Frictional strength and stability of laterally heterogeneous faults under fluid diffusion conditions

Rock heterogeneity and fluids are widespread in natural fault zones. How they interplay to affect fault strength and frictional stability remains inadequately understood, particularly taking fluid diffusion into account. Here, we develop a two-degree-of-freedom spring-slider model based on rate-and-state friction (RSF) to investigate the frictional behavior of heterogeneous faults including quartz- and clay-rich gouges. Our model incorporates lateral fluid diffusion and pore pressure interactions between two fault gouge segments. Our results show that the increase in clay content within a quartz-rich fault causes a progressive weakening in fault strength and enhanced frictional stability. In quartz-dominated faults, high hydraulic diffusivity significantly delays the initiation of stick-slip instabilities. Our results replicate a full spectrum of slip behaviors from stable creep, slow slip, to dynamic rupture with controlled fault composition, friction parameter ratios and fluid diffusivity properties. We demonstrate that pore pressure perturbations, especially inter-patch pressure differences under fluid diffusion conditions, can drive slip behavior transitions. Our findings provide a physical framework for understanding seismic and aseismic slip behavior in fluid-rich, heterogeneous subduction-zone fault gouges.