On the mechanics of earthquake afterslip

Proposes a model for earthquake afterslip based on rate and state variable friction laws. In the model, afterslip is attributed to the interaction of a velocity-weakening region at depth (within which earthquakes nucleate) with an upper region of velocity-strengthening frictional behaviour. Afterslip is the result of relaxation of a stress perturbation within the velocity-strengthening region, which arises when an earthquake propagates into that region from below. The afterslip-time histories of the 1966 Parkfield and 1987 Superstition Hills earthquakes are modeled and the model parameters are related to physical parameters which may govern the rheologic behaviour of the faults. In accord with field observations, the model predicts 1) that proportionally more afterslip occurs for earthquakes in which coseismic surface slip is small compared with coseismic slip at depth. Combining results with those of recent laboratory friction studies indicates that relatively young faults with little accumulated fault gouge should exhibit little afterslip. -from Authors