Fault Gouge Failure Induced by Fluid Injection: Hysteresis, Delay and Shear-Strengthening

Natural faults often contain a fluid-saturated, granular fault-gouge layer, whose failure and sliding processes play a central role in earthquake dynamics. Using a two-dimensional discrete element model coupled with fluid dynamics, we simulate a fluid-saturated granular layer, where fluid pressure is incrementally raised. At a critical fluid pressure level, the layer fails and begins to accelerate. When we gradually reduce fluid pressure, a distinct behavior emerges: slip-rate decreases linearly until the layer halts at a fluid pressure level below that required to initiate failure. During this pressure cycle the system exhibits (a) velocity-strengthening friction and (b) frictional hysteresis. These behaviors, well established in dry granular media, are shown to extend here to shear of dense fluid-saturated granular layers. Additionally, we observe a delay between fluid pressure increase and failure, associated with pre-failure dilative strain and “dilational-hardening.” During this delay period, small, arrested slip events dilate the layer in preparation for full-scale failure. Our findings may explain (a) fault motion that continues even after fluid pressure returns to pre-injection levels, and (b) delayed failure in fluid-injection experiments, and (c) pre-failure arrested slip events observed prior to earthquakes.