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Source Spectral Variation and Yield Estimation Derived from High Frequency P and S Coda

Identifying Underground Nuclear Explosions (UNEs) and discriminating them from natural earthquakes is a critical issue for the verification of the Comprehensive nuclear-Test-Ban Treaty (CTBT). The earthquake source is described as a shear dislocation on a fault plane, which generates both Pand S-wave at the source. On the other hand, the nuclear explosion source is described as an isotropic volumetric expansion, which theoretically generates only P-waves without S-waves. However, significant S-wave generation is commonly observed from explosion sources and the mechanism remains controversial. A number of explanations (e.g., phase interaction and conversion at the free-surface or any other near source boundary, Rg generation by a compensated linear vector dipole source (CLVD), scattering due to topography, spallation, rock damage, etc) were proposed but a unifying model does not present. To enhance the identification and discrimination capability of explosions, a more comprehensive understanding of explosion sources including primary and secondary process are essential. We analyze seismic coda waves from the near-source explosions to better understand the generation and properties of the scattered P and S wavefields. The wellinstrumented experiments provide us with excellent data from which to document the characteristic spectral shape (e.g., tamped single-fired with variety explosives, ripplefired), relative partitioning between P and S-waves.