Fault gouge failure induced by fluid injection: Hysteresis, delay and shear-strengthening

TL;DR

This study uses a coupled discrete element and fluid dynamics model to explore how fluid pressure influences fault gouge failure, revealing hysteresis, delay, and shear-strengthening behaviors relevant to earthquake processes.

Contribution

It demonstrates that velocity-strengthening friction and hysteresis extend to fluid-saturated granular layers, and uncovers the role of dilational-hardening and delayed failure in fault mechanics.

Findings

Fault gouge exhibits hysteresis and shear-strengthening with fluid pressure cycles.

Failure delay is linked to pre-failure dilative strain and dilational-hardening.

Small arrested slip events occur before full failure, explaining delayed earthquake triggering.

Abstract

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 (1) velocity-strengthening friction and (2) 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 the delay period, small, arrested slip events dilate the layer in preparation for full-scale failure. Our findings may explain (i) fault motion that continues even after fluid pressure returns to pre-injection levels, and (ii) delayed failure in fluid-injection experiments, and (iii) pre-failure arrested slip events observed prior to earthquakes.