Effect of Rheological Boundaries and Weaker Patches on Supershear Rupture in 3D Simulations of Earthquake Sequences and Aseismic Slip

We study supershear transition and propagation of dynamic rupture through simulations of earthquake sequences and aseismic slip in a 3D fault model. Our simulations reproduce all stages of spontaneous fault slip, from accelerating slip before dynamic instability, to rapid dynamic propagation of earthquake rupture, to post-seismic slip, and to slow interseismic slip between dynamic events. In the model, a planar strike-slip fault is governed by rate and state friction laws with the aging evolution equation. The fault contains a potentially seismogenic velocity-weakening region surrounded by velocity-strengthening regions. We find that the rheological boundary between the velocity-weakening and velocity-strengthening regions promotes supershear transition. During interseismic periods, velocity-strengthening regions move with slip velocity comparable to the plate loading rate, while the velocity-weakening region is essentially locked. This disparity in slip concentrates shear stress next to the rheological boundary. Once earthquake rupture nucleates, it propagates faster over these areas of higher prestress than over the rest of the seismogenic region, transitioning to supershear speeds in some cases. Since the presence of such rheological boundaries on natural faults can be inferred from laboratory studies and fault observations, this factor may be important for supershear transition on natural faults. The occurrence of supershear transition in our 3D model depends on friction properties and fault stress that develops in the model before large earthquakes and can be explained by the distribution of the effective seismic ratio (e.g., Andrews, 1976) on the fault before large events. The phenomenon of supershear transition due to rheological boundaries could not be established in prior studies, as it can only be observed in simulations that include all of the following factors: (i) inertial effects to enable supershear transition; (ii) a 3D model to include rheological boundaries in the direction of rupture propagation; and (iii) long-term slip history to establish the corresponding stress distribution on the fault before large events. We also find that supershear transition in 3D models of long-term slip can be further promoted by favorable compact fault heterogeneity, as suggested by the 2D single-event study of Liu and Lapusta (2008). Our simulations show that adding a fault patch of lower effective peak frictional resistance can qualitatively modify the behavior of the simulated fault, resulting in occasional supershear earthquakes in a model that has no supershear events without the patch. Supershear transition occurs at the location of the heterogeneity, as advocated by Liu and Lapusta (2008).