Probing the DPRK Nuclear Test Site down to Low‐Seismic Magnitude

On 3 September 2017, the Democratic People’s Republic of Korea (DPRK) carried out its sixth declared underground nuclear test (NK6) at the Punggye‐ri test site. With body‐wave magnitude 6.1, this explosion was significantly larger than any of the previous five explosions, and it has been followed by numerous smaller seismic events. The explosion generated seismic waves dominated by significantly lower frequencies than the earlier tests, which makes accurate measurement of relative time delays using cross‐correlation challenging. Finding a frequency band at which one observes common features in the NK6 signals and the corresponding signals from an earlier event can result in reduced signal‐to‐noise ratio (SNR). Classical double‐difference location estimates for NK6 show a significant spread, depending on the set of measurements used. We treat the first five declared DPRK explosions as a source array and demonstrate, using a geometrical argument about the relative time shifts visible between the signals on pairs of stations, that NK6 was very close to the 9 September 2016 explosion (NK5), assumed to be close to the maximal overburden beneath the summit of Mount Mantap. In addition to the magnitude 4.1 presumed collapse event 8 min after NK6, numerous other small events have been observed at or close to the test site since September 2017. We demonstrate how the test site is monitored to magnitudes below two using multichannel correlation templates from all existing observations. Processing all available historical data from the KSRS and USRK arrays reveals a few small events in 2013, 2014, and 2016 that are similar in nature to those observed in late 2017. This suggests that the more recent low‐magnitude events are not simply a direct result of NK6. We urge caution in the interpretation of the correlation functions between the signals from different events at or close to the test site because the signals are a function of both the source term and of near‐source structure, with the effects of topography likely to be significant.