Mainshock and Aftershock Sequence Simulation in Geometrically Complex Fault Zones

Aftershocks seem to be located along the trace of the mainshock fault; however, due to the location error, we do not know their exact location relative to the mainshock fault. Here, we hypothesize that most aftershocks occur on small subsidiary faults instead of the mainshock fault, and they are triggered by the local increase of stress due to the rough geometry of the mainshock fault. To explore this scenario, we perform 2‐D earthquake sequence simulations considering a rough main fault and numerous subsidiary faults that obey the rate and state friction law. We show that many aftershocks occur at the side of the main fault, delineating the main fault trace. We also show that the roughness of the main fault decreases the concentration of aftershocks around the tip of the mainshock fault. Our numerical simulation reproduces the Omori‐Utsu law for the temporal decay of aftershocks and reveals the log‐time expansion of the aftershock zone. The variability of focal mechanisms of aftershocks strongly depends on the friction coefficient. Large aftershocks are limited in the tips of the mainshock fault. This is one of the first earthquake sequence simulations based on the continuum mechanics framework that reproduces realistic spatiotemporal aftershock activities. Key Points We reproduce aftershocks on small faults surrounding the mainshock fault in the framework of 2‐D quasidynamic earthquake sequence simulations Roughness of the mainshock fault is necessary in the simulations to explain the fault‐side aftershocks Omori‐Utsu's law and expansion of the aftershock area are observed