The Role of Clay in Limiting Frictional Healing in Fault Gouges.

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. Plain Language Summary: Healing is a time‐dependent strengthening process that makes it possible for faults to regain strength after an earthquake and fail again in future events. Faults are commonly hosted in clay‐rich, fine‐grained gouges. While there is a wealth of data on the frictional behavior of these gouges, there is a relative lack of data regarding their healing behavior. Here, we measure the change in frictional healing with increasing clay content using experimental deformation of synthetic fault gouges. We find that the amount and rates of healing decrease with clay content. This decrease is likely caused by the tendency for clay grains, which are elongate and plate‐shaped, to align and limit the ability for the fault gouge to increase the area of contact between grains, thus limiting healing. This small healing, together with their low strength and tendency toward stable behavior, may promote small frequent earthquakes—or inhibit earthquakes—on faults hosted in clay‐rich gouges. Key Points: We conducted experiments on a suite of quartz‐smectite mixtures to investigate the role of clay in frictional healingIncreasing smectite content reduces both healing and frictional strength, interpreted to result from saturation of real area of contactLow healing rates in clay‐rich gouges may favor more frequent and smaller failures than in quartz‐dominated gouges [ABSTRACT FROM AUTHOR]