A model for flash weakening by asperity melting during high-speed earthquake slip

Recent results from laboratory experiments on a broad range of mineral systems exhibit dramatic drops in the effective friction coefficient m once the slip rate exceeds a critical level V-w, which is typically O(0.1) m/s. This flash weakening'' has been attributed to the effects of localized heating at highly stressed microscopic asperities. We extend previous phenomenological treatments to assess whether melting at asperity contacts can explain the observed changes in strength. Using physical parameters obtained from the literature on the phase behavior and mechanical properties of quartz, albite, dolomite, gabbro, Westerly granite, and serpentinite, the predictions of our simplified model are in reasonable agreement with available experimental data. We derive approximate analytical expressions that suggest that strength changes are insensitive to the melt viscosity under conditions that likely include those during earthquake slip along major fault systems. Instead, the primary controls on m are the ratio of slip rate V to Vw and the Stefan number S, which is defined as the ratio of the latent heat of fusion to the sensible heat required to raise the temperature from ambient levels. The phase behavior during the short lifetimes and at the high confining pressures of asperity contacts is a significant source of uncertainty in the parameter choices, as are the presence and availability of water. Nevertheless, our results are encouraging for further efforts to incorporate the microphysics of fault zone processes into earthquake simulations.