No Earthquake Had Been Recorded Within 50 Kilometers of Mount Mantap in a Century. After North Korea's Six Nuclear Tests, Seismologists Counted 1,399 Small Quakes, Still Accelerating Eight Years After the Last Blast, Along One Known Fault and One Nobody Had Mapped.

Between 2006 and 2017, North Korea detonated six nuclear devices deep inside Mount Mantap in North Hamgyong province, and then stopped. Underground explosions normally set off a burst of small earthquakes that fades within weeks. What happened at Mount Mantap was the opposite: the ground has kept shaking for eight years, and the tremors are getting more frequent, not less, according to a study published in the journal Science on Thursday.

The region had been geologically quiet. No crustal earthquake had been documented within 50 kilometers of the site for more than a century. To find out what changed, an international team led by Xingli Fan of Chengdu University of Technology analyzed 17 years of continuous seismic recordings, from 2008 to 2025, captured by monitoring stations in China and South Korea sitting 80 to 200 kilometers away. Because the tremors were too small for routine networks to flag, the researchers used matched-filter detection, a pattern-matching technique that searches the raw data for waveforms resembling known events, to pull hidden quakes out of the noise.

They found 1,399 of them. Rather than dying away after the final test in September 2017, activity near the mountain steadily increased and accelerated through 2025. "Seismicity at Mt. Mantap continued to increase for years after the final test," the authors wrote. When the team pinned down precise locations for 955 of the events, they were not scattered randomly. They lined up along two roughly parallel tracks running away from the test site. One followed the projected extension of a fault geologists already knew about. The other traced a fault that had never been mapped.

The puzzle was the delay. Explosions that ended years ago should not still be driving faults today. To explain it, the researchers modeled how the weight and shape of Mount Mantap itself load the rock beneath it, and found the mountain's steep, heavy slopes keep the crust under intense natural stress. The six blasts fractured that already-strained rock and weakened the surrounding crust. Instead of settling, the stress is slowly redistributing through the damaged zone, and as it migrates it is pushing nearby faults past their breaking point, years after the last detonation. "The Mt. Mantap sequence demonstrates that UNEs can produce long-lasting crustal effects in a heterogeneous and critically stressed environment," the team wrote, using the shorthand for underground nuclear explosions.

The finding has consequences beyond geology. The Comprehensive Nuclear-Test-Ban Treaty relies on seismic monitoring to catch clandestine tests, and the researchers note that a persistent, growing swarm of small earthquakes around an old test site makes it much harder to tell aftereffects of past blasts apart from natural quakes, or from something new. North Korea announced it was dismantling the Punggye-ri site in 2018, but satellite imagery has since shown renewed construction there, and the country has never signed the treaty.