Effects of Oscillating Pore Pressure of Fluid Injection on Fault Slip Described by Rate and State Friction

Abstract Injection of pore fluid can substantially affect fault slip, often resulting in seismic activity. We simulate the axisymmetric compression experiments of Noël, Passelégue, et al. (2019), https://doi.org/10.1029/2019jb018517 on a saw‐cut specimen of Fontainebleau sandstone subjected to periodic oscillations of fluid pore pressure. We adopt a simple spring‐block model obeying rate and state frictional sliding. Results depend strongly on , the ratio of the period of pore pressure oscillation to time scale of rate and state effects. For small pore pressure oscillation dominates and simulations with constant rate and state parameters show that peak velocities align with pore pressure peaks but not for every peak. For large rate and state effects dominate: multiple velocity peaks occur for each pore pressure period and velocity peaks can occur near the trough or the peak of pore pressure oscillation. In the experiments, measured values of the rate and state parameters varied with velocity. When this variation is included in the simulations, results are consistent with observations at higher confining stress imposed in the experiments. However, for the lower confining stress, it is necessary to include a dependence of the parameters on the effective normal stress for the simulations to be consistent with the observations. In both cases, the magnitude of velocity peaks increases with pore pressure amplitude but the magnitude is less for the higher confining stress. This is consistent with the fewer number of stick‐slip and acoustic emission events observed in the experiments at higher confining stress.