North Korea’s underground nuclear tests at Mt. Mantap have had long-lasting and delayed effects on the region’s surrounding crust, according to a new study, triggering seismic activity that has persisted – and even intensified – for years after the detonations ended. The findings have implications for monitoring former nuclear test sites. They show that these explosions have the potential to reactivate seismically quiet faults and produce earthquakes that may be difficult to distinguish from naturally occurring tectonic activity. North Korea’s Punggye-ri nuclear test site beneath Mt. Mantap hosted six underground nuclear tests between 2006 and 2017. The final test – the largest – occurred in September 2017. This explosion had an estimated yield of 100 to 250 kilotons and was reported by the US Geological Survey (USGS) as a magnitude 6.3 seismic event. Moreover, satellite radar measurements reported substantial ground deformation of Mantap’s summit. Most previous studies of these nuclear tests have focused on determining their locations, timing, and sizes. However, their broader effects on the regions’ seismicity remain poorly understood. 

To address this gap, Xingli Fan and colleagues analyzed seismic data recorded in China and South Korea since 2008, including from stations located 80 to 200 kilometers from the test site, to trace how earthquake activity around Mt. Mantap has changed over time. Fan et al. identified 1,399 local earthquakes between 2008 and 2025 – far more than previous earthquake catalogs had recorded. Notably, the authors found that after the 2017 nuclear test, seismic activity around Mt. Mantap did not behave like previous post-explosion sequences, which tend to produce short-lived seismic sequences that decay rapidly after testing ceases. Instead, after the 2017 test, earthquakes increased about three weeks after the detonation and continued to grow in both frequency and magnitude through 2025. High-precision locations showed that the earthquakes were concentrated along two previously existing or previously unrecognized, roughly north-northwest–trending fault structures, indicating persistent and organized fault reactivation rather than random seismic activity. Lin et al. argue that repeated nuclear explosions progressively damaged the shallow crust and altered its internal stress field, allowing faults that were already close to failure to become active gradually over several years.