North Korea’s 2017 Test and its Nontectonic Aftershock J. Liu et. al March 2018

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2018GL077095

North Korea’s 2017 Test and its Nontectonic Aftershock
J. Liu L. Li J. Zahradník E. Sokos C. Liu X. Tian
First published: 14 March 2018 https://doi.org/10.1002/2018GL077095

Geophysical Research Letters.

Abstract
Seismology illuminates physical processes occurring during underground explosions, not all yet fully understood. The thus‐far strongest North Korean test of 3 September 2017 was followed by a moderate seismic event (mL 4.1) after 8.5 min. Here we provide evidence that this aftershock was a nontectonic event which radiated seismic waves as a buried horizontal closing crack. This vigorous crack closure, occurring shortly after the blast, is studied in the North Korea test site for the first time. The event can be qualitatively explained as rapid destruction of an explosion‐generated cracked rock chimney due to cavity collapse, although other compaction processes cannot be ruled out.

Plain Language Summary
North Korea detonated its strongest underground nuclear test in September 2017. It attracted the public interest worldwide not only due to its significant magnitude (6.3 mb) but also because it was followed 8.5 min later by a weaker event. Was the delayed shock a secondary explosion, an earthquake provoked by the shot, or something else? We answer these questions, thanks to unique data from near‐regional broadband stations. We basically solve a simple problem—fitting observed seismograms by synthetics. The good fit means that we understand why and how the seismic waves are radiated. According to our model, the explosion created a cavity and a damaged “chimney” of rocks above it. The aftershock was neither a secondary explosion nor a triggered tectonic earthquake. It occurred due to a process comparable to a “mirror image” of the explosion, that is, a rock collapse, or compaction, for the first time documented in North Korea’s test site. Interestingly, shear fault motions, typical for natural earthquakes, were extremely small both in the explosion and in the aftershock. Small natural earthquakes also occur at the test site, and geotechnical works might trigger them. Thus, all studies related to rock stability of the site, and prevention of radioactive leakage, are important.

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