Velocity dependence and frictional healing of fault gouge under heterogeneous saturation conditions

Water infiltration into faults can trigger reactivation by altering the moisture content of fault gouge, yet the frictional behavior of gouge under heterogeneous saturation remains poorly understood. Here we report velocity-step and slide-hold-slide experiments on gouge-filled faults under dry, fully saturated, and locally wetted conditions, using ultrasonic monitoring and digital image correlation to track contact evolution and host-rock deformation. Our results show that the frictional response during wetting is governed by the coupled evolution of moisture heterogeneity and fault-wall deformation, rather than by water content alone. Frictional healing proceeds through two distinct stages, marking a transition in the dominant mechanism that causes frictional strengthening, ultrasonic healing, and creep relaxation to deviate from the conventional logarithmic law. Wetting direction has little influence on healing, but fault stability during wetting is path-dependent: rear-end wetting promotes velocity-strengthening whereas front-end wetting suppresses it. Our findings imply that models of fluid-induced fault reactivation must explicitly account for moisture heterogeneity and host-rock deformation, and highlight the need to consider infiltration direction relative to fault geometry - a factor that governs whether wetting promotes or suppresses seismic hazard.