Velocity- and slip-dependent weakening in simulated fault gouge: Implications for multimode fault slip

In addition to the velocity dependence of friction, slip dependence may play a major role before and during earthquake slip in fault zones. We performed laboratory friction experiments on simulated fault gouges, measuring both the velocity and slip dependence of friction in velocity step tests. The pure velocity-dependent component of friction measured over short displacements shows both velocity strengthening and velocity weakening friction, depending on the amount of slip considered. However, we observe that increases in sliding velocity can induce slip weakening behavior which overwhelms the velocity dependence resulting in large overall weakening, especially at rates > 1 mu m/s. On natural tectonic faults, this suggests that a velocity perturbation, such as coseismic rupture propagating onto a fault patch, could induce instability via large slip weakening. Therefore, a fault which is experiencing a transient slip or slow earthquakes may be more easily induced to slip coseismically if a dynamic rupture from large earthquake propagates onto the fault.