This research will map the 3-D P- and S-wave velocity structures in the crust of southern California by tomographically inverting over 1.5 million P- and S-wave first-arrival times from over 80,000 local events recorded by the Caltech/USGS network since 1981. The massive data size and the demand for a full 3-D raytracing due to known large lateral inhomogeneities in the crust have imposed a major computational challenge to conventional methods. By an innovative Master Station Method of incorporating 3-D ray tracing, however, the P and S structures will be imaged simultaneously in this project with relocation of summarized events. The primary objectives are to construct P and S crustal velocity models for southern California at one order higher resolution than the latest P velocity model, to relocate earthquakes and re-examine first- motion focal mechanisms in areas of large bias by routine processing, and to gain insight into the crustal seismo-tectonic processes through integrated analysis of all known geophysical and geological information. This research is a component of the National Earthquake Hazard Reduction Program.
