Last September, North Korea conducted its largest underground nuclear test beneath Mount Mantap. The mountain appeared to have collapsed as a result of the explosion and by studying the geological changes, geologists from South Korea and California have learned a lot about what happened.
As reported in Science, researchers have shown that the surface of the mountain was pushed outwards by about 3.5 meters (11.5 feet) when the explosion happened, before the mountain sank by about half a meter (1.6 feet). The yield of the atomic bomb was between 120 and 300 kilotons of TNT equivalent. For scale, the bomb dropped on Hiroshima is estimated to have been about 15 kilotons.
Mount Mantap is 2,200 meters (7,220 feet) high and the team has estimated that the nuclear device was detonated directly under the summit at a depth of 400-660 meters (1,300-2,160 feet). They were also able to pinpoint that seismic activity that followed 8.5 minutes after the explosion happened just 700 meters (2,300 feet) away. All of this analysis was done remotely, away from the North Korean facility for obvious reasons.
"This is the first time the complete three-dimensional surface displacements associated with an underground nuclear test were imaged and presented to the public," said lead author Teng Wang, of the Earth Observatory of Singapore at Nanyang Technological University, in a statement.
The game-changing technology that allowed this analysis is called Synthetic Aperture Radar, or SAR. The team combined seismic data with before-and-after satellite observations from Germany’s TerraSAR-X and Japan’s ALOS-2, both of which have a SAR on board.
"As opposed to standard optical imaging satellite imagery, SAR can be used to measure earth deformation day and night and under all weather conditions," co-author Roland Bürgmann, a UC Berkeley professor of earth and planetary science, added. "By precisely tracking the image pixel offsets in multiple directions, we were able to measure the full three-dimensional surface deformation of Mt Mantap."
Therefore, the team worked out the most likely scenario. They believe that on September 3, 2017, a nuclear device was detonated inside the mountain, vaporizing granite rock and forming a cavity the size of a football stadium. The event produced a 5.2-magnitude earthquake and raised the mountain. After a few minutes, a nearby cavity collapsed producing a smaller earthquake. Afterward, the rock began compacting, causing the whole mountain to sink slightly.
The researchers show that the technology they used can provide a lot of details regarding underground nuclear testing, although we hope that it won’t have to be used again as the de-nuclearisation of the Korean Peninsula continues.