Effects of Dip Angle on Rupture Propagation Along Branch Fault Systems

ABSTRACT An important consideration in assessing seismic hazards is determining what is likely to happen when an earthquake rupture encounters a geometric complexity such as a branch fault. Previous studies showed parameters such as branch angle, stress orientation, and stress heterogeneity as key factors in the self-determined rupture path on branch faults. However, most of these studies were conducted in 2D or 3D with perfectly vertical faults. Therefore, in this study, we investigate the effects of dipping angle on rupture propagation along a branch fault system. We construct 3D finite-element meshes where we vary the dip angles (nine geometries in total) of the main and secondary faults, the stressing angle (Ψ=20°, 40°, and 65°), and the hypocenter location with nucleation on both the main and secondary segments. We find that for Ψ=40°, a rupture on the main fault is most likely to propagate across the branch intersection when the secondary fault is dipping. In addition, for Ψ=65°, a rupture on the secondary fault is most likely to propagate to the main fault when the secondary fault is shallowly dipping. This is caused by a fast rupture speed on the secondary fault and the dynamic stress effect that develops with the interaction of the free surface and the dipping secondary fault. These results indicate that dip angle is an important parameter in the determination of rupture path on branch fault systems, with potentially significant impact for seismic hazard, and should be considered in future dynamic rupture modeling studies.