We have used a 3D finite-difference method to simulate ground motion from elastodynamic propagating ruptures with constant slip on faults in the metropolitan area of Los Angeles, California. Simulations are carried out for hypothetical M 6.75 earthquakes on the Palos Verdes and Elysian Park faults and, for comparison, an approximation to the 17 January 1994 M 6.7 Northridge earthquake. The dominant subsurface features of this area are the deep sedimentary Los Angeles basin and the smaller and shallower San Fernando basin. Simulated ground motions are restricted to the frequency range 0.0 to 0.4 Hz.
Ground-motion time histories show that, in general, sites associated with the largest particle velocities and cumulative kinetic energies are located (1) in the epicentral area, (2) above the deepest parts of the basins, and (3) near the steepest edges of the Los Angeles basin. We find maximum particle velocities for the Palos Verdes, Elysian Park, and Northridge simulations of 0.44, 0.67, and 0.58 m/sec, respectively. In each case, both the directivity of the rupture and the lower impedance of the basins significantly amplify the ground motion. Although the gross radiation pattern from these ruptures is observable, the 3D basin structure distorts the wave field and becomes a source for edge-generated waves. Signal durations at some basin sites last beyond 90 sec due to Love waves and refracted S waves that propagate into the sediments from the basin edges. Compared with the Palos Verdes event, the durations are generally smaller for the Elysian Park earthquake due to a smaller amount of Love waves generated at the basin edges.
A simple approximation to the Northridge earthquake reproduces the overall spatial pattern of the long-period particle velocities, successfully predicts the timing of late-arriving waves, and matches the peak velocities with discrepancies generally less than a factor of 2. However, for localized areas immediately north and south of the Santa Monica Mountains, the computed ground motion underpredicts the observed horizontal peak velocities but matches the vertical ones. The pattern of simulated total cumulative kinetic energies is similar to that for the damage intensities observed near the epicenter of the Northridge event. While the Northridge earthquake caused damage in the Los Angeles area, the 3D simulations show that earthquakes with the same magnitude on the Palos Verdes or Elysian Park faults produce more severe ground shaking in the Los Angeles basin.