The coefficient of friction of chrysotile gouge at seismogenic depths

We report new strength data for the serpentine mineral chrysotile at effective normal stresses, σ_n between 40 and 200 MPa in the temperature range 25°-280°C. Overall, the coefficient of friction, μ (= shear stress/effective normal stress) of water-saturated chrysotile gouge increases both with increasing temperature and σ_n, but the rates vary and the temperature-related increases begin at ~100°C. As a result, a frictional strength minimum (μ = 0.1) occurs at low σ_n at about 100°C. Maximum strength (μ = 0.55) results from a combination of high normal stress and high temperature. The low-strength region is characterized by velocity strengthening and the high-strength region by velocity-weakening behavior. Thoroughly dried chrysotile has μ = 0.7 and is velocity-weakening. The frictional properties of chrysotile can be explained by its tendency to adsorb large amounts of water that acts as a lubricant during shear. The water is progressively driven off the fiber surfaces with increasing temperature and pressure, causing chrysotile to approach its dry strength. Depth profiles for a chrysotile-lined fault constructed from these data would pass through a strength minimum at ~3 km depth, where sliding should be stable. Below that depth, strength increases rapidly as does the tendency for unstable (seismic) slip. Such a trend would not have been predicted from the room-temperature data. These results therefore illustrate the potential hazards of extrapolating room-temperature friction data to predict fault zone behavior at depth. This depth profile for chrysotile is consistent with the pattern of slip on the Hayward fault, which creeps aseismically at shallow depths but which may be locked below 5 km depth.