Preliminary Analysis of Source Physics Experiment Explosion‐Triggered Microseismicity Using the Back‐Projection Method

A series of four chemical explosions were detonated in a deep borehole within the Yucca Flat Dry Alluvium Geology (DAG) at the Nevada National Security Site between 2018 and 2019. The two larger chemical explosions of 50 tons (DAG‐2) and 10 tons (DAG‐4) TNT equivalent yield triggered energetic aftershock sequences numbering 1392 and 347 microearthquakes, respectively, within the first 10 days. No significant aftershock activity was observed for the two smaller 1‐ton explosions (DAG‐1 and DAG‐3). We used a back‐projection method based on travel‐time migration and stacking of signal‐to‐noise ratio traces to detect, associate and locate aftershocks from a subset of 22‐geophones within a larger 2 × 2 km seismic array surrounding the borehole. The aftershocks located within 300 m of the borehole and the depths were above the working points of 300 and 50 m depths of DAG‐2 and DAG‐4, respectively, ruling out triggering slip on geologic faults or disturbances beneath neighboring collapse craters. DAG‐2 and DAG‐4 aftershocks decayed at similar rates, with power‐law exponents of p = 1.48 and p = 1.49, respectively. These decay rates are comparable to aftershocks sequences triggered by earthquakes and historical nuclear explosions at Yucca Flat. A smooth power‐law aftershock decay within the first 10 days suggests a triggering mechanism from explosion generated stress relaxation due to the diffusion of high gas pressures in the cavity and radial fractures. A more random and episodic aftershock rate would be expected due to cavity collapse or falling rubble in chimney formation. Plain Language Summary Large earthquakes trigger sequences of smaller earthquakes which are clustered in space and time and are known as aftershocks. Explosions also trigger aftershocks that behave similar to earthquake‐triggered aftershocks. However, not all explosions trigger aftershocks and if they do, the aftershocks are fewer, smaller in size, and limited to a few kilometers from the point of the explosion. Detection of explosion‐triggered aftershocks after a suspected nuclear test helps international inspectors locate an area for on‐site‐inspection and differentiate explosions from earthquakes which reduces false alarms. Four chemical explosions were conducted at the Nevada National Security Site as an experiment to study the physics of underground explosions for improving nuclear nonproliferation verification and monitoring capabilities. We detected 1392 explosion‐triggered aftershocks