After North Korea set off its sixth and most powerful nuclear test beneath Mount Mantap on Sept. 3, 2017, the ground around the site did not settle down. It got more active. A study published Sept. 17 in the journal Science identified 1,399 small earthquakes around the Punggye-ri test site between 2008 and May 2025, and both their frequency and their strength increased for years after the final blast.

That runs against decades of experience. Underground tests in Nevada during the Cold War caused small earthquakes that were typically short-lived and confined to the area around ground zero, CNN reported on the nearly 1,400 quakes. "It's completely different from our expectation or textbook cases," study co-author Kwang-Hee Kim, a seismologist at Pusan National University who led the research team, told Science's news report on the North Korean earthquake study.

The finding also raises a practical problem. The study authors wrote that persistent quakes at a former test site could blur the distinction between explosion-related and tectonic events. That makes it harder for monitors to tell whether a new tremor is a natural quake or a sign of another test.

A Mountain That Would Not Settle

Before testing began, the area appeared seismically quiet. Historical earthquake records show no seismic activity around Mount Mantap before North Korea began nuclear testing at Punggye-ri in 2006, according to the Korea Times.

The pattern changed in stages. Researchers detected about 60 earthquakes between the first test in 2006 and the sixth test in 2017, with activity gradually increasing after the third test in 2013. Seismic activity intensified about 20 days after the 2017 blast, and about 1,330 more earthquakes have been detected since, concentrated along two north-northwest-trending fault zones stretching about 24 kilometers (15 miles).

The quakes have also grown stronger. Magnitudes rose from about 1.0 to 1.3 in the early years to above 3.0 in 2023 and 2024, and the largest reached magnitude 3.4. Most were unusually shallow, within about 1 kilometer of the surface, compared with the roughly 15-kilometer depth typical of Korean earthquakes. Science reported that small quakes were still occurring in 2025 along two suspected faults, some as far as 20 to 30 kilometers (12 to 19 miles) from the test site.

Finding Faint Quakes Without Setting Foot on the Site

The team had no access to the test site and no seismic data from North Korea. Instead, researchers from Pusan National University and four Chinese institutions analyzed 17 years of continuous seismic records from stations in South Korea and northeastern China.

To find quakes too faint for routine catalogs, the team used the recorded waveforms of known earthquakes as templates and scanned years of data for fainter matching signals buried in background noise. This approach revealed the full catalog of 1,399 events, of which 955 were precisely located. Most were tiny, below magnitude 2.

The researchers concluded that repeated tests progressively damaged the surrounding rock and changed the stresses on pre-existing faults. "Repeated underground nuclear tests [have] progressively disturbed and damaged the surrounding rock and changed the stresses acting on pre-existing faults," Kim told Scientific American. He said the 2017 test caused the most damage and set off a domino effect along those faults.

Outside experts see a broader lesson. "I think the broader lesson is that short-lived disturbances can have consequences that persist for years or even decades," Sunyoung Park, a geophysicist at the University of Chicago who was not involved in the study, told CNN. The study did not examine what role groundwater might have played.

The finding joins a growing list of human-caused earthquakes. Science noted that wastewater injection from oil and gas operations, mining, reservoir filling, geothermal projects and underground nuclear detonations have all been shown capable of triggering quakes. What stands out at Mount Mantap is how long the activity has lasted.

A Harder Job for Nuclear Test Monitors

The monitoring challenge is concrete. Networks watch Punggye-ri closely because any seismic event there could signal a new test. When a former test site keeps producing quakes that grow stronger over time, each tremor needs closer analysis before it can be ruled in or out.

Kim told CNN the team found no evidence of any nuclear test since 2017. North Korea invited international journalists in 2018 to witness explosions that appeared to demolish parts of the site, but the International Atomic Energy Agency has since reported signs that the site was being reopened, starting in 2022.

The evidence has limits. This is an observational study of one site, based on distant stations with no ground access. The earthquakes recorded so far have been small, and none has been reported to cause damage. Kim estimated that the 24-kilometer fault zone could generate an earthquake of up to magnitude 6.4 if it were fully reactivated at once, the Korea Times reported, a similar figure also reported by the Seoul Economic Daily. That is a worst-case estimate of the fault zone's potential, not a forecast, and the researchers gave no timing.

The bottom line: nearly a decade after the last known test, the ground beneath Mount Mantap is still active, and the trend has been rising rather than fading. Scientists do not yet know whether the faults will stabilize. Continued regional monitoring will show whether the activity levels off or keeps growing.

What Readers Want to Know

What did the study find?

It identified 1,399 small earthquakes near North Korea's Mount Mantap test site between 2008 and May 2025. Their frequency and magnitude increased for years after the 2017 nuclear test.

Why is that surprising?

Earthquakes after underground nuclear tests are usually short-lived and stay near ground zero, as they did after Cold War tests in Nevada. At Mount Mantap, activity grew instead.

How did scientists detect the quakes without visiting the site?

They analyzed 17 years of records from stations in South Korea and northeastern China, using the waveforms of known quakes as templates to find fainter matching signals.

How strong are the earthquakes?

Most have been below magnitude 2 and very shallow. The largest reached magnitude 3.4, and no damage has been reported.

Has North Korea tested a nuclear weapon since 2017?

The researchers said they found no evidence of any test since 2017. The IAEA has reported signs since 2022 that the site was being reopened.

Why does this matter for nuclear monitoring?

Ongoing quakes at a former test site can blur the line between explosion-related and natural events, which makes new tremors harder to interpret.

Could a larger quake occur?

Kim estimated that the fault zone could produce up to a magnitude 6.4 quake if it were fully reactivated at once. That is a worst-case estimate, not a forecast.