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Effects of Large-Scale Surface Topography on Ground Motions, as Demonstrated by a Study of the San Gabriel Mountains, Los Angeles, California

Department of Earth Science and Institute for Crustal Studies, University of California, Santa Barbara, California 93106

Morgan T. Page

Department of Physics, University of California, Santa Barbara, California 93106

^* Present address: Department of Geophysics, Stanford University, Panama Mall 397, Stanford, California 94305-2215.

We investigate the effects of large-scale surface topography on ground motions generated by nearby faulting. We show a specific example studying the effect of the San Gabriel Mountains, which are bounded by the Mojave segment of the San Andreas fault on the north and by the Los Angeles Basin on the south. By simulating a M_w 7.5 earthquake on the Mojave segment of the San Andreas fault, we show that the San Gabriel Mountains act as a natural seismic insulator for metropolitan Los Angeles. The topography of the mountains scatters the surface waves generated by the rupture on the San Andreas fault, leading to less-efficient excitation of basin-edge generated waves and natural resonances within the Los Angeles Basin. The effect of the mountains reduces the peak amplitude of ground velocity for some regions in the basin by as much as 50% in the frequency band up to 0.5 Hz. These results suggest that, depending on the relative location of faulting and the nearby large-scale topography, the topography can shield some areas from ground shaking.

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