Improved Understanding of Broadband Lg/P Ratio and Its Application toDiscriminating Earthquakes from Explosions, Including Decoupled Shots

摘要

An understanding of the mechanism of generation of Lg from underground nuclearexplosions is essential for seismic monitoring and source discrimination. Near-source scattering of explosion-generated Rg into S appears to be responsible for the low-frequency Lg from nuclear explosions at both NTS and Kazakh test sites. The source of Rg has a strong compensated linear vector dipole (CLVD) component, and the prominent low-frequency null in the observed Lg is due to Rg from a CLVD source. In this study, the dependence of spectral nulls on source parameters is first investigated by analyzing Lg from a large number of NTS explosions. Most Lg spectra indicate prominent nulls at frequencies which vary with shot depth and are in remarkably good agreement with those expected from Rg due to a CLVD source at about one-third the shot depth. Analysis of low-frequency Lg from explosions at Kazakh, Lop Nor, and Azgir test sites also show that the observed spectral nulls depend strongly on shot depth and local velocity structure, and a comparison with theory provides useful source information, including accurate shot depth. By providing an improved understanding of Lg and its usefulness for obtaining source information, these results make an important contribution to seismic monitoring of the Comprehensive Test Ban Treaty.