The Role of Shear Stress in Fault Healing and Frictional Aging

The seismic cycle involves repeated failure and strength recovery of tectonic faults, known as fault healing. In Earth's crust, where fault strength is dominated by friction, frictional healing is the primary agent of fault healing. Laboratory studies document the role of contact aging and cementation as key mechanisms of frictional healing; however, the role of shear stress is not well understood. We describe a laboratory study of frictional healing as a function of shear stress for bare granite surfaces and simulated fault gouge composed of Westerly granite and quartz. Faults are sheared at normal stresses of 5–25 MPa, 100% relative humidity and room temperature. We quantify frictional healing in slide-hold-slide tests for shear stresses from 0 to the sliding friction value and monitor the amplitude of elastic waves propagating across the fault zone. Healing of gouge shows a strong negative dependence on shear stress, whereas healing of bare surfaces does not vary systematically with shear stress. In gouge, frictional healing involves shear stress-dependent compaction and particle rearrangement, while for bare surfaces frictional strength is derived primarily from contact area evolution. The magnitude of compaction in gouge varies with shear stress, which explains the dependence of healing on shear stress. Elastic wave amplitude increases with hold time and varies inversely with shear stress, consistent with fault zone compaction, shear dilatancy, and their impact on strength evolution and healing. Fault zone compaction during shear unloading and reloading operates with contact aging to dictate frictional healing and fault restrengthening following shear failure.