A REFERENCE DATA SET FOR VALIDATING 3-D MODELS

High-resolution cluster analysis (multiple-event relocation) of earthquakes and other seismic sources has been developed as a tool for assembling catalogs of reference events, especially those whose locations can be determined with an accuracy of 5 km or better (ground truth [GT] 5). We use the Hypocentroidal Decomposition (HDC) method of Jordan and Sverdrup (1981), which is well suited to the rigorous statistical analysis required for this task. An ongoing task is the identification and collection of potential reference events monitored by local seismic networks or recorded during temporary aftershock deployments. Cluster analysis is used with arrival-time data reported to the International Seismological Centre (ISC) and United States Geological Survey National Earthquake Information Center (USGS/NEIC) at regional and teleseismic distances to validate these reference events and to generate new GT5 reference events. The resulting catalog and derived parameters, such as empirical estimates of source-station path anomalies, provide a reference data set that can be used in experiments designed to validate three-dimensional (3-D) models of the region of interest. To date HDC analyses have been performed on 26 earthquake and explosion sequences across Eurasia and North Africa. We examine the characteristics of our cluster data using, for example, plots of combined Pn/P residuals (corrected for origin time offset) at regional distances. For residuals in the range +/-15 sec, both raw and trimmed data show similar variations of median and spread as a function of distance, with a maximum spread near 12-13 degrees. Similar plots of empirical source-station path anomalies show a range of -7.5 sec to +5.0 sec, with maximum differences occurring at distances of 11 to 18 degrees where the influence of interfering travel-time branches makes phase identification difficult. The University of Colorado (CUB) group has constructed from surface wave dispersion data a global 3-D S-wave velocity model of the crust and the upper mantle. S-wave speeds were converted into P-wave speeds based on laboratory-measured properties of mantle minerals and an average compositional model of the upper mantle. Regional path anomalies predicted by the CUB 3-D model are in much better agreement with empirical path anomalies derived by cluster analysis than with predictions made using a 1-D reference model (AK135). Moreover, agreement with the 3-D model predictions is significantly improved by re-identification of Pn and P phases (set during cluster analysis) based on the insights gained by ray tracing through the CUB model. The CUB 3-D model is also used to relocate reference events either known exactly or validated by cluster analysis as GT5 or better. Culled Pn/P arrival-time data at regional distances are used with a grid search technique and with Source-Specific Station Corrections (SSSC's) derived from the CUB model to relocate the events. In a preliminary validation test, we find that the 3-D model improves locations relative to locations based on the 1-D model AK135 in 70-85% of all cases, with details depending on the number of reported phases used for relocation. Typically, the 3-D model reduces the location errors to about half the values attained with the 1-D model. 261 24th Seismic Research Review – Nuclear Explosion Monitoring: Innovation and Integration