Seismic Wave Generation and Propagation from Complex 3d Explosion Sources

The objective of this new project is to investigate the generation of complex seismic waves by explosions in media with realistic three-dimensional (3D) heterogeneity, including topography. To do this, we use information on subsurface heterogeneity from nuclear test sites to develop 3D models that have the characteristics of the emplacement media for actual nuclear tests. We then run calculations in these structures and compare results with near-field, regional, and teleseismic data. Since we will never have complete information on the subsurface structure, we do not expect exact matches of the data, but if the heterogeneous characteristics of the media are correct, we expect the characteristics of the calculated data and the variations in the calculated data to be similar to those of the observed data. This is particularly true for the near-field data where additional complexities caused by the travel path are expected to be less pronounced. One of the objectives of this project is to address one of the most fundamental problems in nuclear monitoring seismology: why are shear waves generated by all underground nuclear explosions? This project follows a recent project on 3D numerical modeling and wave propagation. In that project, we did the following: (1) developed a 3D nonlinear finite-element code designed for calculation of explosions in 3D heterogeneous media with well-tested material models and including gravity; (2) implemented interface codes, using the representation theorem, that propagate the numerical solution from the source region to regional and teleseismic distances; and (3) used the code to evaluate the effect of near-source heterogeneity, including nonlinear material property variations, elastic variability, and topography. The reason that 3D calculations are important for understanding shear wave generation is that symmetry constraints imposed by one-dimensional (1D) and two-dimensional (2D) calculations act to suppress shear waves. To see this, consider trying to do an explosive experiment in which 2D axisymmetry is imposed. This requires that the motion generated by the explosion be identical in all horizontal directions through propagation of the shock wave, nonlinear deformation of the surrounding material, and rebound to a final state. Since the motion is never identical in all directions, shear waves will always be generated, and in fact shear waves are observed from all underground explosions. One specific task of this project is to perform a 3D nonlinear calculation of the Nonproliferation Experiment (NPE). We have previously modeled this event with a 2D axisymmetric calculation and have rerun that calculation in 3D. We have also incorporated the actual topography of the site and run the calculation with 3D topography. The results show generation of an SH wave not present in the calculation without topography. 2011 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies