Abstract

The Southern California San Jacinto fault is geometrically complex, consisting of several major strands with smaller scale complexity within each strand. The two northernmost strands, the Claremont and the Casa Loma–Clark, are separated by a 25‐km‐long extensional stepover with an average of 4 km separation between the strands. We use a combined modeling method to assess probable rupture and ground‐motion behaviors for this stepover. First, dynamic rupture modeling on geometrically complex fault strands embedded in a state‐of‐the‐art 3D crustal velocity model is used to generate a series of scenario earthquakes. We then use the resulting near‐fault low‐frequency (<1  Hz) ground‐motion time histories to generate broadband synthetic seismograms with a hybrid approach. These synthetics are then compared with a distribution of precariously balanced rocks (PBRs) near the fault to constrain our results and assess shaking hazard for the region surrounding the fault. Our dynamic models produce sources between Mw 5.4 and 6.9, with rupture limits imposed by sharp contrasts in fault stress or by geometrical barriers. The main stepover serves as a primary barrier to rupture in our model, producing event sizes that are consistent with the historical behavior of the San Jacinto fault. The largest broadband synthetics are a good match to leading ground‐motion prediction equations and are generally consistent with the distribution of PBRs, none of which experience accelerations that produce toppling probabilities significantly higher than zero. Thus, although the PBRs do not rule out any of our model scenarios, they confirm that our models produce realistic rupture extents and shaking.

Online Material: Figures of total slip for additional rupture models, low‐frequency intensity plots, synthetic seismograms, and comparison with ground‐motion prediction equations.

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