Analysis of Source Physics Experiment Explosion Triggered Aftershocks at the Nevada National Security Site

Session: Explosion Seismology Advances
Type: Oral
Date: 4/29/2020
Time: 11:00 AM
Room: 240
Description: 

Four chemical explosions were detonated in a deep borehole within the Yucca Flat alluvium at the Nevada National Security Site between July 2018 and June 2019. The two larger chemical explosions of 50-tons and 10-tons triggered vigorous microearthquake aftershock sequences numbering in the 1000’s and 100’s respectively, while no aftershocks were observed for the two smaller 1-ton chemical explosions. We use a back-projection method based on travel-time migration and stacking of signal-to-noise (SNR) traces to detect, associate and locate aftershocks using an array of 22 geophones within 2-km surrounding the borehole. The aftershocks located near the borehole above the emplacement depths of 300-m for the 50-ton and 52-m for the 10-ton explosions. The number of aftershocks decayed quickly similar to aftershock decay rates of historical nuclear explosions.

There are two possible triggering mechanisms: cavity stress relaxation and falling rubble due to chimney formation. We conducted a ball-drop experiment to examine the falling rubble theory. A 31-pound concrete ball, with the size of an NBA basketball, was dropped down the borehole from the surface to 150-m depth landing on sand fill. We detected this ball-drop with the geophone array using the back-projection method measuring an SNR typically observed for the smallest aftershocks. The estimated energy release for this ball-drop is similar to a magnitude ‑0.6. Waveform correlation analysis with the ball-drop as a template found only a few similar matches with aftershocks. Future work is needed to further model the ball-drop and aftershocks to determine if one or both mechanisms can be identified. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL release number LLNL-ABS-800462.

Presenting Author: Gene Ichinose

Authors