Scientists have observed a strange increase in seismic activity at North Korea’s Mount Mantap, where Pyongyang conducted underground nuclear tests between 2006 and 2017.
The findings could point to long-lasting, delayed effects from repeated nuclear explosions on the region’s landscape.
Chinese researchers found that seismic activity has persisted, and even intensified for years – even after underground detonations at the mountain ceased.
Rather than suggesting any more recent activity, it likely points to the fact that underground nuclear explosions have the potential to reactivate seismically quiet faults, scientists say.
Earthquakes produced from such explosions may be difficult to distinguish from naturally occurring tectonic activity, they warn.
The findings could have implications for monitoring former nuclear test sites, according to the study published in the journal Science.
North Korea’s Mantap nuclear site hosted six underground tests between 2006 and 2017, with the largest and final known test conducted by Pyongyang in September 2017.
The last explosion had an estimated yield of 100 to 250 kilotons, and registered a magnitude 6.3 seismic event, according to the US Geological Survey.
Satellite radar measurements also reported substantial deformation of the mountaintop by about 3 metres following the explosion.
Until now, the broader effects of these explosions from 2006 to 2017 on the region’s seismic activity have remained poorly understood.
In the new study, scientists analysed seismic data recorded in China and South Korea since 2008 to trace how earthquake activity around Mount Mantap has changed over time.
The data included records from seismic stations located about 80km to 200km from the test site.
Overall, scientists analysed data from 1,399 local earthquakes between 2008 and 2025, far more than previous earthquake catalogues had recorded.
Researchers found that after the 2017 nuclear test, seismic activity around Mount Mantap appeared to be fundamentally altered.
Before the 2017 test, quakes in the area produced short-lived seismic sequences that decayed rapidly after testing ceased.
Earthquakes increased dramatically about three weeks after the detonation and continued to grow in both frequency and magnitude through until 2025, scientists found.
The earthquakes were concentrated along two roughly north-northwest–trending fault structures.
This indicates persistent and organised fault reactivation rather than random seismic activity, scientists say.
Researchers suspect that the repeated nuclear explosions at the mountain progressively damaged the region’s shallow crust.
The test likely altered the way the crust tolerated stress and led to faults that were already close to failure to become active gradually over several years, they say.
