Fluid‐Driven Cohesive Strengthening: Critical Role of Reaction Kinetics as the Determinant for Frictional Stability

Following an earthquake, faults lock and regain strength via a combination of healing mechanisms that include pressure solution, contact growth, and cementation. Fault healing dictates strength recovery during the seismic cycle and is therefore a key factor controlling earthquake recurrence intervals, stress drop, and other source properties. Field and laboratory studies indicate that healing derives from frictional and chemical processes acting in concert from the atomic to the fault scale. Although field studies and theoretical considerations suggest that fault healing involves thermally activated and chemically assisted processes, few laboratory studies have directly examined these mechanisms. Here, we investigate fault‐healing mechanisms via experiments on anhydrite gouges under dry, saturated, and fluid‐pressurized conditions. Our results illuminate the role of both frictional and cementation processes and show that the presence of fluids significantly enhances fault healing in anhydrite gouge, ultimately promoting a time‐dependent increase in fault cohesive strength. The increase in fault strength observed in the presence of fluids is driven by the hydration reaction of anhydrite. We propose a coupled frictional–chemical healing model that accounts for both the stress dependence and chemical kinetics of hydration at frictional contacts. We incorporate cohesion into the rate‐and‐state friction (RSF) stability criterion (Kc) and show that it provides a mechanism for earthquake nucleation that operates even in velocity‐strengthening lithologies, with broad implications for the seismogenic potential of fault zones.