Seismic Calibration of Group One International Monitoring System Stations in Eastern Asia for Improved Event Location

A consortium of institutions that includes SAIC, the Massachusetts Institute of Technology MIT Earth Resources Laboratory ERL, Weston Geophysical Corporation, the Russian Institute for Dynamics of the Geospheres IDG, and the Chinese Seismological Bureau of Sichuan Province is engaged in a research program directed toward the seismic travel-time calibration of the 30 Group 1 International Monitoring System IMS stations of eastern Asia. We have assembled a preliminary 3-D velocity model of the entire region, which is composed of a global background model on a 5-by-5 grid derived from surface wave analyses, supplemented by more detailed models in regions where they are available. At present, such detailed models have been identified for a large portion of the Former Soviet Union FSU for which Deep Seismic Sounding DSS data have been used to define a 3-D velocity model of the crust and upper mantle on a roughly 40-km by 40-km grid, and for an approximately 25-by-30 area centered on the PakistanAfghanistan region for which a 3-D velocity model has been defined on a 1 by 1 grid. Regional phase travel times through these 3-D models are being computed using the Podvin and Lecomte finite difference algorithm to obtain preliminary SSSC estimates for the IMS stations in this region. These initial estimates are being tested using various calibration data sets that have been assembled for this study. These include a unique set of regional arrival-time data at FSU permanent network stations from some 60 Soviet Peaceful Nuclear Explosion PNE tests, as well as numerous Semipalatinsk and Novaya Zemlya explosions with precisely known locations and origin times.