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Fault Geometry, Dynamics, and Precarious Rocks in the Claremont-Casa Loma Stepover of the San Jacinto Fault, Southern California

LOZOS, J. C., University of California, Riverside, Riverside, CA, jlozo001@ucr.edu; OGLESBY, D. D., University of California, Riverside, Riverside, CA, david.oglesby@ucr.edu; BRUNE, J. N., University of Nevada, Reno, Reno, NV, brune@seismo.unr.edu; OLSEN, K. B., San Diego State University, San Diego, CA, kbolsen@sciences.sdsu.edu

The extensional stepover between the Claremont and Casa Loma strands of the San Jacinto Fault in southern California is an area in which fault geometry may have a controlling effect on both rupture propagation and ground motion. This region also has many precariously balanced rocks near the fault trace (Brune, 2002), which may be used to validate or constrain ground motions – and some characteristics of the rupture behavior – calculated in numerical models. Using the 3D finite element method, we model several ruptures on a simplified representation of the Claremont-Casa Loma stepover, each with a different nucleation point. These dynamic models also include calculations of ground motion in the near-source region. We find that the location of the forced nucleation along strike of the first fault segment is important in determining whether or not rupture can even jump across the 4 km stepover. The further the forced nucleation point is from the stepover region, the more likely rupture is to jump, and the more energetic the renucleated rupture on the second fault segment can become. Intensity of ground motion, and distribution of the strongest motion, is directly tied to this. More energetic ruptures and stronger directivity effects produce higher ground motions; motion in the stepover region is strongest when forced nucleation on the first fault is as far from the stepover as possible. We overlay our model ground motion results on maps showing the distribution of precariously balanced rocks around the Claremont-Casa Loma stepover as a real-world check for ground motion intensities, and as a way to infer possible directivity and nucleation locations for past earthquakes on this part of the San Jacinto.

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Last Modified: 2011 Aug 10

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