Ground-Based Nuclear Explosion Monitoring Technologies 525

This paper reports on research activities for the second year of this project developing new explosion source models that include the effects of deep-seated, shock-induced tensile failure. Free surface interactions and the dynamics of shock-wave rebound are responsible for modes of tensile and shear failures leading to permanent deformations in a conical volume over the shot point. A vast majority of nuclear explosions worldwide were conducted under containment conditions that facilitated shock-induced tensile failure. The new model is a superposition of monopole + tectonic release + shock-induced tensile failure sources, the latter represented by a compensated-linear-vector dipole (CLVD) with vertical axis of symmetry. This model was used to explain why Ms for the North Korean test of  9 October 2006 is anomalously large for its mb. The model also applies to the recent test of 25 May 2009. Research this past year has been directed at three outstanding observations/problems that challenge our understanding of seismic wave generation by the explosion source. (1) Anomalous surface waves for Balapan nuclear tests. In the 70’s and 80’s, long-period surface waves from these tests were noted for large Love waves and Rayleigh waves with polarity reversals and significant time delays. Repeated attempts to model these observations with just tectonic release were not completely successful. New observations based on deglitched waveforms of the Borovoye archive (Baker et al., these Proceedings) bridge the frequency gap between long-period fundamental modes and higher modes making up the low frequency Lg spectrum. This additional bandwidth is needed to resolve source parameters of new models that include tensile failure as well as tectonic release. (2) Source-related spectral modulations. Modulations were first observed in the mid-90’s for Lg spectral ratios of Yucca Flats explosions. Since then they have been observed in Pg and Lg coda-wave spectra for Pahute and Yucca Flats explosions. The nearfield RgS scattering hypothesis explains spectral modulations as the result of interference between Rg waves excited by explosion and CLVD sources, causing modulation in the composite Rg wavefield which imprints its spectrum onto the scattered P and S waves making up regional phases. The strength of the CLVD relative to the monopole decreases with yield due to the increasing force of slapdown, which compacts weak materials and reduces static deformations associated with the CLVD. As such, the RgS scattering hypothesis predicts the existence of modulations in spectral ratios taken between shallow and deep explosions on Pahute Mesa. This prediction was validated by observations, which are being modelled for time histories of the CLVD source. (3) Direct versus indirect generation of S waves. A recent publication used plane-layered theory to argue against the importance of near-field scattering (indirect mechanisms) relative to direct generation by the CLVD source. Our studies show that the published results are in error due to (a) deficiencies of Earth models lacking low velocities near the free surface where Rg is excited and propagates, and (b) inadequacies of plane-layered theory for scattering estimates. 2009 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies