Analysis and Simulation of Far-Field Seismic Data from the Source Physics Experiment

The Source Physics Experiment (SPE-N) at the Nevada National Security Site (NNSS) provides new data for investigating the excitation and propagation of seismic waves generated by buried explosions. A particular target is the generation of S waves. The far-field ground motions during the first two SPE explosions reveal complex features, such as pronounced azimuthal variations in P- and S-wave arrival times, amplitudes and scattered energy. Substantial energy is observed on the tangential component. We pursue multiple approaches for analyzing the effects of three-dimensional (3D) structure and scattering on wave propagation. These include determinations of the source characteristics, comparisons of waveforms from local earthquakes with the SPE waveforms, refinements of the 3D velocity model, and numerical simulations of the source using both standard moment tensor representations and coupled hydrodynamic codes. A comparison of the SPE waveforms with nearby earthquakes shows clear differences in phase amplitude ratios between the shots and the earthquakes. A full waveform moment tensor inversion using a 1D velocity model and seismograms recorded at a distance of approximately 2 km yielded a poor fit to the tangential components suggesting significant 3D velocity variations. Anelastic finite-difference synthetic seismograms were calculated using the WPP computer program and a 3D seismic model. The 3D seismic model includes surface topography. It is based on regional geological data, with material properties constrained by shallow borehole data. The model was progressively increased in complexity by adding random heterogeneities to evaluate the effect of wave scattering on waveform complexity. Two types of sources were tested: a moment tensor representation and an extended source generated by GEODYN, which is a 3D hydro-dynamic code. Finally we use simulations of the SPE explosions and local earthquakes to examine the frequency behavior of P/S ratios as function of source size and geology.