The objective of this research is to investigate lateral heterogeneity in the Earth's mantle and is a continuation of work begun three years ago. In combination with previous results, the research will produce a three-dimensional map of shear velocity heterogeneity over length scales of about 500 km beneath North and South America, the East Pacific, the North Atlantic, North Africa and the Asian continent. The approach is to collect all available tangential component seismic recordings with sources and receivers in the study area. These records will be modeled using synthetic seismograms to determine the ray paths and travel times corresponding to observed arrivals. Tomographic techniques then produce the three-dimensional shear wave map. This work has important application to mantle models of convection and resulting stresses in the lithosphere. It is therefore important to the National Earthquake Hazard Reduction Program.
