Evidence from coseismic slip gradients for dynamic control on rupture propagation and arrest through stepovers

Analysis of historic slip distributions from large‐magnitude continental strike‐slip earthquake ruptures reveals a pronounced correlation between the distance over which slip decreases as a rupture approaches a structural “step” in the fault and the ability of the rupture to propagate through the step. Our analysis of coseismic slip gradients near these stepovers indicates that in earthquakes in which slip decreases gradually toward a step, rupture will not continue on the next segment. Conversely, in earthquakes in which slip decreases abruptly, rupture commonly renucleates on the next segment. The fact that ruptures that stopped had low slip gradients near stepovers, relative to those that continued, indicates that rupture dynamics control the propagation of rupture through stepovers. These results corroborate dynamic rupture models that show that ruptures in which slip decreases abruptly at a step generate strong seismic waves that serve to renucleate rupture on the opposite side of the structural step. There are several potential causes for gradual decrease of displacement as rupture approaches a stepover, including transference of deformation onto subsidiary structures, rheological contrasts that could dictate favored rupture propagation directions, the existence of stress shadows from previous earthquakes within the system, or other material or frictional heterogeneities. All of these are potentially observable, and, if mapped systematically, could provide the basis for a strong constraint on the likely end points of future earthquakes. Inasmuch as earthquake magnitude is strongly dependent on the size of the rupture, such predictions would be of great utility as a basic component of scenario‐based earthquake rupture forecasts.