North Korean leader Kim Jong Un at an undisclosed location and date provided by the Korean Central News Agency in Pyongyang September 3, 2017. Credit: KCNA / Reuters
For nearly eight years, North Korea has not announced another nuclear test at Mount Mantap. But the ground beneath the mountain has not gone quiet.
Scientists have detected hundreds of small earthquakes around the Punggye-ri test site, where North Korea carried out six underground nuclear explosions between 2006 and 2017. The last produced a magnitude 6.3 seismic event and had an estimated yield of roughly 100 to 250 kilotons.
More surprisingly, the tremors did not simply fade with time. They became more frequent and released more seismic energy years after the final and largest blast.
While it may be tempting to think that the highly closed-off North Korean government is simply making more nuclear tests, something weirder is at play.
A new study suggests the previous nuclear explosions may have permanently altered the surrounding crust, weakening rock and reawakening old faults that had remained quiet for generations. The result is an unusual seismic afterlife for a nuclear test — one that is still unfolding years after the explosions stopped.
“I think the broader lesson is that short-lived disturbances can have consequences that persist for years or even decades,” Sunyoung Park, a University of Chicago geophysicist who was not involved in the work, told CNN. “The Earth’s crust has a long memory.”
A seismic sequence that refused to die
Satellite imagery of North Korea’s Punggye-ri nuclear test site. Credit: CNN.
For the new study, Xingli Fan of Chengdu University of Technology, Kwang-Hee Kim of Pusan National University and colleagues analyzed continuous seismic records from 2008 through 2025. The stations, in China and South Korea, sat 80 to 200 kilometers from the test site.
Using a pattern-matching method that can recover faint signals missed by routine earthquake catalogs, they found 1,399 local earthquakes. The first appeared in 2013, after the third nuclear test, but activity remained sparse until 2017. Sustained seismicity began about three weeks after the final blast and accelerated rather than decayed.
That is not how classic nuclear-test aftershock sequences usually behave. Following the 1968 Boxcar underground test in Nevada, for instance, scientists recorded thousands of small aftershocks during the next six weeks. At Mount Mantap, activity continued building for years.
Locations of the six nuclear tests by DPRK at Mt. Mantap and the distribution of seismic stations used in this study. Credit: Science (2026).
The setting makes the pattern more striking. Mount Mantap lies within stable continental crust, far from an active plate boundary. Major regional and global catalogs contained no documented crustal earthquakes within 50 kilometers of the test site from 1904 until the 2017 test.
When the team precisely relocated 955 earthquakes, the events traced two roughly parallel structures running north-northwest from the site. One followed the projected continuation of a mapped fault. The other had the geometry of a fault that had not previously been mapped.
“We were basically very shocked,” Kim said. “The seismic trend was so clear and linear.”
Why the mountain itself is important
The researchers do not think the final explosion simply shoved nearby faults hard enough to make them slip years later. The strongest acceleration came too late, and organized fault activity emerged 10 to 30 kilometers from the detonation points.
Proposed evolution of delayed topography-conditioned fault-zone seismicity beneath Mt. Mantap. Credit: Science (2026).
Instead, they propose that the six explosions progressively fractured and weakened the shallow crust. Stress then redistributed through that damaged rock, gradually bringing preexisting faults closer to failure.
Mount Mantap’s shape may have helped determine where. The mountain is steep and asymmetric. Computer models suggest its topography creates large variations in stress in the shallow crust, locally on the order of tens of megapascals. The authors argue that the uneven stresses may have left some fault segments closer to slipping before the explosions occurred.
Each dot represents an earthquake recorded by the new study. Credit: Xingli Fan and Kwang-Hee Kim/Science
Important uncertainties remain. With monitoring stations far from the site, the researchers say the earthquakes’ absolute depths are poorly constrained; their strongest evidence comes from the events’ map pattern and how it changed through time. They also had no direct access to Punggye-ri, for very obvious reasons.
Alternatively, Park raised the possibility that blast-created fractures could let water migrate through rock, raising fluid pressure and making faults easier to slip. The new study did not test that mechanism.
A problem for nuclear-test monitoring
Seismic monitoring is central to detecting underground nuclear explosions. The Comprehensive Nuclear-Test-Ban Treaty Organization’s global verification system combines seismic stations with infrasound, hydroacoustic and radionuclide sensors. Thanks to this global monitoring network, concealing a nuclear test is virtually impossible.
But if an old nuclear test can generate fault earthquakes years later, analysts may face a harder attribution problem. A tremor near a former test site could be a natural earthquake, a delayed consequence of an earlier explosion or something more recent. The study’s authors say sustained monitoring will therefore be important at legacy test sites.
The new study appeared in the journal Science.
