North Korea’s Nuclear Tests Reactivated Faults Near Mount Mantap, Study Finds

The key finding is not simply that North Korea’s underground nuclear tests caused immediate shaking. The research indicates that the six tests at Punggye-ri may have left Mount Mantap’s surrounding crust mechanically altered for years, with the largest test in 2017 followed by an unusual, growing sequence of small earthquakes on faults that had previously been quiet. 1

What the researchers found

The study cataloged 1,399 local earthquakes between 2008 and 2025 near Mount Mantap. Historical records had not documented crustal earthquakes within 50 kilometers of the mountain between 1904 and 2017, making the later activity notable. 1

The most consequential change followed North Korea’s sixth and final nuclear test on 3 September 2017. Earlier research placed that explosion at roughly 0.5 kilometers depth and estimated a yield near 191 kilotons of TNT equivalent, though yield estimates depend on the method used. 6

According to the later analysis, seismicity began to rise around three weeks after the 2017 detonation. That pattern differs from a conventional aftershock sequence, in which earthquake activity normally declines after the initial disturbance. Instead, small earthquakes continued and intensified over subsequent years, with activity still detected in 2025. 1

Evidence for reactivated faults

Of the 1,399 detected events, researchers precisely located 955 along two inferred faults trending north-northwest. The clustering supports the interpretation that the earthquakes reflect slip on pre-existing structures rather than only localized disturbance around the explosion cavity. 1

The authors also modeled a larger—but conditional—hazard scenario: rupture of an approximately 24-kilometer fault segment could produce an earthquake of about magnitude 6.4. That is an estimate of what the fault geometry could allow, not a prediction that such an earthquake will occur. 1

Why the earthquakes may have been delayed

The study’s explanation is a chain of cumulative geological effects. Repeated underground blasts may have fractured and weakened shallow rock beneath and around Mount Mantap while also changing stress on nearby faults. 1

Fracturing can create or connect pathways for water. If water enters those fractures, increased pore pressure can reduce the effective friction holding a fault in place. In the researchers’ interpretation, this combination of damage, stress change, and fluid pressure helped faults begin slipping after a delay instead of producing only an immediate burst of aftershocks. 1

Why the result matters

The findings suggest that the consequences of underground nuclear testing may extend beyond the explosion and its short-term seismic signature. At Punggye-ri, the evidence points to a long-lived, evolving local earthquake hazard after testing stopped. 1

It also matters for nuclear-test monitoring. A population of later, fault-driven earthquakes can complicate efforts to distinguish a new explosion from natural or test-triggered tectonic events. The study therefore reinforces the need to analyze seismic event type, location, and timing rather than interpreting every nearby signal as evidence of another nuclear test. 1

The evidence does not show that a future test will necessarily trigger a damaging earthquake. It does show that the 2017 test was followed by persistent seismic activity consistent with fault reactivation—an important geological risk to consider alongside the site’s nuclear-monitoring significance. 1