Abstract

The generation of Lg from underground nuclear explosions is still not fully understood. Near-source scattering of explosion-generated Rg into S appears to be responsible for the low-frequency Lg from nuclear explosions at both Nevada test site (NTS) and Kazakh test site (KTS) (Gupta et al., 1991a, 1992). This mechanism has recently been supported by the analysis of regional data from several Yucca Flats, NTS explosions by Patton and Taylor (1995), who further indicate that the prominent low-frequency spectral null in Lg is due to Rg from a compensated linear vector dipole (CLVD) source. In this study, the dependence of spectral nulls on shot depths and other source parameters is first investigated by analyzing Lg from a large number of Yucca Flats (NTS) explosions, with known ground truth, recorded at several broadband stations. Analysis of low-frequency Lg from nuclear explosions at KTS also indicates spectral nulls that are in most cases distinct and identifiable, although generally not as strong as for the NTS shots. Methods of analysis include narrow bandpass filtering, spectral ratios, network averaging, and comparison with synthetic seismograms. Most Lg spectra indicate prominent nulls showing strong dependence on shot depth, and the network-averaged spectral nulls are in remarkably good agreement with those expected from Rg due to a CLVD source at about one-third the shot depth. By providing an improved understanding of Lg from explosions and its usefulness for obtaining source information, these results make an important contribution to regional discrimination and seismic-monitoring capabilities.

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