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The mainshock-aftershock sequence is one of the fundamental characteristics of seismogenesis, yet the physical mechanism of aftershock generation remains poorly understood. The simple explanation that aftershocks are caused by the mainshock's redistribution of strain energy is not always applicable, especially for aftershocks within the mainshock slip area, where strain energy is released. Here we show that the genesis of aftershocks can be modeled using a frictionally heterogeneous fault system. We conducted quasi-dynamic numerical simulations of fault rupture cycles on a finite fault governed by a rate- and state-dependent friction law. Aftershocks are observed around and within the mainshock rupture area when the frictional heterogeneity varies significantly along the fault. On the other hand, aftershocks are not produced when along-fault variations in the frictional heterogeneity are small, which mimics the observed lack of aftershocks for repeating earthquakes.

Plain Language Summary A large earthquake is often followed by many smaller earthquakes, which occur both within and outside the rupture area of the mainshock. Although the stress redistribution due to rupture of a mainshock explains aftershocks around the rupture area, physical mechanism generating aftershocks within the rupture area is still under debate. We model such aftershocks activity using the fault with the frictional heterogeneity. Our model can reproduce aftershocks both within and outside the rupture area of a mainshock when the mainshock ruptures only a part of the fault due to the along-fault variations of the frictional property.