Japan’s meteorological agency analyse the seismic activity after the blast Getty

On Sunday at exactly noon local time, a massive vibration emanated from the mountains of Kilju County in North Korea. It felt like a short earthquake, a magnitude 6 or so tremor, but the disturbance was in fact caused by an underground nuclear weapons test – and it could signal a significant leap forward for the nation’s nuclear capabilities. It might be their first “proper” hydrogen bomb.

“The nuclear explosion was seen around the world – the surface waves generated really do cover the globe,” says Steven Gibbons, senior research geophysicist at Norwegian monitoring agency NORSAR, which combines data from its own sensors with that of other institutions.

The rumbling was even registered by a seismometer set up by a school in Nottingham, England.


“I was surprised by the sheer size of the event,” says Gibbons, explaining an early morning tweet. “It towers above all the others.”

The “others” are the five previous nuclear weapons tests thought to have been carried out by North Korea between 2006 and 2016. The latest is likely to be an order of magnitude bigger than the last one, about a year ago. While it is difficult to estimate the explosive power, or yield, of the bomb used because the exact geology of the site is unknown, NORSAR’s calculation suggests around 120 kilotons – nearly 10 times the size of the bomb dropped on Hiroshima in 1945.

Most experts agree that such a large detonation was probably caused by a hydrogen bomb, or H-bomb – a nuclear weapon that uses a two-stage explosion. The process involves a small fission reaction that emits X-rays that then trigger the fusion of hydrogen isotopes in a separate, secondary compartment.

Kim Jong-un pictured with the bomb that may have been behind the blast Getty

Last year North Korea claimed it had developed an H-bomb but most experts believed that instead it was a “boosted fission” device – a standard fission weapon spiked with heavy isotopes of hydrogen. Now it looks as if they might have the real deal.

That’s certainly the interpretation North Korea wants us to make. Leader Kim Jong-un was pictured with a bomb design the day before the test that suggested a two-stage device, dubbed “the peanut” by one researcher.

It’s not actually possible to differentiate between H-bombs and pure fission detonations on seismic data alone, but the size of the explosion is a good indicator, says Gibbons.

Further evidence may be collected in the form of radionuclides that leak into the atmosphere from the underground test site. These unstable atoms are produced in a nuclear explosion and may be detectable by aircraft fitted with special instruments.

Sniffing for evidence

So far China says it has not detected any radioactive material. The radionuclides would reveal unequivocally whether it was an H-bomb as they differ depending on what materials were used in the explosion.

Either way, the incident has huge diplomatic ramifications, not least because North Korea’s capabilities have evolved so quickly. The country now appears to have intercontinental ballistic missiles (ICBMs) that could in theory deliver a nuclear warhead to targets in the US.

Sunday’s tremors were felt underfoot by residents in China and Russia to the north of the test site, according to reports collected by the US Geological Survey. This may hasten North Korea’s powerful neighbours’ willingness to work with the US on diplomatic solutions to the crisis, says Catherine Dill at the James Martin Center for Nonproliferation Studies in California.

But she adds that a “pre-emptive strike” option to disarm North Korea is too risky to properly execute.

“In my opinion it’s not reasonable to assume that we can destroy all their [nuclear weapons] sites even if we know where they are,” she says.