Ground-motion amplification in the Santa Monica area: Effects of shallow basin-edge structure

The 1994 Northridge earthquake produced ground motions in the northwest portion of the Los Angeles basin that were significantly larger than rock-site motions observed at locations just north of the basin. The Santa Monica area was hit particularly hard, with numerous structures being damaged or destroyed by the strong ground shaking. In this region, the basin-edge geology is controlled by the active strand of the east-west-striking Santa Monica fault, and virtually all of the structural damage occurred at or south of the fault location. We have used 2D finite-difference ground-motion simulations to investigate the effect of the basin-edge structure in amplifying ground response. Constraints on the basin-edge structure come from geologic cross sections, geophysical data, and seismological observations. Our simulations indicate that the shallow basin-edge structure (1 km deep) formed by the active strand of the Santa Monica fault creates a large amplification in motions immediately south of the fault scarp, in very good agreement with mainshock damage patterns, recorded ground motions, and locations of elevated site response. This large amplification results from constructive interference of direct waves with the basin-edge-generated surface waves and is quite similar to the basin-edge effect associated with the 1995 Kobe earthquake. In addition, we find that focusing effects created by the deeper basin structure (3 to 4 km deep) cannot explain the large motions observed immediately south of the fault scarp. This strongly suggests that the deep-basin focusing models proposed by Gao et al. (1996) and Alex and Olsen (1998) are not likely explanations of the observed pattern of ground-motion amplification in the Santa Monica area.