Interplay between seismic and aseismic slip along subduction zone megathrusts: Insights from numerical modeling

Earthquakes in nature are incredibly complex, and the controls on their behavior remain largely unknown. Particle dynamics models can provide insight into the mechanical behavior of earthquake slip and help us understand the controls on complexity in the earth. In this study, we use the discrete element method (DEM) to create numerical analogs to subduction megathrusts with heterogeneous fault friction. Displacement boundary conditions are applied in order to simulate tectonic loading, which in turn, induces slip along the fault. Our models reproduce many natural phenomena, most significantly (1) slip self organizes into remarkably complex spatiotemporal patterns similar to foreshocks, mainshocks, and aftershocks and (2) slow slip in the model emerges systematically prior to large events, potentially preparing the fault for subsequent seismic slip. To gain insight into the physical processes that govern these phenomena, we probe the kinematics and stress evolution of the system. We find that prolonged, localized differential stress drops precede dynamic failure, which we attribute to the gradual unlocking of contacts as particles dilate prior to rupture. We also propose that slip stability in our system is governed primarily by geometrical phenomena, which allow both slow and fast slip to take place at the same areas along the fault. Given the similarity in slip behavior between our simulated fault and real subduction zones, we argue that the physical processes inferred in the model are also at work in nature, and that some of these processes may not detectable in natural systems.