Shear zone broadening driven by rate-strengthening friction on granular contact surfaces

As a result of the comminution that takes place over numerous earthquake cycles, mature faults are characterized by thick layers of pulverized gouge with finite porosity that is saturated with water at seismogenic depths. At a given total slip-rate, increases to the strain rate that accompany a reduction in shear-zone width serve to raise the temperature faster and enhance the pressurization of pore fluids so that the fault strength is reduced. This localizing influence of thermal pressurization can be counteracted by rate-strengthening frictional behavior on granular contact surfaces. A linear stability analysis shows that such rate-strengthening friction favors broader shear-zone widths that lower strain rate for a given total slip-rate. A transition from pronounced rate weakening to rate-strengthening friction at high contact slip-rates is an important shared feature of recent models for progressive flash heating by the development and growth of thermally-activated weak layers along contact surfaces, and by particle dynamics models for shear at low effective stresses. Building upon this framework, we describe a model that incorporates the interactions between changes in friction on granular contact surfaces and the localization-pressurization phenomenon. When the total slip-rate exceeds a threshold, the numerous transient slips occurring on granular contact surfaces are kinematically unstable, promoting the activation of previously undeformed regions so that the shear-zone thickness increases. Model predictions are consistent with the predictions of the linear stability analysis. Importantly, the transition to rate-strengthening friction that promotes shear-zone broadening takes place at lower contact slip-rates when the ambient temperature is high or the effective stress is low. We use our model to describe the evolution of shear-zone thickness, temperature, and pore pressure during model earthquakes along a mature fault.