This research was supported by the National Science Foundation's Grand Challenges in High Performance Computing and Communications program, under grant CMS-9318163. Funding comes from the Directorate for Computer and Information Science and Engineering, the Directorate for Engineering, and the Directorate for Earth and Atmospheric Sciences. In addition, NSF funding was supplemented with funds from the Advanced Research Projects Agency. Computing services on the Pittsburgh Supercomputing Center's Cray T3D and DEC 8400 were provided under PSC grant BCS-960001P. We thank Harold Magistrale and Steve Day of San Diego State University for providing the material property model of the San Fernando Valley.