Far-Field Ground Motion Characteristics from Underground Chemical Explosions in Dry Alluvium

Session: Explosion Seismology Advances [Poster]
Type: Poster
Date: 4/29/2020
Time: 08:00 AM
Room: Ballroom
Description: 

We examined characteristics of recorded near-field ground motion from four underground chemical explosions detonated in dry alluvium during the second phase of the Source Physics Experiment (SPE), also known as Dry Alluvium Geology experiment (DAG) performed at the Nevada National Security Site (NNSS). Ground motions from these explosions and other seismic events were recorded by a dense network of temporal and permanent seismic stations with locations conveniently selected in order to densely sample seismic waves in the near-field and far-field, as well as at local and regional distances.

One of the scientific objectives of far-field seismic recordings during DAG was understanding of wave-path effects on seismic waves for shallow chemical explosions, with the intention to refine source discriminants used in local nuclear explosion monitoring. We found that regardless of depth and yield of the explosion source, the waveforms recorded in the far-field (starting from 100m from the source) contain substantial shear motion. The observed radiation patterns for both P and S waves, beyond 500 m from the source, are complex but very similar between the four sources. Cross-correlations between waveforms recorded at circular arrays around the source demonstrate a correlation between azimuthal variation of P and S wave amplitude with near-source complexity of underground structure of alluviums, shaped by the Yucca Fault. Due to their shallow depth, DAG explosion sources generate seismic waves that are trapped and scattered in the shallow part of the Yucca Flat basin. As a consequence, conversions of P to S and S to Rg waves and focusing and defocusing become dominant mechanisms for generating ground motion with relatively large amplitude on the transverse component. These features may explain the decrease in performance of P/S discriminants between earthquakes and SPE explosions observed at local distances.

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

Presenting Author: Arben Pitarka

Authors