Velocity- and slip-dependent weakening in simulated fault gouge: Implications for coexistence of slow slip with fast slip

Abstract:

Evaluation of the frictional behavior of fault gouges in order to predict a wide range of natural phenomena, including earthquakes, steady creep, afterslip, or transient slow slip events, depends critically on laboratory shearing experiments. Here we report that in friction experiments on simulated fault gauges, an increase in sliding velocity can induce a change from steady-state frictional strength or slip hardening, to slip-weakening behavior. Here we explicitly consider slip-weakening or hardening trends in laboratory friction experiments. We use a mixture of silt-sized quartz and commercially obtained clay-rich sediment as an analogue fault gouge. Experiments are conducted in a single-direct shear apparatus under normal stress of 2 MPa, with total shear displacements of up to ~16 mm. We evaluate both the velocity- and slip-dependence of friction by extracting the velocity-dependent friction parameters a, b, and D_c, and measuring the rate of change in friction coefficient with shear displacement as the slip-dependence of friction. Our results show that the slip weakening is dominantly observed during our experiments. This suggests a large total difference in strengths can result from a change slip velocities due to the combination of velocity- and slip-dependence of friction. On natural tectonic faults, this suggests that a slip hardening fault area at low slip rates will tend to change into a slip weakening area with a velocity perturbation, such as coseismic rupture propagating onto a fault patch. Therefore, it may be possible that a fault which is experiencing a transient slip, or stable sliding may be more easily induced to slip coseismically if dynamic rupture from large earthquake propagated to the fault.