The slip-rate, state-, temperature-, and normal-stress-dependence of fault friction

The mechanics of slow-slip events and earthquakes is controlled by the constitutive behavior of rocks in active fault zones, which is sensitive to many factors encompassing lithology, temperature, confining and pore-fluid pressure, and slip-rate, among others. Understanding the frictional properties of faults is crucial to predicting many aspects of the seismic cycle, from the source characteristics and recurrence patterns of earthquakes to the mechanics of remote triggering. Here, we describe a constitutive model that explains the slip-rate-, state-, temperature-, and normal-stress-dependence of fault friction for a wide variety of rock types, explaining the evolution of frictional stability under various barometric and hydrothermal conditions relevant to natural and induced seismicity, encompassing the brittle-ductile transition. The frictional strength is controlled by the area of contact junctions that form along a rough interface or by grain-to-grain contact in fault gouge and follows a nonlinear function of normal stress. The physical model explains the direct and evolutionary effects following perturbations in temperature, normal stress, and slip-rate, and the dependence of the frictional parameters on ambient physical conditions. The competition among healing and deformation mechanisms explains the dependence of fault stability on temperature, slip-rate, and effective normal stress for a wide range of rocks. The brittle-to-flow transition at the bottom of the seismogenic zone is caused by the thermobaric activation of semi-brittle deformation mechanisms. The model unifies and extends previous formulations, providing a single framework to explain rock deformation in Earth’s brittle and ductile layers.