Project Abstract
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Studying the likelihood of through-going earthquake ruptures along the San Gorgonio Pass (SGP) section of San Andrea fault (SAF) is of great importance to seismic hazard assessment in Southern California. Previous numerical modeling studies [e.g., Olsen et al., 2009] suggested that large northward-propagating events on the SAF that breaks through the SGP will likely result in substantial increase of ground-motion intensity in the greater Los Angeles area. We focus on the physical plausibility of such large through-going ruptures from a mechanical point of view by performing 3-D simulations of dynamic rupture along the southern SAF that encompasses the SGP area. We incorporate complex fault geometries in our modeling based on the SCEC Community Fault Model version 4.0 (CFM-v4) [Plesch et al., 2007]. In particular, the Banning Fault in the SGP region is assumed to be the active strand for through-going ruptures. We simulated scenarios of ruptures nucleated from either the San Bernadino (SB) segment of the SAF (north of SGP) or the Coachella Valley (CV) segment of the SAF (south of SGP) to investigate the effects of fault geometry and stress condition on rupture dynamics and ground motions. Results from two end-member sets of calculations using different approaches of assigning initial fault traction showed that the fault geometry can play distinctly different roles regarding the plausibility of through-going ruptures in the SGP region depending on the initial fault traction condition: (1) With homogeneous volume stress input and off-fault plasticity, rupture fails to break through the SGP section regardless of propagation direction or speed. In this case, fault geometry dictates the prestress/traction on the fault and has a major effect on rupture dynamics; (2) with heterogeneous fault tractions and pure elasticity, rupture properties do not seems to be affected much by the relatively long-wavelength geometric irregularities of the Banning fault as is represented in the CFM-v4. In this case, fault geometry has little effect on rupture dynamics. On the other hand, the ground motion patterns showed clear signatures of the complex fault geometry in all cases. We can infer from this study the nonhomogeneous stress field that the SGP section of southern SAF is subject to is the determining factor in the likelihood of such large through-going ruptures along the southern SAF. Future observational studies on the stress conditions in the SGP area will be essential in finding a definitive answer to the physical plausibility of such rupture scenarios. |
