Earthquake nucleation requires reduction of frictional strength τ = ¿ (σ - p) with slip or slip rate, where ¿, σn, and p are the friction coefficient, normal stress, and fluid pressure, respectively. For rate state ¿ at fixed (σ - p), instabilities can occur when d ¿ss/dv 0 are linearly stable at all wavelengths to adiabatic perturbations when v is near a plate rate if the wall rock permeability exceeds a critical value that is orders of magnitude less than inferred. Thus shear heating alone cannot then nucleate unstable slip; frictional weakening is required. However, shear heating can produce inertial instability on velocity strengthening faults following strong stress perturbations. On faults with d¿ss/dv < 0, shear heating increases pore pressure faster than is dissipated by Darcy flow at slip speeds of order 1 mm s-1. For faults bounding half-spaces with uniform thermal and hydraulic properties, ¿ $dot{p}$ exceeds $dot{mu}$ (σ - p) during nucleation for slip speeds in excess of 10-2 to 101 mm s-1, depending on parameters chosen. Thus thermal effects are likely to dominate late in the nucleation process, well before seismic waves are radiated, as well as during fast seismic slip. By the time shear heating effects dominate, inertial slip is imminent (~10-1 s), so that time-to-failure calculations based on rate state friction are not biased by thermal pressurization. |