At 9 am Indian time on Sunday, seismographs around the world detected an artificially induced earthquake in Punggye-ri, the mountainous region where North Korea has conducted all its nuclear tests. The tremor came at exactly noon, North Korean time.
Eight minutes later, a second, smaller tremor was detected, believed to be the result of a mountain collapse at the underground test site.
A few hours later North Korean television confirmed what was already clear: Kim Jong-un’s isolated country had conducted its sixth nuclear test. The test, according to the North Koreans, was a two-stage thermonuclear device that would be mated to the Hwasong-14 ICBM.
North Korea has previously claimed its two tests in 2016 involved thermonuclear devices, though the relatively low estimated yields from those tests suggest they either failed or were in fact boosted fission devices.
Sunday’s test was different. The Japan Meteorological Agency and the US Geological Survey detected 6.1 and 6.3 magnitude events respectively. By one estimate, this could mean a 100 kiloton explosion, making it very likely that North Korea did indeed test a thermonuclear device.
The previous day, North Korean authorities released a photograph showing Kim Jong-un inspecting what it claimed was a thermonuclear weapon. The device (or mock-up) had a shape that resembled a typical two-stage H-bomb.
On Monday, the South Korean military said it had carried out live-fire exercises in response to its neighbour’s nuclear test, firing Hyunmoo short range ballistic missiles and deploying F-15K jets on simulated ground attack missions.
Separately, South Korea’s National Intelligence Service reportedly told lawmakers that the North could fire an ICBM towards the North Pacific.
Sunday’s nuclear test came just days after North Korea launched a Hwasong-12 intermediate range ballistic missile over Japan. The test, which was carried out in the early hours of August 29, resulted in the missile flying over Hokkaido and splashing into the North Pacific.