Effects of Fault-Adjacent Topography on Supershear Rupture Behavior Along Strike-Slip Branch Fault Systems

Complex fault systems are often surrounded by topography. Although it is known that nearby topography can affect on-fault rupture dynamics, the particulars of its effect on supershear rupture and behavior at fault intersections have not been fully explored. In this study, we conduct 3D dynamic rupture simulations comparing the rupture behavior between a branch fault system with a flat free surface and one with asymmetric fault-adjacent topography. To isolate the effects of topography, we implement simplified uniform on-fault traction and illustrate that the fault-adjacent topography causes the free surface-induced supershear to propagate much more slowly and asymmetrically compared with the corresponding flat free surface case. As an example, for a right-lateral fault system along the x direction, the daughter crack is slightly faster in the positive x direction as opposed to the negative x direction. In addition, depending on the initial shear stress conditions, the normal stress perturbations introduced by the nearby topography can fully suppress the formation of the daughter crack in the slower direction, leading to a unilateral supershear rupture in the other direction. Furthermore, topography can facilitate throughgoing rupture onto secondary fault segments at fault intersections. The results from this simple dynamic modeling study suggest that topography can have significant implications for earthquake dynamics and may explain why some strike-slip earthquakes do not experience supershear rupture.