Scientists detected an unexpected and prolonged increase in seismic activity at North Korea’s Punggye-ri nuclear test site since the country’s final and most powerful test in 2017. North Korea conducted six underground tests between 2006 and 2017 at the site at Mount Mantap in North Hamgyong province. The final test, with an estimated explosive yield of 160 kilotons, unleashed a magnitude 6.3 earthquake and is believed to have resulted in an on-site collapse, perhaps of a tunnel.

According to seismic wave data from 2008 to 2025 collected and analyzed by Kwang-Hee Kim in South Korea and his colleagues in China, the area around Mount Mantap experienced 1,399 earthquakes, with the number and magnitude increasing over time. The earliest earthquake occurred in 2013, after the third nuclear test, but seismicity remained sparse until 2017, when activity increased markedly after the sixth and largest detonation. Unlike typical post-explosion patterns, seismicity at Mount Mantap continued to increase for years after the final test.

The earthquakes were not powerful, mostly of a magnitude between 2 and 3, and occurred at shallow depth. Before this century, the area had been quiet tectonically, with earthquakes virtually unknown. The team identified preexisting intraplate faults—ancient fractures in Earth’s crust far from active tectonic plate boundaries—and suspects the increase in seismicity shows the crust did not return to normal after the 2017 test, but instead at least two preexisting faults became active.

Mount Mantap’s irregular topography may have left some faults closer to failure, and the underground explosions served as a trigger. The real rate of earthquakes could be higher as the study excluded data from only one or two monitoring stations.