Fault dynamics in non-isothermal conditions

The evolution of frictional resistance on faults largely affects the style and characteristics of seismic ruptures. A wide range of rupture styles, from slow-slip events to fast earthquakes, can be explained under isothermal rate- and state- friction. However, laboratory experiments on quartz-rich gouge indicate that the direct and evolutionary effects also depend on temperature. Velocity-weakening, which is the setting for the seismogenic zone, is often accompanied by temperature strengthening, meaning that the steady-state friction coefficient increases as temperature rises. The effect of temperature on friction has been long known in the laboratory, yet its impact on earthquake ruptures is not well understood. Here, we discuss how thermally activated friction affects rupture behavior in quasi-dynamic models of seismic cycles. The temperature dependence is controlled by the activation energy of the direct effect and the activation enthalpy of healing. The evolution of temperature is modulated by shear heating at the source and diffusion in the surrounding medium. Although the thermo-mechanical system involves many physical parameters, their effect can be compounded in fewer non-dimensional parameters that control the transition to different rupture styles. The degree of temperature strengthening impacts a transition from crack-like to pulse-like ruptures, but these rupture behaviors can also be found with isothermal friction. In contrast, stronger temperature-strengthening induces unique behaviors that do not occur under any combination of parameters under isothermal conditions with a single homogeneous asperity, including earthquake swarms and tremogenic slow-slip events, both characterized by strong interactions between slow and fast ruptures. Temperature-strengthening effects can contribute to complexity of earthquake rupture behavior in the seismogenic zone and may be key to explain an even wider spectrum of rupture behaviors observed in nature.