An underground North Korean nuclear test in September last year exploded with 10 times the energy of the atomic bomb that exploded over Nagasaki in 1945.
It also caused parts of the overlying mountain peak to sink by half a metre and shift about 3.5 metres south.
These are conclusions drawn by geophysicists, who used satellite radar and instruments that pick up waves travelling through the earth, to calculate the explosion’s depth and strength.
In the journal Science today, they also report signs that a subterranean tunnel system at the test site collapsed 8.5 minutes after the bomb detonated.
In the past, satellite technology — called synthetic radar aperture imagery — has mapped how the ground stretches and warps after earthquakes.
But this is the first time it has been used to examine a nuclear bomb test site, according to Teng Wang, study co-author and a geophysicist at Singapore’s Nanyang Technological University.
Since the United Nations General Assembly adopted the Comprehensive Nuclear-Test-Ban Treaty in 1996, nine nuclear tests have taken place.
Six of these were by North Korea, five of which were at its Mt Mantap facility in the country’s north.
The bombs were detonated in chambers tunnelled into the mountain itself — a granite peak that extends upwards just over 2,200 metres.
But this means the details of the tests, such as the energy produced by the bombs, have been largely unknown outside North Korea — until now.
Eye in the sky, ear to the ground
Dr Wang and his colleagues suspected they could deduce the strength and precise location of the bomb test on September 3 last year, which triggered a magnitude-6.3 earthquake.
Clandestine nuclear activities are tracked by a global monitoring system of sensors that pick up the faint shivers and shudders generated by distant underground blasts and earthquakes.
But while these instruments are capable of picking up the wave signature of a bomb blast thousands of kilometres away, more information is needed to pinpoint exactly where an explosion has taken place.
So in the weeks after the September North Korean bomb test, Dr Wang and his colleagues received images of the Mt Mantap terrain before and after the test, snapped by the German TerraSAR-X satellite.
To map the bumps and dips on the Earth’s entire surface, TerraSAR-X pings radar towards the ground and measures the time it takes for the signal to bounce back up again.
“As long as the ground is deformed, we can measure it from space using synthetic radar aperture,” Dr Wang said.
Combined with a bit of nifty mathematical modelling — the first time anyone’s modelled an underground nuclear test with radar data — he and his colleagues got a fix on the exact location of the detonation site.
This is a highlight of the work, said Hrvoje Tkalcic, a geophysicist at the Australian National University, who was not involved in the study.
“What’s always difficult is pinpointing an exact location [of a bomb test],” Professor Tkalcic said.
Dr Wang and his team calculated that the top of the mountain subsided about half a metre after the September test, and parts of it shuffled south.
To manage this deformation, the bomb released the energy equivalent to between 109,000 and 276,000 tonnes of TNT in a chamber 450 metres below Mt Mantap’s peak.
The “Fat Man” bomb that exploded over Nagasaki yielded an energy level equivalent to 20,000 tonnes.
Among the data, they found the seismic shivers of a second, smaller event — an aftershock that appeared 700 metres south of, and 8.5 minutes after, the explosion.
The waves produced by the aftershock weren’t consistent with an explosion; rather, it looked like the ground had imploded.
This, the geophysicists suggest, “likely indicates the collapse of the tunnel system of the test site”.
Australia’s bomb monitor
While Dr Wang and his team used data from seismic monitoring systems in China and the surrounding area, Australia has one of the best in the world, Professor Tkalcic said: the Warramunga monitoring station in the Northern Territory, near Tennant Creek.
It’s almost smack bang in the centre of the continent, in an incredibly quiet part of the world, seismically speaking; far from tectonic plate edges, cities and the shoreline, where waves crashing on the coast create seismic noise.
It uses an array of buried instruments to pick up waves that travel through the ground, acting as a giant antenna to amplify weak signals.
“They’re used in the same way as astronomers use arrays of antennas to look at deep space. It’s just that our antennas are pointed to the centre of the earth,” Professor Tkalcic said.
There is also an infrasound detection system at Warramunga station, which detects waves that travel through the atmosphere produced by bomb blasts.
The data is transmitted by satellite to the Comprehensive Nuclear-Test-Ban Treaty Organisation in Vienna, where it is monitored round the clock.
So how do geophysicists know if seismic waves are caused by bomb blasts and not, say, an earthquake or landslide?
In a subterranean explosion, the ground is pushed outwards and compressed, sending a particular type of wave through the ground, Professor Tkalcic said.
An earthquake’s seismic signature is different. If two plates collide, rub against each other or slip, they send out another type of wave.
“We can tell if the first motion was predominantly a compression or if it was a shear type of motion,” Professor Tkalcic said.