Effects of normal stress perturbations on the frictional properties of simulated faults

We report on laboratory experiments to investigate the frictional response of creeping faults to sudden changes in normal stress. Experiments were conducted on layers of quartz powder, bare surfaces of Westerly granite, and layers of a 50/50 mixture of quartz powder and smectite clay powder. The tests were carried out at room temperature and controlled humidity using a servo‐controlled double‐direct shear configuration. Normal stress perturbations, corresponding to loading and unloading of tectonic fault zones, were applied during steady sliding at constant loading rate from 3 to 1000 μm/s (shear strain rates of 1.5 × 10 −3 to 0.5 s −1 ). Sudden changes in normal stress resulted in a linear elastic response of shear stress followed by a transient evolution of friction over a characteristic displacement. The transient, inelastic response is quantified as α = (Δτ α /σ)/ln(σ/σ 0 ), where Δτ α is the transient change in shear stress following a step change from initial normal stress σ 0 to final normal stress σ. We find that α is independent of sliding velocity and varies with ambient relative humidity and shear loading history. For unloading, we document a transition from stable to unstable behavior as a function of net slip in the range 3 to 30 mm (shear strains of 1.5 to 15). Increased humidity led to higher values of α for pure quartz gouge, but smaller α for the quartz‐clay gouge. The effects of shear displacement and humidity are discussed in the context of particle characteristics and gouge fabric development. The extended rate‐ and state‐dependent friction laws, using one state variable and the Ruina evolution law with normal stress variation, describe our observations.