Lateral heterogeneity of the crust combined with the complexity of an extended earthquake source may account for observations of strong ground motion not easily explained by simplified source models in a laterally homogeneous medium. Both the effects of an extended source and lateral heterogeneity can be examined using a forward modeling technique that combines distributed, nonuniform slip on a fault plane with dynamic ray tracing in a three-dimensional medium. Using this technique, velocity seismograms are calculated at ranges less than 10 km for a finite source, which occurs in a wedge-shaped zone of low velocity. This structure is chosen to approximate the structure of fault zones found in reflection and refraction surveys across active faults in California. Iterative paraxial ray tracing is used to solve the two-point ray tracing problem.
The low velocity zone surrounding the fault causes arrivals in the laterally varying structure to be delayed and rays to leave the fault more closely to the fault normal. At most locations examined, the effect of the three-dimensional structure on isochrone velocity is small compared to the effects on geometric spreading and radiation pattern. Because rays are bent substantially in the near-source region, the seismograms calculated for the laterally varying structure vary substantially in amplitude and polarity from seismograms calculated for the laterally homogeneous structure.