Researchers, including those from Nanyang Technological University in Singapore and University of California, Berkeley in the US, showed how the explosion altered the mountain above the detonation.
The nuclear test on September 3, 2017 took place under Mt Mantap at the Punggye-ri nuclear test site in the country’s north, rocking the area like a 5.2-magnitude earthquake, according to the study published in the journal Science.
Based on seismic recordings from global and regional networks, and before-and-after radar measurements of the ground surface from Germany’s TerraSAR-X and Japan’s ALOS-2 radar imaging satellites, researchers showed that the underground nuclear blast pushed the surface of Mt Mantap outward by as much as 3.5 metres and left the mountain about 0.5 metres shorter.
By modelling the event on a computer, they were able to pinpoint the location of the explosion and its depth, 400-600 metres below the peak.
They also located more precisely another seismic event, or aftershock, that occurred 8.5 minutes after the nuclear explosion, putting it some 700 metres south of the bomb blast.
This is about halfway between the site of the nuclear detonation and an access tunnel entrance and may have been caused by the collapse of part of the tunnel or of a cavity remaining from a previous nuclear explosion.
“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 Teng Wang of the Earth Observatory of Singapore at Nanyang Technological University.
Putting all of this together, the researchers estimate that the nuclear test, North Korea’s sixth and the fifth inside Mt. Mantap, had a yield between 120 and 300 kilotons, about 10 times the strength of the bomb dropped by the US on Hiroshima during World War II.
That makes it either a small hydrogen, or fusion, bomb or a large atomic, or fission, bomb.
The explosion occurred more than a 450 metres below the summit of Mt Mantap, vaporising granite rock within a cavity about 50 metres across – the size of a football stadium – and damaging a volume of rock about 300 metres across.
The blast likely raised the mountain two metres and pushed it outward up to 3-4 metres, though within minutes, hours or days the rock above the cavity collapsed to form a depression.
Eight and a half minutes after the bomb blast, a nearby underground cavity collapsed, producing the 4.5-magnitude aftershock with the characteristics of an implosion.
Subsequently, a much larger volume of fractured rock, perhaps 1-2 kilometres across, compacted, causing the mountain to subside to about 0.5 metres lower than before the blast.
While it is possible to discriminate explosions from natural earthquakes using seismic waveforms, the uncertainty can be large, researchers said.
Explosions often trigger nearby earthquake faults or other natural rock movements that make the seismic signals look earthquake-like, confusing the analysis.