Simulations of Wave Propagation Effects on Far-Field Ground Motions from the SPE-5 Underground Chemical Explosion

Explosions are traditionally discriminated from earthquakes using the relative amplitude of compressional and shear waves at regional and teleseismic distances. This technique has been shown to be less robust at shorter distances that are necessary to detect small magnitude earthquakes and low-yield explosions. The disparity is largely due to ground motion from small, shallow sources being scientifically impacted by near-surface structural complexities. To understand the wave propagation effects that generate shear motion from isotropic sources and make discrimination challenging, we performed several simulations of the Source Physics Experiment (SPE) underground chemical explosion using 1D and 3D velocity models of the Yucca Flat basin, covering an area of 37 km x 22 km. All simulations are performed using isotropic point sources in the frequency range 0-5Hz. We isolate the effect of large scale geological structure and small scale scattering using the Geologic Framework Model (GFM) 3D earth model and GFM-S models derived from the GFM by adding correlated stochastic velocity perturbations. A parametric study of effects of small scale velocity variations on wave propagation, computed using 1D velocity models with stochastic perturbations, shows that the correlation length and depth of stochastic perturbations impact wave scattering effects on wave type conversions and generation of shear waves. Comparisons of recorded and simulated waveforms for the SPE-5 explosion using 3D velocity models demonstrate that the shallow structure of the Yucca Flat basin contributes to generation of shear motion observed at basin sites. The inclusion of 3D wave scattering, simulated by correlated small scale stochastic velocity perturbations in the 3D model improves the fit between the simulated and recorded waveforms. In addition, a relatively low intrinsic attenuation, combined with small scale velocity variations in our models, can explain wave trapping and coda waves with very long duration observed at basin sites.

LLNL-ABS-783217

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.