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Wave Propagation from Complex 3D Sources using the Representation Theorem
SCIENCE APPLICATIONS INTERNATIONAL CORP SAN DIEGO CA
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In spite of extensive prior research on generation of seismic waves by underground nuclear explosions, it is still not possible to provide a complete explanation for the observed wavefields, particularly at regional distances. Spherically symmetric explosion models embedded in layered elastic media effectively model the P phases generated by explosions, and the major characteristics of some reflected and transmitted phases. Nonlinear axisymmetric finite difference calculations of explosions including gravity and the effect of the free surface can model a more realistic explosion source that directly generates shear waves. These models explain more characteristics of explosion-generated seismic waves, including some aspects of regional shear phases. However, it is clear that linear and nonlinear near-source 3D effects are important in many cases. SH waves are commonly observed within a few km of explosions, too close to have been generated by simple conversion of vertical and radial components, and often larger than those components. Furthermore, it has not been established what impact 3D effects have on discriminants and on explosion yield estimates. It is important, therefore, to be able to model and understand how 3D source and source region heterogeneity affect the seismic wavefield, and what impact this has on parameters used for nuclear monitoring. We are in the second year of a project to develop and test a three-dimensional nonlinear finite element code CRAM3D, which will be used to calculate nonlinear explosion sources that have both 3D source geometry and may occur in a 3D heterogeneous medium. The code includes the same well-tested material models that have been used in earlier axisymmetric calculations. In addition, we are developing algorithms based on the representation theorem to propagate the motion from these source region calculations to any desired distance. We have implemented a technique that allows us to propagate the results of near source 3D
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