Crustal Phase Propagation: Discrimination and Corrections
MAXWELL TECHNOLOGIES INC SAN DIEGO CA SYSTEMS DIV
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The primary objective of this work is to improve the accuracy of discrimination of nuclear explosions from earthquakes, by developing and implementing a transportable path correction algorithm for the LgPg discriminant. A secondary result is the development of a depth discriminant based on Lg group velocity. A key to understanding Lg and Pg amplitude changes, which can be dramatic over just 10s of kilometers, is using observations of such changes over short distances. We analyze changes in Lg and Pg along short paths between southern California seismic network SCSN stations for relationships between propagation and statistics of topography, gravity, and crustal thickness. A second key element of our approach is the use of categorical statistical techniques, which frees us from the common and generally unsupported assumption of a linear relationship between geophysical parameters and propagation. The principle underlying this approach is that distinct crustal or path types that have distinct effects on propagation can be characterized by geophysical parameters. The path types can include directionally dependent parameters. Corrections based on calibrated path types are then transported to uncalibrated areas with similar path types. This approach complements empirical methods that reduce the variance of existing data. We use an inversion approach to extend the path correction techniques developed in southern California, to global data reported in the Reviewed Event Bulletins REB, while maintaining the key elements of identifying path types and using short path segments. We divide long source-station paths into short segments, with each segment identified with a particular path type, and then invert for an LgPg ratio correction factor for each path type.
- Geology, Geochemistry and Mineralogy
- Numerical Mathematics
- Seismic Detection and Detectors