North Korea’s nuclear tests may have reactivated seismic faults

By Katie Hunt, CNN

(CNN) — Scientists said they have detected an unexpected and prolonged increase in seismic activity at North Korea’s sprawling Punggye-ri nuclear test site since the country’s final and most powerful test in 2017.

North Korea conducted six underground tests between 2006 and 2017 at the site at Mount Mantap in North Hamgyong province. The final test, with an estimated explosive yield of 160 kilotons, unleashed a magnitude 6.3 earthquake and is believed to have resulted in an on-site collapse, perhaps of a tunnel.

Tests of nuclear bombs, such as those conducted in Nevada during the Cold War, have long been known to cause small, detectable earthquakes. However, the effects were typically short-lived, occurring in the days and weeks after the explosion and confined to the area around ground zero, the test site.

The tests did not appear to affect Earth’s crust in a significant way, said Kwang-Hee Kim, lead author of a study published Thursday in the journal Science.

However, according to seismic wave data from 2008 to 2025 collected and analyzed by Kim in South Korea and his colleagues in China, the area around Mount Mantap experienced 1,399 earthquakes, with the number and magnitude increasing over time.

“I think the broader lesson is that short-lived disturbances can have consequences that persist for years or even decades,” said Sunyoung Park, an assistant professor in the University of Chicago’s department of the geophysical sciences who was not involved in the research. “The Earth’s crust has a long memory.”

Earthquake detection at nuclear test site

The earliest earthquake occurred in 2013, after the third nuclear test, but seismicity — the relative frequency and intensity of the quakes — “remained sparse until 2017, when activity increased markedly after the sixth and largest detonation,” the study authors wrote. Unlike typical post-explosion patterns, seismicity at Mount Mantap continued to increase for years after the final test. “We were basically very shocked,” said Kim, a professor in Pusan National University’s department of geological sciences in South Korea. “The seismic trend was so clear and linear.”

The earthquakes were not powerful, mostly of a magnitude of between 2 and 3, and Kim said they appeared to be at a shallow depth.

Before this century, according to the study, the area had been quiet tectonically, with earthquakes “virtually unknown.” “We have more than 2,000 years of historical record in China and Korea. We have one event about 100 kilometers away,” Kim said. “I cannot say there’s no earthquakes, but the seismic activity here is very low.”

Unlike Japan to the east, the Korean Peninsula sits on stable continental crust. However, the team identified preexisting “intraplate” faults — ancient fractures in Earth’s crust far away from active tectonic plate boundaries.

The team suspects the increase in the rate and intensity of earthquakes shows that Earth’s crust did not return to normal after the final test in 2017, but instead seismicity increased as at least two preexisting faults became active, Kim said. Mount Mantap’s irregular topography — it’s a large, steep and asymmetric mountain — may have left some faults closer to failure than others and the underground explosions served as a trigger.

The real rate of earthquakes could be higher as the team only included earthquakes detected by multiple monitoring stations but excluded data if only one or two monitoring stations recorded it, Kim said.

The observations detailed in the study were convincing and the increase in seismicity “genuinely surprising,” Park said.

“In most previous cases, earthquakes triggered by underground nuclear tests declined relatively quickly after the initial disturbance,” Park said in an email. “Here, the opposite seems to be happening: seismicity has persisted and even increased over time.”

However, she noted that delayed and long-lasting seismic activity caused by humans is not unprecedented. In the Netherlands, more than 1,700 earthquakes have occurred since gas extraction took place in the 1960s. The seismic activity has damaged property. In Oklahoma, oil and gas production between 2009 and 2015 resulted in wastewater injection. The process, which stashes fluids underground, drove a marked increase in seismic activity in the state.

The explosions may have fractured a large volume of rock, Park said, “creating new cracks and pathways for water to move through.”

“As water infiltrates those fractures, it increases pore pressure within the rock,” she said. “That pressure effectively pushes faults apart and makes them easier to slip.” And because water moves slowly, the seismic response can be delayed by years, Park added. The study made no mention of groundwater and what role it might have played in triggering the earthquakes.

The discovery that underground nuclear tests can trigger intensifying seismic activity could complicate long-term monitoring of underground nuclear test sites, blurring the distinction “between explosion-related and tectonic events,” the study authors wrote.

Kim said the researchers had no evidence that further nuclear tests had taken place since 2017. The team had no access to the Punggye-ri nuclear testing site or any seismic data from North Korea.

CNN was among international journalists invited to North Korea in 2018 to witness explosions at the site and observe its apparent destruction. However, the International Atomic Energy Agency reported that North Korea began restoring test tunnels in March 2022, and a 2025 US Defense Intelligence Agency report stated North Korea had restored its nuclear test site.

South Korean research has documented possible signs of radiation exposure in defectors from North Korea who lived near the test site, but scientists said the cause remains unclear.

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