Strength Recovery in Quartzite Is Controlled by Changes in Friction in Experiments at Hydrothermal Conditions up to 200°C.

The rate of fault zone restrengthening between earthquakes can be influenced by both frictional and cohesive healing processes. Friction is dependent on effective normal stress while cohesion is independent of normal stress, potentially explaining—in part—the lack of depth dependence of earthquake stress drops. Although amenable to laboratory testing, few studies have systematically addressed the normal stress dependence of restrengthening rate. This is partially due to difficulty in separating relative contributions of friction and cohesion in recovery of fault strength. We present results from a series of slide‐hold‐slide tests on thin layers (≤10 휇m) of ultrafine quartz gouge that develop during shearing of initially bare‐surface quartzite. Tests were conducted at 10 MPa constant pore pressure, 20–200 MPa constant effective normal stress, and temperatures of 22°–200°C. Restrengthening, defined as the difference between peak shear stress measured after resumption of sliding and steady‐state sliding shear stress, increases with the log of hold duration. The 200°C healing rate, 0.014 per e‐fold increase in time, is comparable to that determined from seismological observations along the Calaveras Fault, California. Construction of Mohr‐Coulomb failure envelopes shows that changes in cohesion are small (<1 MPa) and independent of hold durations to 105 s, indicating that the increased strength is due to changes in the friction coefficient. These experimental results are inconsistent with the hypothesis that cohesive healing explains the depth independence of earthquake stress drop, but higher temperatures, longer time‐scales, and more complex mineralogy could facilitate cohesive healing in natural fault systems. Plain Language Summary: Faults recover strength, or heal, following earthquakes. This occurs due to processes that can increase the frictional or cohesive strength of the fault surface, but the relative contributions of friction and cohesion to fault strength are difficult to constrain. By conducting a series of slide‐hold‐slide friction experiments, where a simulated fault in quartzite alternates between periods of shear displacement and quasi‐stationary contact, at a range of temperatures and effective normal stresses, we can independently determine the magnitude of frictional and cohesive healing. The strength of the simulated faults increases with time that the surfaces are held in contact. We show that cohesive healing is negligible and the rate of frictional healing is comparable to earthquake observations. This study has implications for our understanding of earthquake recurrence and fault stability. Key Points: Strength recovery is similar at 22° and 200°C but minimized at 100°C, indicating the mechanism is not a simple Arrhenius‐type processCohesive healing in these experiments is negligible and strength recovery is predominately due to frictional healing processesExperimentally determined frictional healing rate is comparable to seismological observations [ABSTRACT FROM AUTHOR]