Small intermediate fault segments can either aid or hinder rupture propagation at stepovers

Large-scale geometrical complexities along faults are known to be likely endpoints for coseismic rupture, as suggested by analysis of historic ruptures and corroborated by models of rupture on bent or discontinuous faults. However, natural faults also include smaller-scale complexities. We use the 3D finite element method to model dynamic ruptures on strike-slip fault stepovers with a smaller intermediate fault between the main strands. We find that such small faults can have a controlling effect on rupture behavior and ground motion intensity and distribution. In particular, the intermediate fault can either aid or prevent rupture propagation across the stepover, depending on its length and basal depth. The results have important implications for hazard assessment of faults with large- and small-scale geometrical complexities, and also suggest that more site-specific modeling studies may be necessary to develop realistic rupture scenarios for individual complex fault systems.