In recent decades, it has become increasingly evident that various human activities, including water waste injection, hydraulic fracturing, and geothermal energy production, can contribute to seismic events. To effectively mitigate the risks associated with induced seismicity, it is crucial to comprehend how fluid injection-related factors influence seismic response and evolution. Therefore, gaining an understanding of the effects of these parameters is vital in developing strategies to manage induced seismicity. Experimental and numerical studies indicate that varying injection patterns and rates can mitigate seismicity. However, the mechanism involved in seismicity mitigation of faulted porous medium remains unclear. In the current study, we perform injection-driven fault reactivation experiments on the faulted (saw-cut) Red Felser sandstone to provide new insight into the effect of fluid pressure cycling and rate on fault slip behavior and seismicity evolution. Three different injection rates were applied: low, medium, and high rates of 2 MPa/min, 1 MPa/min, and 0.2 MPa/min, respectively. Three types of injection patterns were also used, including cyclic recursive, monotonic, and stepwise injections. Our results show that the rate at which pore pressure increases plays a more significant role in determining the likelihood of triggering earthquakes than the magnitude of the pressure itself. For seismicity mitigation, a low injection rate in terms of b-value, slip velocity, number of events, and total AE energy is desirable. In addition, results from samples subjected to different injection patterns show that the cyclic recursive pattern can induce fault dilation and comparison that changes the critical stiffness of the fault, and by increasing the number of injection cycles available hydraulic energy budget, maximum slip velocity, and shear stress drop increases. Besides compared to the stepwise and monotonic injection patterns, the cyclic recursive injection scenario showed higher peak slip velocity. A proper injection strategy needs to consider various factors, such as fault drainage, critical shear stress, injection rate, and injection pattern (frequency and amplitude). Our results demonstrate that selecting proper stress/pressure amplitude, waiting time, and pressurization rate for injection design strategy can help to reduce seismicity risk.
