Creep modulation of Omori law generated by a Coulomb stress perturbation in a 3-D rate-and-state asperity model

We present numerical simulations conducted with a quasi‐dynamic, 3‐D rate‐and‐state asperity model and an analytical approach in order to study the behavior of a seismic asperity surrounded by aseismic creep in response to external Coulomb stress perturbations. This work is inspired by the observation of Omori decay characterizing the recurrence time of isolated repeating earthquakes, such as at the Parkfield segment of the San Andreas Fault during the postseismic phase of the 2004 Mw6 event. Based on the numerical results and on an analysis of phase diagrams, we identify two possible regimes that characterize the response of an asperity surrounded by aseismic creep to a stress step, depending on an effective steady state friction parameter A. For the specific perturbation used in this study, we observe that when A is positive, the relaxation of the system is governed by the response of the creeping segments of the fault, and the asperity ruptures in an Omori sequence. In this regime, we demonstrate that the duration of the relaxation process depends on A. Furthermore, we show that this effective strengthening behavior is equivalent to a subcritical density of asperities meaning that the shape of the Omori decay is controlled by the relative proportion of seismic and aseismic material within the fault. On the other hand, a fault characterized by effective steady state weakening (A