Effects of local underground structure and surface topography on waveforms from explosions recorded at the Source Physics Experiments site

The far-field ground motions recorded along five radial lines during the first three SPE explosions are characterized by complex features, such as large azimuthal variations in P- and S-wave amplitudes, as well as substantial energy on the tangential component of motion. Shear wave energy is also observed on the tangential component of the near-field motion (epicentral distance < 20 m) suggesting that shear waves were also generated at or very near the source. These features become more pronounced as the waves propagate away from the source. In this study we investigate the effects of underground structure, including large-scale geologic features, a weathered granite surface layer and surface topography on wave propagation near the source region by modeling and analyzing the SPE waveforms. Anelastic finite-difference synthetic seismograms were calculated using WPP program and a 3D model. The 3D model is based on a regional geological model developed in EarthVision, with material properties constrained by shallow borehole data and tomography. By progressively including various geological features into the 3D model we were able to analyze separately their effects on wave propagation and scattering as well as their contributions to S-wave generation. In addition to modeling results obtained with an isotropic explosion source we will also show results of WPP simulations that use near-field synthetic motion calculated with GEODYN-L a Lagrangian hydrodynamic code for modeling the response of earth materials to explosion loading. The GEODYN-to-WPP coupling allows for a better representation of the physics of seismic energy generation and its propagation in the earth's crust.