Underground Nuclear Tests Reactivated Quiet Faults Beneath North Korea’s Mt. Mantap

More than eight years after North Korea’s largest underground nuclear test, the ground beneath Mt. Mantap has still not gone quiet. A new study published in Science shows that the detonations conducted at the Punggye-ri nuclear test site did not simply shake the mountain and fade away; instead, they set off a slow, persistent reactivation of faults in the surrounding crust, producing earthquakes that have continued to increase in both frequency and magnitude long after the explosions stopped. The finding reshapes how scientists understand the aftermath of underground nuclear testing and raises difficult questions for anyone charged with monitoring former test sites around the world.

Between 2006 and 2017, North Korea carried out six underground nuclear tests at the Punggye-ri facility, which lies beneath Mt. Mantap in the country’s northeast. The final and largest of these explosions, conducted in September 2017, had an estimated yield of 100 to 250 kilotons. The United States Geological Survey registered the event as a magnitude 6.3 earthquake, and satellite radar measurements revealed substantial deformation of Mantap’s summit, evidence that the blast had physically warped the rock above the test cavity. For years afterward, most scientific attention focused on the standard questions of nuclear test monitoring: where the explosion occurred, when it happened, and how large it was.

The new research, led by Xingli Fan and colleagues, takes a broader view. Rather than characterizing individual explosions, the team set out to understand how the tests changed the seismic behavior of the region as a whole. To do so, the researchers analyzed seismic data recorded in China and South Korea since 2008, drawing on stations located between 80 and 200 kilometers from the test site. That distance matters: instruments far from the epicenter record the region’s background seismicity rather than just the violent, short-lived signals of the detonations themselves, allowing the team to track subtle changes in earthquake activity over nearly two decades.

The scale of what they found surprised even the authors. By combing through the continuous data streams, Fan and colleagues identified 1,399 local earthquakes around Mt. Mantap between 2008 and 2025, far more than previous earthquake catalogs had recorded. Many of these events were small enough to escape earlier detections, but together they paint a picture of a crust that has been fundamentally unsettled by the testing program. The sheer number of events suggests that the mountain and its surroundings experienced a level of seismic disturbance that conventional monitoring approaches had substantially underestimated.

What makes the sequence remarkable is its timing. After most underground nuclear explosions, seismologists observe a familiar pattern: a burst of aftershocks in the immediate vicinity of the blast cavity that decays rapidly over days or weeks, much like the aftershock sequences that follow natural earthquakes. Mt. Mantap refused to follow the script. Instead of decaying, seismic activity after the September 2017 test began roughly three weeks after the detonation and then continued to grow in both frequency and magnitude through 2025, intensifying years after the last explosion. This is not the signature of a crust settling back into equilibrium; it is the signature of an ongoing process.

High-precision relocation of the earthquakes revealed an equally striking organizational pattern. The events were not scattered randomly through the rock but concentrated along two roughly north-northwest–trending fault structures, some of which had existed before the testing began while others had gone unrecognized until now. That alignment indicates persistent and organized fault reactivation rather than the chaotic shattering one might expect from simple blast damage. The faults, in other words, appear to have been switched on as coherent structures, slipping repeatedly along their length in the years following the detonations.

The physical explanation proposed by the researchers is a gradual one. According to the study, the repeated nuclear explosions progressively damaged the shallow crust around Mt. Mantap and altered its internal stress field. In a region where many faults already sat close to failure, even modest perturbations to the stress balance could tip them into sliding. Rather than failing all at once, the faults became active gradually over several years as stresses redistributed through the damaged rock, producing the slow escalation in seismicity that the team documented. The 2017 explosion, with its enormous yield and the pronounced ground deformation it caused, likely delivered the decisive push.

The implications extend well beyond a single mountain on the Korean Peninsula. The study demonstrates that underground nuclear explosions have the potential to reactivate faults that were previously seismically quiet, awakening geological structures that no monitoring program would have flagged as hazardous. Earthquakes triggered in this way may be difficult to distinguish from naturally occurring tectonic activity, since they occur on real faults and can continue for years with no obvious temporal link to the explosion that caused them. For organizations tasked with verifying compliance with nuclear test bans, that ambiguity is a serious concern: a cluster of earthquakes near a former test site could represent natural tectonics, lingering explosion effects, or something else entirely.

Monitoring former nuclear test sites, the authors suggest, will require longer horizons and finer tools than have typically been applied. Dense regional seismic networks capable of detecting and precisely locating small events, combined with sustained observation over many years, appear essential to capturing delayed sequences like the one at Mt. Mantap. The study’s catalog of 1,399 earthquakes, assembled from stations hundreds of kilometers away, shows what such sustained analysis can reveal even at considerable distance from the source. As more nations conduct and then abandon underground nuclear testing, the geological legacies they leave behind may keep generating earthquakes, and scientific attention, long after the political headlines have moved on. For now, Mt. Mantap stands as the clearest demonstration yet that the consequences of nuclear testing are written not only in treaties and diplomacy but in the slow, patient mechanics of the Earth’s crust itself.

Subject of Research: Delayed earthquake activity caused by nuclear explosion–induced fault reactivation at North Korea's Mt. Mantap test site

Article Title: Mt. Mantap nuclear testing triggered delayed seismicity via fault reactivation

Article References: Mt. Mantap nuclear testing triggered delayed seismicity via fault reactivation. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Mt. Mantap, Punggye-ri, nuclear test monitoring, fault reactivation, delayed seismicity, induced earthquakes, seismology, North Korea, intraplate faults, crustal stress, Science journal, nuclear explosions