3-DIMENSIONAL SIMULATION OF EARTHQUAKES ON THE LOS-ANGELES FAULT SYSTEM

We have used a 3D finite-difference method to simulate ground motion f rom elastodynamic propagating ruptures with constant slip on faults in the metropolitan area of Los Angeles, California. Simulations are car ried out for hypothetical M 6.75 earthquakes on the Pales Verdes and E lysian Park faults and, for comparison, an approximation to the 17 Jan uary 1994 M 6.7 Northridge earthquake. The dominant subsurface feature s of this area are the deep sedimentary Los Angeles basin and the smal ler and shallower San Fernando basin. Simulated ground motions are res tricted to the frequency range 0.0 to 0.4 Hz. Ground-motion time histo ries show that, in general, sites associated with the largest particle velocities and cumulative kinetic energies are located (1) in the epi central 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 Pales Verdes, Elysian Park, and Northridge simulati ons of 0.44, 0.67, and 0.58 m/sec, respectively. In each case, both th e directivity of the rupture and the lower impedance of the basins sig nificantly amplify the ground motion. Although the gross radiation pat tern from these ruptures is observable, the 3D basin structure distort s 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 an d refracted S waves that propagate into the sediments from the basin e dges. Compared with the Pales Verdes event, the durations are generall y smaller for the Elysian Park earthquake due to a smaller amount of L ove waves generated at the basin edges. A simple approximation to the Northridge earthquake reproduces the overall spatial pattern of the lo ng-period particle velocities, successfully predicts the timing of lat e-arriving waves, and matches the peak velocities with discrepancies g enerally less than a factor of 2. However, for localized areas immedia tely north and south of the Santa Monica Mountains, the computed groun d motion underpredicts the observed horizontal peak velocities but mat ches the vertical ones. The pattern of simulated total cumulative kine tic energies is similar to that for the damage intensities observed ne ar the epicenter of the Northridge event. While the Northridge earthqu ake caused damage in the Los Angeles area, the 3D simulations show tha t earthquakes with the same magnitude on the Pales Verdes or Elysian P ark faults produce more severe ground shaking in the Los Angeles basin .