LOW-FREQUENCY LG FROM NTS AND KAZAKH NUCLEAR-EXPLOSIONS - OBSERVATIONS AND INTERPRETATION

The generation of Lg from underground nuclear explosions is still not fully understood. Near-source scattering of explosion-generated Rg int o S appears to be responsible for the low-frequency Lg from nuclear ex plosions at both Nevada test site (NTS) and Kazakh lest site (KTS) (Gu pta et al., 1991a, 1992). This mechanism has recently been supported b y the analysis of regional data from several Yucca Flats, NTS explosio ns by Patron and Taylor (1995), who further indicate that the prominen t low-frequency spectral null in Lg is due to Rg from a compensated li near vector dipole (CLVD) source. In this study, the dependence of spe ctral nulls on shot depths and other source parameters is first invest igated by analyzing Lg from a large number of Yucca Flats (NTS) explos ions, with known ground truth, recorded at several broadband stations. Analysis of low-frequency Lg from nuclear explosions at KTS also indi cates spectral nulls that are in most cases distinct and identifiable, although generally not as strong as for the NTS shots. Methods of ana lysis include narrow bandpass filtering, spectral ratios, network aver aging, and comparison with synthetic seismograms. Most Lg spectra indi cate 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 sho t depth. By providing an improved understanding of Lg from explosions and its usefulness for obtaining source information, these results mak e an important contribution to regional discrimination and seismic-mon itoring capabilities.