North Korea's 2017 Test and its Nontectonic Aftershock

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.