Influence of Injection rate and slip-induced dilatancy on the propagation of fluid-driven slip front

A growing amount of evidence indicate that aseismic transients driven by overpressure play an important role in the triggering of induced seismicity. Understanding the physical control on aseismic slip development is thus important for seismic hazard assessment. We conducted an investigation into the propagation dynamics of a fluid-driven slip front along a laboratory frictional interface composed of granite. The experiments were carried out under a confining pressure of 90 MPa, with an initial uniform fluid pressure of 10 MPa. Fault reactivation was initiated by injecting fluids through a borehole directly connected to the fault.

Our findings reveal that the peak fluid pressure at the borehole leading to reactivation exhibits an increase proportionate to the injection rate. Employing three fluid pressure sensors and eight strain gauges strategically positioned around the experimental faults, we performed an inversion analysis to image the spatial and temporal evolution of (i) hydraulic diffusivity and (ii) kinematic fault slip during each injection experiment. Our inversion methods integrated both deterministic and Bayesian procedures, facilitating the tracking of the fluid pressure front along the fault interface and the subsequent propagation of the slip front over time.

The migration pattern shares many similarities with natural slow slip events suspected to play a role in the development of natural and induced earthquake swarms or aftershock sequences.  We demonstrate that increasing the fluid injection rate induces a transition from a quasistatic propagation of the slip front correlated with the increase in fluid pressure to a dynamic scenario where the slip front outgrows the fluid pressure front, accelerating during its propagation. Furthermore, we establish that temporarily shutting off fluid pressure during injection induces the propagation of a pore-pressure back-front, which halts the propagation of the slip front, aligning with theoretical expectations.