Cohesion-driven fault instability

Fault healing is a fundamental process in the seismic cycle, allowing faults to relock and restrengthen during the interseismic period. Numerous geophysical studies have shown that the rate of fault healing plays a key role in controlling both earthquake magnitude and recurrence interval, in laboratory experiments as well as in natural fault systems. At the laboratory timescales (1–10⁵ s), fault healing is predominantly frictional and results from the time-dependent growth of contact area due to plastic deformation of the contact asperities. In contrast, seismic cycles in nature occur over much longer timescales, allowing additional healing mechanisms, often driven by chemically assisted processes, to become dominant.

Field observations reveal that chemically cemented fault rocks, such as cataclasites, are commonly present within the cores of several exhumed faults. Despite their widespread occurrence, the interplay between chemically-driven healing processes and fault stability remains poorly constrained by laboratory studies, largely due to the limited experimental timescales.

Here we present a suite of laboratory friction experiments specifically designed to overcome these limitations. We use analogue fault gouges composed of highly reactive materials, including hydraulic cement and anhydrite, tested under both nominally dry and fluid-saturated conditions. This approach allows us to investigate the combined and competing effects of frictional and chemically driven healing on fault slip behavior.

Microstructural and geochemical analyses reveal the formation of newly precipitated mineral phases under fluid-saturated conditions, consistent with the expected reaction for both gouge materials. Compared to purely frictional healing, chemically driven healing produces larger, non-log-linear fault restrengthening and a time-dependent increase in fault cohesive strength. Moreover, faults undergoing chemically driven healing exhibit unstable fault slip, characterized by recurrent stick–slip cycles.

These results indicate that chemically-driven healing processes play a fundamental role in interseismic fault restrengthening and may critically influence fault stability over geological timescales. Our results also suggest that these chemically-driven healing processes may favor the development of favorable conditions for unstable slip even at shallow depths, with relevant implications for natural and induced seismicity.