Effects of Large-Scale Surface Topography on Ground Motions, as Demonstrated by a Study of the San Gabriel Mountains, Los Angeles, California by Shuo Ma * , Ralph J. Archuleta, and Morgan T. Page Abstract 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 si- mulating 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 metropo- litan Los Angeles. The topography of the mountains scatters the surface waves gen- erated 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. Introduction It has long been known that surface topography can sig- nificantly affect earthquake ground motions (Geli et al., 1988). Structures situated at the tops of hills, ridges, and can- yons suffer more intensive damage than structures situated at the bases of hills or on level surfaces, as evidenced in past earthquakes such as the 1909 Lambesc, France, earthquake (Angot, 1910), the 1976 Friuli, Italy, earthquake (Brambati et al., 1980), the 1980 Irpinia, Italy, earthquake (Siro, 1982), the 1985 Chile earthquake (Celebi and Hanks, 1986), the 1989 Loma Prieta earthquake (Hartzell et al., 1994), and the 1999 Athens, Greece, earthquake (Assimaki et al., 2005). Higher levels of ground shaking on topographic sites have also been extensively documented from seismic recordings (for example, Shakal et al., 1988, 1994; Hartzell et al., 1994; Spudich et al., 1996; Assimaki et al., 2005). An abundant amount of theoretical, numerical and ex- perimental work has dealt with the amplification of seismic waves at a topographic site, which has been reviewed by Geli et al. (1988) and Bouchon et al. (1996). Less attention, however, has been accorded to how topography affects ground motion for nearby sites that are not situated on the topography. We demonstrate this phenomenon by focusing on the San Gabriel Mountains (SGM)—a macroscopic feature with about 3 km elevation and 30–60 km horizontal expansion that sits north of the Los Angeles Basin (LAB). The San Andreas fault (SAF), which drives much of the seismic ha- zard for Southern California (Working Group on California Earthquake Probabilities, 1995; Frankel et al., 2002; Fialko, 2006), slices through the northern boundary of the SGM. The critical feature of this geometry, relevant to this study, is that most of the mountain range lies on the south-southwest side of the fault (Fig. 1). Later we show that the SGM can reduce the severity of the shaking experienced in the LAB from faulting on the Mojave segment of the SAF . The importance of the subsurface velocity structure on earthquake ground motions has been well documented for basins (Frankel and Vidale, 1992; Olsen et al., 1995; Olsen and Archuleta, 1996; Graves, 1998; Pitarka et al., 1998; Komatitsch et al., 2004; Liu et al., 2004; Krishnan et al., 2006; Olsen et al., 2006). While the effect of the LAB on ground motions has been studied, most of this work neglects a prominent surface feature in the Los Angeles area: the sig- nificant topography surrounding the basin. Surface topogra- phy has been included in regional (Komatitsch et al., 2004; Liu et al., 2004; Krishnan et al., 2006) and global (Koma- titsch and Tromp, 2002a,b) seismic-wave propagation; how- ever, the effects of surface topography on ground motions have not been clearly identified in these studies. Ji et al. (2005) attributed the anomalous wave packets recorded by the Southern California Seismic Network during the 2002 Denali earthquake to the reflections of minor-arc Rayleigh waves off the Oregon Coast and the Rocky Mountains. * Present address: Department of Geophysics, Stanford University, Pana- ma Mall 397, Stanford, California 94305-2215. 2066 Bulletin of the Seismological Society of America, Vol. 97, No. 6, pp. 2066–2079, December 2007, doi: 10.1785/0120070040