Explosion Shear‐Wave Generation in Low‐Velocity Source MediaExplosion Shear‐Wave Generation in Low‐Velocity Source Media

This paper investigates which mechanism of Lg generation dominates in low-velocity source media, which is important because of the central role of Lg in discrimination and yield estimation of nuclear explosions. The mechanisms investigated are surface P-to-S conversion (pS), generation directly by the nonspherical component of the explosion source volume, and Rg-to-S scattering. We identify and quantify observations that distinguish between mechanisms. We also specifically test the assumptions of previous work that concluded that Rg scattering is the dominant mechanism. To do so, we analyze and simulate records of adjacent, normally buried and overburied Nevada test site (NTS) explosions, and analyze deep seismic sounding (DSS) explosion Quartz 3 data. The data analyses and simulations consistently indicate that pS is the dominant source of explosion Lg in low-velocity source media, that nonspherical source components could also contribute significantly to Lg, and that scattered Rg contributes less, except possibly at very low frequencies. For NTS overburied versus normally buried explosions, we compare Lg-to-Pg spectral ratios, corner frequencies, and tangential versus vertical and radial Lg spectral nulls. We perform simulations for the NTS to compare the contributions to Lg of pS, direct S from a CLVD, and scattered Rg. Quartz 3 data show that Rg spectral nulls vary with azimuth and differ from corresponding Sg and Lg spectral nulls, counter to assumptions required by the Rg scattering hypothesis.