Slip Stability of Gouge-Filled Faults Under Fluid Injection

Natural faults commonly contain fluid-saturated gouge layers, in which fluid injection can modify pore pressure and porosity evolution, thereby affecting slip stability and induced seismicity. Here we develop a spring-slider model based on rate-and-state friction (RSF) to investigate fault slip evolution under dry, fluid-saturated, and fluid injection conditions. Our model incorporates gouge dilatancy/compaction and fluid-related pore pressure effects. Our results show pronounced differences in the onset time of the first dynamic instability and the peak slip rate among these cases. Compared to the dry case, fluid saturation without injection delays instability and slightly lowers the peak slip rate, whereas rapid injection-induced pressurization triggers earlier dynamic slip and higher peak slip rates. Without injection, increasing the dilatancy coefficient systematically delays instability. Under rapid injection, however, the onset time becomes much less sensitive to dilatancy, indicating a gradual transition from a dilatancy-influenced to an injection-dominated nucleation regime as the injection-dilatancy competition number increases. Linear stability analysis further suggests that fault stability can be characterized by a generalized critical stiffness that combines the effects of effective normal stress, pressurization rate, and dilatancy/compaction feedback. These results indicate that fluid effects on fault rupture arise from the competition between stabilizing dilatancy hardening and destabilizing time-dependent pressurization, highlighting that injection-induced seismicity can be understood as an injection-rate-driven stability problem. Our findings provide a physical framework for understanding the transition from dilatancy-sensitive to injection-controlled fault slip within the explored net dilatant regime under different fluid environments.