The effects of realistic fault geometry on ground motion on the Claremont - Casa Loma stepover of the San Jacinto Fault
Abstract
The Claremont and Casa Loma strands of the San Jacinto Fault in southern California are separated by a 25 km long extensional stepover, which bounds a sedimentary basin. Both individual strands are themselves geometrically complex. A smaller fault strand is located within the stepover, approximately halfway between the two main strands. The width of the larger stepover approaches 4 km, which has been shown by prior observational and modeling work to be close to the upper limit for stepover width through which rupture may jump. The region within and surrounding the Claremont-Casa Loma stepover is densely populated, which further emphasizes the importance of understanding the rupture and shaking hazard associated with this fault system. We use the 3D finite element method to model dynamic ruptures on the Claremont-Casa Loma stepover, incorporating geometrical complexities based on the USGS Quaternary Fault Database and the SCEC Community Fault Model, in order to investigate the ability of the rupture to propagate through the geometrical complexities in this region, as well as the resulting ground motions. As compared to a stepover model with planar segments, the overall intensity and distribution of strong motion for the complex model is reduced, but there remains a region of decreased peak motions between the end of the nucleating fault and the second fault strand. In addition, rupture directivity is diminished near each bend in the fault. Due to this break in directivity, peak ground motions near the fault are weakest immediately after the rupture turns a bend, and are strongest and more widespread immediately before the next bend. Unlike the case of a stepover with planar segments, the shape and intensity of the ground motion distribution is not symmetrical across the fault; an alternating pattern of fault bends produces an alternating pattern for which side of the fault trace experiences higher ground motion. Future work will involve incorporating a complex 3D velocity structure based on the SCEC Community Velocity Model, which should significantly improve the accuracy of our ground motion estimates.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.S21B2515L
- Keywords:
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- 7209 SEISMOLOGY / Earthquake dynamics;
- 7212 SEISMOLOGY / Earthquake ground motions and engineering seismology;
- 7290 SEISMOLOGY / Computational seismology;
- 8118 TECTONOPHYSICS / Dynamics and mechanics of faulting