Rupture propagation and ground motion of strike-slip stepovers with intermediate fault segments

Field studies of historic rupture traces show that fault stepovers commonly serve as endpoints to earthquake ruptures. This is an effect that is corroborated by past dynamic modeling studies. However, field studies also show a great deal of complexity in fault-zone structure within a stepover, which is often simplified out of modeling studies. In the present study, we use the 3D finite-element method to investigate the effect of one type of smaller-scale complexity on the rupture process: a smaller fault segment positioned between the two primary strands of a strike-slip fault stepover. We find that such small faults can have a controlling effect on whether or not a rupture is able to jump the stepover and on the resulting ground motions from these ruptures. However, this effect is neither straightforward nor linear: the length of the intermediate segment and its basal depth, as well as whether the stepover is extensional or compressional, all contribute to the rupture behavior and ground-motion distribution. These results have important implications for assessing the probability of a rupture propagating through small- and large-scale discontinuities in faults, as well as for evaluating ground-motion intensities near fault stepovers. Because of the sensitivity of results to so many parameters, these results also suggest that modeling studies on idealized fault geometries may not be sufficient to describe the rupture behaviors of specific complex fault systems. Site-specific modeling studies, where possible, will provide better inputs and constraints for probabilistic rupture length assessments as well as for ground-motion estimates.