The Role of Clay in Limiting Frictional Healing in Fault Gouges

Abstract

Frictional healing is fundamental to the seismic cycle and plays a role in the energy balance, dynamics, and recurrence interval of earthquakes and slow slip events. Although the healing behavior of quartz has been studied extensively, the role of clay content is less understood. We tested synthetic mixtures of quartz and smectite (10%–100% smectite) in a double-direct shear configuration to measure frictional healing. We show that the magnitude and rate of healing decreases systematically with higher clay content (from 0.008/decade at 10% smectite to 0.002/decade at 100% smectite). Healing scales with both the magnitude of stress relaxation during holds and layer-normal compaction of the gouge. We suggest this reflects the alignment of clay minerals, leading to saturation of the real area of contact that limits restrengthening during holds. The low healing rates of clay-rich faults—together with rate-neutral to rate-strengthening friction—should promote frequent, small failures or stable sliding.