North Korea's 6th nuclear test awakened dormant faults, study finds

North Korea's sixth nuclear test in 2017 reactivated dormant faults around Mantap Mountain near the Punggye-ri test site, a new study has found.

Pusan National University said Friday that a research team led by Kim Gwang-hee, a professor in the Department of Geological and Environmental Sciences, worked with researchers from Chengdu University of Technology, the Chinese Academy of Sciences, the China Earthquake Administration and Shandong University to document long-term seismic activity around Mantap Mountain and trace the reactivation of pre-existing faults in the area.

The findings were published in the international journal Science.

North Korea conducted six underground nuclear tests at Punggye-ri between 2006 and 2017. Small earthquakes and underground collapses typically follow such tests immediately, but at sites including the Nevada Test Site in the United States and the former Soviet Semipalatinsk test site, that activity is known to subside within days to weeks.

The research team analyzed 17 years of continuous seismic waveform data recorded in South Korea and northeastern China. The data showed that seismic activity increased for years after the sixth test in 2017, with earthquakes clustering along two previously quiet fault zones oriented in a north-northwest direction. Over time, the locations of those earthquakes became increasingly concentrated along the fault lines.

The team interpreted this as the cumulative effect of repeated nuclear tests fracturing and damaging underground rock while altering the surrounding stress state. As stress within the crust slowly redistributed following each explosion, it promoted movement along existing faults that were already primed to slip.

The findings show that seismic activity can persist long after a nuclear explosion ends, driven by residual damage to underground rock and ongoing stress redistribution.

The study is significant because it challenges the assumption that post-test seismic activity is a short-term phenomenon. Accurately identifying the causes of earthquakes that occur years after a nuclear test requires comprehensive analysis of long-term observational data, fault distribution and stress changes, the researchers said.

The team said the findings are expected to inform not only monitoring systems designed to verify compliance with the Comprehensive Nuclear-Test-Ban Treaty, but also risk assessments for induced seismicity linked to human activities such as underground development and energy extraction.

"What we found particularly striking was that seismic activity did not die down quickly after the 2017 test," Kim said. "The earthquakes continued for years, and over time they became increasingly concentrated along pre-existing faults."

He added that continuous seismic monitoring and the sustained accumulation and preservation of observational data are essential even after nuclear explosions cease. "Safety assessments for underground spaces must extend from before development begins through operations and beyond decommissioning," he said.

Kim's team previously published research in Science in 2018 suggesting that the Pohang earthquake was most likely triggered by fluids injected during geothermal energy development.