A Model of Aftershock Migration Driven by Afterslip

Aftershocks region have been extensively reported to expand logarithmically with time. The associated migration velocity is typically of the order of several km/decade but can be much larger, especially when observing early aftershock sequences, seismic swarms, or tremors. We present here a model for the expansion of aftershock zones based on the idea that aftershocks are triggered as afterslip grows with time along the fault. One of the model assumptions is that aftershocks are triggered when a critical level of afterslip is reached. We predict that the migration velocity V-p at time t following the mainshock is given by V-p = zeta A'/Delta X-tau c/t, where A' is a frictional parameter, Delta tau a characteristic value for the stress perturbation, c the radius of the stress perturbation, and zeta a constant of order unity. The scaling V-p alpha 1/t implies a logarithmic expansion of the aftershock zone with time. The migration velocities predicted by our model are in quantitative agreement with the observations reported following aftershock sequence of small and large earthquakes in various tectonic settings, seismic swarms, and tremor sequences. Plain Language Summary Aftershocks are shown to migrate with time away from the rupture area of the mainshock. This migration typically occurs as the logarithm of time. We present here a model based on the idea that afterslip drives aftershocks. The model is able to predict the migration as the logarithm of time, predicting apparent propagation velocities consistent with the observations. The propagation velocity is simply related to the physical parameters of the model.