Analyses of Small Seismic Events Around the Underground Nuclear Test Site in the Democratic People’s Republic of Korea

Following the 2016–2017 underground nuclear explosions at the Punggye-ri test site (NKTS), a sequence of small seismic events (M _L 2.5–3.4) has continued for several years in a region with little prior seismicity. In this study, we examine this sequence using regional seismic data recorded at distances of ~ 350–600 km. Relative relocation based on Lg -wave cross-correlation constrains 78 events with horizontal uncertainties of approximately 0.2 km and reveals spatially coherent clusters, including linear alignments that may delineate previously unmapped fault structures. Focal mechanism solutions for the largest events indicate shallow sources with both strike-slip and normal faulting. Event type is independently evaluated using three-component P/S spectral amplitude ratios; network-averaged ratios in the 6–9 Hz band place all analyzed events as earthquakes, clearly distinguishing them from explosions. Furthermore, waveform similarity identifies several repeating event pairs, implying localized and recurrent slip. The spatial distribution of seismicity, source characteristics, and the lack of prior seismicity indicate that this sequence may be associated with the 2017 explosion.