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Institute of Tectonics Department of Earth Sciences University of California, Santa Cruz, Santa Cruz, California 95064
Abstract
Regional seismic signals play an important role in identifying low-magnitude explosion and earthquake sources for monitoring the Comprehensive Test Ban Treaty (CTBT). To enhance the performance of seismic discriminants, regional phases need to be corrected for propagation effects of large- and intermediate-scale crustal wave-guide heterogeneity. Multivariate regression analysis using parametric path characteristics has shown promise for reducing the propagation-induced scatter in short-period regional phase amplitude ratios. While path length is an important parameter for most seismic phase amplitudes, other parametric features of crustal wave-guide structure are often significantly correlated with regional amplitude ratios of P-wave energy (Pn, Pg) to S-wave energy (Sn, Lg) as demonstrated by our prior work on station WMQ. A question remains as to whether these effects can be regionalized so that they apply to all observations in a given region. We explore the viability of regionalization of wave-guide effects on regional seismic discriminants for frequencies of 0.75 to 6 Hz in central Asia and southwest China by analyzing single- and multi-station data. Linear regressions for single-station data indicate that the significance of specific path parameters extracted from available crustal models varies from one station to another and from one subregion to another. In general, corrections for optimal combinations of wave-guide parameters obtained from multivariate regression reduce the data variance more than conventional corrections for path length alone for both single-station and multi-station data. However, in central Asia, several stations show path dependence on certain variables with opposite signs, so regionalization with multi-station data does not perform well. This is due to highly path-specific effects. In southwest China, there is greater commonality in behavior among stations, and fairly good regionalization can be achieved. However, the resulting variance reduction is still less than that for individual stations because some path effects appear to be direction dependent. Our results indicate that individual station calibration is the preferred procedure, but this should include corrections for path effects beyond simple distance correction.
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