Explosion or Tectonic Earthquake? Evidence That an ML 3.7 Seismic Event Occurred near North Korea’s Nuclear Test Site on 11 February 2022

Abstract Discriminating between tectonic earthquakes and underground explosions in the monitoring of possible underground nuclear tests has become more challenging following the increase in seismicity at the Punggye-ri nuclear test site after North Korea’s sixth nuclear test conducted in 2017. In this article, we discuss how to distinguish between explosions and moderate-magnitude tectonic earthquakes around the Punggye-ri test site. For an ML 3.7 seismic event that occurred near the Punggye-ri test site on 11 February 2022, we precisely relocated the source epicenter, determined the possible depth, identified the P first motions, measured the P/S amplitude ratios, and compared the seismic waveform details with those of North Korea’s historical nuclear test explosions and tectonic earthquakes. The results showed that (1) the epicenter is farther away from the tunnels under Mt. Mantap, and no tunnel entrances and shafts are found around the epicenter in the latest satellite images; (2) the source depth is 5.5 km, which is greater than the ordinary depth of underground nuclear tests; (3) negative P first motions were observed; (4) the P/S amplitude ratios are significantly different from those of North Korea’s previous nuclear tests; and (5) the waveform details are similar to previous tectonic aftershocks rather than historical nuclear test explosions. Our findings suggest that this seismic event was a tectonic earthquake rather than an underground explosion.