Analysis of Sequences of Aftershocks Initiated by Underground Nuclear Tests Conducted in North Korea on September 9, 2016 and September 3, 2017

Abstract

The article presents the results of analyzing aftershock sequences initiated by underground nuclear tests conducted in North Korea in 2016 and 2017. Waveform cross-correlation and a multimaster method applied to phase association allowed us to find and analyze 91 aftershocks and group them into clusters associated with the underground explosions on September 9, 2016 (DPRK-5) and September 3, 2017 (DPRK-6). The duration, intensity, and sporadic nature of these two sequences indicate specific mechanisms of seismic energy release, which are probably related to the interaction of the destruction zones of DPRK-5 and DPRK-6 explosions and the collapse of their cavities with the gradual advance of the collapse columns to the free surface. The aftershock activity in 2021 suggested that the collapse process was not yet completed and predicted the occurrence of new aftershocks in the near future, which could end in the collapse column reaching the free surface. These assumptions were confirmed; during the preparation of this work, a strong aftershock with a magnitude of 3.85 was recorded on February 11, 2022, which, according to preliminary estimates, indicates the beginning of the final stage of the advance of the collapse column to the surface, the probability of which is discussed in this study.