High‐Precision Relocation and Event Discrimination for the 3 September 2017 Underground Nuclear Explosion and Subsequent Seismic Events at the North Korean Test Site

On 3 September 2017, a strong seismic event occurred at the North Korean nuclear test site near the border between China and North Korea. Using P/S‐type spectral ratios calculated from regional seismic data, we identified this event as an explosion. Using a high‐precision relative location method, with the first North Korean nuclear test as the master event, we obtained that the epicenter of this explosion was at 41.3018° N and 129.0696° E with a relative location precision of $∼87m$ based on the error ellipse. This explosion was followed by three moderate seismic events, which occurred 8 min after the explosion, on 23 September and 12 October 2017, respectively. The relocation suggests that the first postexplosion event occurred under the same mountain as the nuclear tests, but the other two events were located about 8 km to the northeast of the explosion. Based on their spectral ratios, these postexplosion events are clearly distinguished from the explosions.