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Explosion Source Model Development in Support of Seismic Monitoring Technologies: New Models Accounting for Shock-Induced Tensile Failure
LOS ALAMOS NATIONAL LAB NM
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The traditional source model for long-period seismic waves from nuclear explosions consists of a monopole releasing tectonic strain. Tectonic release has been studied since the 1960s, and numerous studies have shown that linear superposition of monopole double-couple sources can explain many observations of Rayleigh and Love waves. Free surface interactions and the dynamics of shock-wave rebound are responsible for modes of tensile failure which can also lead to permanent deformations affecting long-period excitation. Indeed, the vast majority of nuclear explosions worldwide were conducted under containment conditions that facilitated shock-induced, deep-seated tensile failure. A new source model, which is a superposition of monopole tectonic release shock-induced tensile failure, is proposed, the latter source represented by a compensated-linear-vector dipole CLVD with vertical axis of symmetry. This CLVD source does not excite Love waves. I draw upon the Toksoz-Kehrer 1972 model for tectonic release where F is an index measuring long-period source strength of the release relative to monopole moment MI. A new index K, analogous to F, is introduced, providing a relative measure of MCLVD, the source strength of tensile failure. MCLVD vanishes for K 1, and is 0 in the case of extensional deformation along the vertical axis, e.g., K 1. Rayleigh waves from the CLVD destructively interfere with waves from the monopole, and polarity reversals occur on all azimuths for K 3 in Poisson media. Most Nevada Test Site NTS observations support 1 K 3, and as such the new model predicts lower Ms compared to the traditional model involving just tectonic release. This effect of tensile failure on Ms improves mb - Ms discrimination and suggests that anomalously large Ms compared to mb for the North Korean test of 9 October 2006 is due to the absence of tensile failure on this explosion.
APPROVED FOR PUBLIC RELEASE