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Effects of Large-Scale Topography on the Ground Motions and Rupture Dynamics in the Simulation of the 1812 Wrightwood, California, Earthquake

S. MA, University of California, Santa Barbara, sma@crustal.ucsb.edu; R. ARCHULETA, University of California, Santa Barbara, ralph@crustal.ucsb.edu

Surface topography has long been known to have a large effect on the ground motions. In the Los Angeles area, the San Gabriel Mountains are a macroscopic feature sitting to the north of the LA basin. The Mojave segment of the San Andreas Fault (SAF), which is believed to have caused the great 1812 Wrightwood earthquake, cuts through the range, where most part of the range lies on the southwest side of the fault. We investigate the effects of the San Gabriel Mountains 1) on the ground motions in the greater Los Angeles area and 2) on the dynamic rupture propagation of the Mojave segment of the SAF. We have simulated a kinematic rupture of the 1812 earthquake using a finite-element method. We simulated ground motions with and without true topography by incorporating the SCEC 3D velocity structure and examined the differences in ground motion purely from the inclusion of the large-scale topographic features. We find that the ground motions in the LA basin barely affected by the existence of the large-scale topography. There are, however, anomalous large-amplified ground motions in the area southeast of the San Gabriel Mountains in our simulation with topography compared to one without topography. At present we have no explanation as to how the inclusion of topography leads to this amplification. We will next model a dynamic rupture source on the Mojave segment by incorporating the realistic bending fault geometry, material contrast across the fault, and true topography, making this the most complete dynamic rupture model to date. The scattered waves from large-scale topography are likely to interact with the stresses on the fault, which in turn will affect the propagation of the rupture. We investigate the effects of large mountains on both the dynamic rupture process and the resultant ground motions. The results can help us to understand how surface topography and material properties affect strong ground motion and seismic hazard in the Los Angeles area.

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Last Modified: 2011 Aug 10

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