An unexpected and prolonged increase in seismic activity at North Korea's extensive Punggye-ri nuclear test site, following the country's latest and most powerful test in 2017, has been identified by scientists. North Korea conducted six underground tests between 2006 and 2017 at the site on Mount Mantap in North Hamgyong province. The final test, with an estimated explosive yield of 160 kilotons, triggered a magnitude 6.3 earthquake and is believed to have resulted in the collapse of the facility, likely a tunnel. Nuclear bomb tests, such as those conducted in Nevada during the Cold War, have long been known to trigger small, detectable earthquakes.
However, the effects were typically short-lived, occurring in the days and weeks following the detonation and limited to the area around ground zero, the test site. The tests did not appear to affect the Earth's crust in a significant way, said Kwang-Hee Kim, lead author of a study published 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 region around Mount Mantap suffered 1,399 earthquakes, with both frequency 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 department of geophysical sciences at the University of Chicago, who was not involved in the research. "The Earth's crust has a long memory."
Seismic activity at a nuclear test site
The first earthquake occurred in 2013, following the third nuclear test, but the seismicity—the relative frequency and intensity of earthquakes—"remained sparse until 2017, when activity increased noticeably after the sixth and largest explosion," wrote the study authors. In contrast to typical post-explosion patterns, seismicity at Mount Mantap continued to escalate for years following the final test. "We were basically very shocked," said Kim, a professor in the department of geological sciences at Pusan National University in South Korea. "The seismic trend was so clear and linear." The earthquakes were not powerful, mostly ranging between magnitudes 2 and 3, and Kim noted that they appeared to occur at shallow depths. Prior to this century, according to the study, the region was tectonically quiet, with earthquakes being "virtually unknown." "We have a historical record of over 2,000 years in China and Korea.
We have one event at a distance of about 100 kilometers," said Kim. "I cannot say there are no earthquakes, but seismic activity here is very low." Unlike Japan to the east, the Korean Peninsula rests on stable continental crust. However, the team identified pre-existing "intraplate" faults—ancient fractures in the Earth's crust located far from active tectonic plate boundaries. The team suspects that the rising rate and intensity of the earthquakes indicate that the Earth's crust did not return to normal following the final test in 2017; instead, seismicity increased as at least two pre-existing faults became active, Kim stated. The irregular topography of Mount Mantap—a large, steep, and asymmetrical mountain—may have left certain faults closer to failure than others, with the underground explosions serving as a catalyst. The actual rate of earthquakes could be higher, as the team included only earthquakes detected by multiple monitoring stations, excluding data if only one or two monitoring stations recorded them, Kim said. The observations detailed in the study were convincing, and the increase in seismicity was "frankly surprising," Park stated. "In most previous cases, earthquakes triggered by underground nuclear tests subsided relatively quickly after the initial disturbance," Park said in an email.
"Here, the opposite appears to be happening: seismicity has persisted and even increased over time." However, she noted that delayed and long-lasting human-induced seismic activity is not unprecedented. In the Netherlands, more than 1,700 earthquakes have occurred since gas extraction began in the 1960s. The seismic activity has caused property damage. In Oklahoma, oil and natural gas production between 2009 and 2015 resulted in wastewater injection. The process, which stores fluids underground, led to a sharp increase in seismic activity across the state.
Pore pressure and long-term monitoring
The explosions may have fractured a large volume of rock, Park stated, "creating new cracks and pathways for water to move." "As water permeates these fractures, it increases pore pressure within the rock," she said. "This pressure essentially pushes the faults apart and makes them easier to slip." And because water moves slowly, the seismic response can be delayed for years, Park added. The study made no mention of underground water or what role it may 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 events and tectonic events," the study's authors wrote. Kim stated that researchers had no evidence that further nuclear tests had been conducted since 2017. The team had no access to the Punggye-ri nuclear test site or to any seismic data from North Korea.
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