Frictional Properties of Simulated Fault Gouges subject to Normal Stress Oscillation and Implications for Induced Seismicity

Under critical conditions where fault slip exhibits self-sustained oscillation in experiments, effects of normal stress oscillation (NSO) on fault strength and stability remain uncertain, as do potential effects of NSO on natural and induced seismicity. In this study, we employed double direct shear testing to investigate the frictional behavior of a synthetic, near velocity-neutral (VN) fault gouge (characterized by self-sustained oscillation under quasi-static shear loading), when subjected to NSO at different amplitudes and frequencies. During the experiment, fault displacement and gouge layer thickness were measured. Transmitted ultrasonic waves were also employed to probe grain contact states within the gouge layer. Our results show that fault weakening and unstable slip can be readily triggered by oscillations, depending on oscillation frequency and amplitude. Interestingly, an amplified shear stress drop and weakening effect were observed when the oscillation frequency fell in a specific range (0.01–0.1Hz). No such effects were seen in a velocity-strengthening gouge. Analysis of transmitted ultrasonic waves in the test on the VN gouge reveals the presence of fault dilatation, accompanied by unstable slip and weakening. By extending an existing microphysical model (the "CNS” model), to account for elastic effects of NSO on gouge microstructure and grain contact state, the mechanical and wave data obtained in our experiment on the VN gouge was reproduced. Assisted by the microphysically-based friction model, resolving the instability criterion of a velocity-neutral fault under perturbation is crucial for understanding and thus predicting the fault behaviors of certain scenarios, like periodic gas storage in deep reservoirs.