Evidence for gradual weakening in earthquake fault dynamics

A number of recent studies suggest that dynamic slip on earthquake faults may trigger consistent frictional weakening or lubrication, a feature enhanced at relatively high slip rates (of the order of 1 m/s). Here we present the first clear seismological evidence of a progressive fault weakening under dynamic earthquake slip. The weakening increases with the estimated amount of heat rate (and resulting temperature increase) generated on the fault by frictional heating, indicating the presence of some thermally-activated weakening processes. The observed effect seems stronger for less mature slip systems, suggesting that confinement of heat or fluids is less effective on faults which possess a wider damage zone.