Physicochemical Processes of Frictional Healing and Lithification: Effects of Normal Stress and Water on Stick-Slip Stress Drop and Friction in Synthetic Fault Gouge

Earthquakes are dynamic phenomena characterized by slip instability along a preexisting zone of weakness (fault zone) within more competent rock. Fault zones are characterized by granular and clay-rich wear material (fault gouge) produced by dynamic and quasi-static slip processes. The mechanical strength, frictional stability, and seismic potential of a fault are strongly influenced by the evolution of grain contacts within the fault zone. In this context, water plays an important role at mineral surfaces and within contact junction via processes such as hydrolithic weakening, adsorption/desorption and pressure solution. To investigate the role of water in faulting, we performed shear experiments using synthetic fault gouge as a function of relative humidity (RH) and normal stress (σn). We sheared layers of glass beads and granular rock of known initial grain size (dia. 105 to 149 μm) in a double direct shear configuration. Normal stress was kept constant during shear at values of 2.5, 5 and 10 MPa. Shear (τ) stress was applied via a constant displacement rate at the layer boundaries, and shearing velocity was varied from 0.3 to 300 μm/s. Careful calibration of apparatus stiffness plus an external DCDT, mounted directly across the shear zone, were used to measure slip velocity of the fault zone. During each experiment, RH was kept constant at values of 5, 50 and 100%. Our experiments were conducted in the stick-slip sliding regime. For σn = 2.5 MPa, stress drop, (Δτ), is constant from 0.3 to 10 μm/s and decreases linearly with log velocity above 10 μm/s. Maximum shear stress τmax shows the same trend. At σn = 10 MPa, Δτ decreases linearly with log velocity over the entire velocity range. We find that maximum friction μmax increases systematically with increasing RH, in contrast to previous results for angular quartz and nominally-bare surfaces. In agreement with previous studies, our results indicate that frictional behavior of simulated fault gouge is strongly influenced by RH and contact properties. In particular, we observe that at low values of σn samples are characterized by velocity neutral behavior until a characteristic velocity, 10 μm/s, after which Δτ and μmax decrease with increasing velocity. Future work will consider the connection between the observed mechanical behavior and the evolution of grain contact properties.