Rock friction experiments and modeling under hydrothermal conditions

Frictional instabilities along tectonic faults produce a full spectrum of slip behaviors including aseismic creep, slow earthquakes, and regular earthquakes. Thermally controlled rock friction provides new insights into the frictional properties of rock faults and associated seismic behaviors with increasing crustal depth. We first review the standard rate and state friction laws. Then we focus on discussing hydrothermal friction experiments on single, mixed, and natural gouges. Steady-state friction and velocity dependence vary significantly with increasing temperature. As temperature increases, steady-state friction coefficients of rock gouges may increase progressively, keep roughly constant, or fluctuate. At high temperatures some gouges may show a brittle-ductile transition. In addition, with increasing temperature, the rock friction switches between velocity-strengthening and velocity-weakening. This variation indicates thermal constraints on the fault stability transition and provides a possible explanation for the temperature and depth distribution of seismic activity. We examine four friction models for hydrothermal conditions: the classical rate-, state-, and temperature-dependent friction model, the friction to flow model, the power-law rate-, state- and temperature-dependent friction, and the extended microphysical (Chen-Niemeijer-Spiers) model. The hydrothermal friction experiments and physically-based models can enhance our understanding of fault friction. Further research is needed on the effects of pore fluids, multiple deformation and healing mechanisms, microstructural evolution at elevated temperatures, and extrapolation of experimental data and friction models to natural and induced earthquakes.