Abstract

Abstract : Most source discriminants make use of the prominent regional phase Lg, so that a complete understanding of the generation and spectral characteristics of Lg from explosions is essential for improved and reliable monitoring of the Comprehensive Test Ban Treaty (CTBT). Our research provides valuable new information regarding the origin of both low and high frequency Lg. The low-frequency (up to about 2 Hz) part of the Lg spectra (including the most prominent peaks and nulls) appears to be due to the near-source scattering of explosion-generated Rg into S. Excellent agreement between observations and theory leaves no doubt regarding the contribution of Rg-to-S scattering to the low-frequency Lg from explosions. The high-frequency Lg appears to originate from explosion-generated cracks around the source region. Evidence for these results comes mostly from analysis of broadband regional data from over 40 Nevada Test Site (NTS) and several Kazakh Test Site (KTS) explosions. The results obtained in this study not only improve our understanding of broadband Lg from explosions, but also provide potentially useful source discriminants. Knowledge of the physical basis of discriminants is important to CTBT monitoring because a physical understanding of how and why they work allows for the prediction of discrimination performance in different geologic settings where adequate seismic data may not be available.

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