Seismic waves from a nuclear test in North Korea. A study uses such waves alongside radar data to measure movement at the mountain test site.Credit: Chung Sung-Jun/Getty

When North Korea detonated its most recent nuclear weapon deep inside its mountain test site, the peak’﻿s flanks heaved up and outwards by a few metres, according to a detailed reconstruction of the event and its aftermath.

The analysis is the first to combine satellite radar images with seismic data to track the effects of the blast on 3 September 2017 at Mount Mantap. It also found that the explosion — which created a seismic disturbance registered at magnitude 6.3 — caused the top of the mountain to sink by about half a metre. The work is published on 10 May in Science1.

“I have mapped surface displacement from many geodynamic processes such as earthquakes, volcano eruptions, landslides, but I have never seen such a large displacement caused by human activity,” says study co-author Teng Wang, a remote-sensing and geodesy researcher at Nanyang Technological University in Singapore.

Despite the mountain’s large lurch, the analysis shows no visible crater at its summit, says Wang. This could ease fears that the mountain might have undergone a substantial collapse that could leave it susceptible to leaking radiation, he says. The test site near Punggye-ri has housed all six of North Korea’s nuclear tests, beginning in 2006.

Researchers say the work is the most detailed study yet of a nuclear test using a technique called synthetic aperture radar (SAR), which is commonly used to make 3D terrain maps. They say the analysis demonstrates how geophysicists could use such data to complement conventional seismic methods when monitoring future nuclear tests.

“Nuclear tests are not common these days,” says Matt Wei, a geophysicist at the University of Rhode Island in Narragansett. “However, monitoring nuclear tests is still one of the most important things that we do.”

Rise, fall and compaction

Most detection and analysis of nuclear blasts has relied on seismic data from observing stations in the region. But measuring the actual displacement of the mountain is difficult without access to the site.

Wang’s team turned to SAR, which creates high-resolution images by sweeping a radar beam over the target from a high-speed moving platform. Wang and his co-authors compared data gathered by the German TerraSAR-X satellite before and after the blast.

They found that the mountain’s slopes had moved outwards horizontally by up to 3.5 metres, with the highest shift on the west and south flanks, where the terrain was steepest.

Using further seismic data and computer modelling, they recreated the sequence of events to simulate what had happened inside the mountain, tuning the model to match their observations. Their analysis showed that the explosion was followed by the partial collapse of the rock directly above the blast cavity, about a second later.

A second, smaller seismic event eight and a half minutes later — which many have interpreted as the collapse of some of the test site’s tunnels — probably did little to move the mountain itself, says Wang.

However, over the next week, the damaged rock above the cavity slowly settled in on itself, according to the team’s models — causing the mountain’s summit to sink by half a metre.

The researchers estimate the bomb’s yield at equivalent to roughly 190 kilotonnes of TNT, or 13 times as powerful as the bomb dropped on Hiroshima, Japan, at the close of the Second World War. That calculation is slightly higher than estimates made soon after the blast.

Confirm and corroborate

Wei says the study shows that observations from space can be accurate enough to act as an independent tool for analysing nuclear tests. A study2 he published last year used a similar SAR technique to look at North Korea’s fourth nuclear test, in January 2016, but couldn’t distinguish between a landslide and actual movement of the mountain. His method was less sensitive to horizontal movements, he says. But Wang’s results show that “the movement is definitely caused by the nuclear explosion”, he says.

In recent weeks, the state of the test site has become the subject of intense speculation. North Korean leader Kim Jong-un has pledged to close the site by the end of May, a possible signal of his willingness to negotiate giving up his country’s nuclear weapons.

Two seismic studies from Chinese researchers, published in March3 and April4, have also suggested that the mountain’s partial collapse could leave it at risk of leaking radiation, possibly rendering the site unusable.

Won-Young Kim, a seismologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York, says that given the amount of displacement reported in the latest study, the facility might be compromised, but he thinks reports that the entire mountain is unstable and could collapse are “overblown”.