Relating high-velocity rock friction experiments to coseismic slip in the presence of melts

The dynamic strength (tau_f) of faults during coseismic slip is a major unknown in earthquake mechanics, though it has crucial influence on rupture properties, dynamic stress drop, radiated energy and heat produced during slip. In order to provide constraints on tau_f, High-Velocity Rock Friction Experiments (HVRFE) are conducted on natural rocks with rotary shear apparatuses, reproducing slip (several meters) and slip rate (0.1–3 m s-1) typical of large earthquakes. Among the various weakening mechanisms possibly activated during seismic slip, we focus on melt lubrication. Solidified, friction-induced melts (pseudotachylytes) decorate some exhumed seismic faults, showing that melt can occur on natural faults, though its frequency is still a matter of debate. In the presence of melt, tau_f undergoes an initial strengthening stage, followed by a dramatic weakening stage (thermal runaway). Field estimates based on pseudotachylyte thickness and experimental measures of tau_f suggest large stress drops once thermal runaway is achieved. These estimates of tau_f are compatible with large dynamic stress drops and high radiation efficiency, as observed for some earthquakes. Moreover, the threshold for the onset of thermal runaway might explain differences between the mechanics of small (M < 4) and large earthquakes. A simple mathematical model coupling melting, extrusion and thermal diffusion reproduces some observed experimental features such as the duration of the weakening stage and the convergence to a steady-state.