perturbations.