North Korea set off an earthshaking explosion -- and claimed it was nuclear. Was it? For scientists, that was not a quick and easy question to answer.
Like earthquakes, large explosions send out shockwaves that can be detected with seismographs. Big nuclear bombs make big waves, with clear signatures that make them fairly easy to detect, analyze and confirm that they were caused by splitting atoms.
But smaller blasts -- as North Korea's appears to have been -- are trickier to break down.
The natural sound of the Earth, caused by tectonic plates grinding together, complicates the task of trying to determine if a blast was caused by conventional explosives or a nuclear bomb, said Xavier Clement of France's Atomic Energy Commission.
He likened the problem to trying to "detect the violins or a flute in a symphony orchestra when you are playing the cymbals."
His agency estimated the North Korean blast at around one kilotonne or less -- equivalent to the explosive force of 1,000 tonnes of TNT. For a nuclear device, that would be so weak that the French defense minister suggested that "there could have been a failure" with North Korea's reported test.
Clement said it could take days before scientists can declare with certainty whether the explosion was nuclear or not.
When blasts are very weak, "we could be in a situation where we cannot tell the difference," he said.
The US, Russia, China, Japan and South Korea are among the countries with equipment strong enough and close enough to monitor a North Korean test, according to Russian nuclear physicist Vladimir Orlov of the PIR Center, a nonproliferation think tank.
"It takes days, dozens of lab hours, to evaluate. Now we have only a rough estimate," he said.
The Comprehensive Nuclear Test Ban Treaty Organization (CTBTO), has about 200 stations worldwide to monitor nuclear tests as part of what it hopes will become the world's most reliable source of data for such tests. But until the treaty comes into force, the data is not made public, only released to governments and vetted partners.
Seismic data comes in almost immediately, and is usually passed to governments within an hour or so. Their scientists must decide what the numbers mean.
With the North Korean blast, there were wide variations. While the French atomic agency estimated around one kilotonne and South Korea's geological institute 0.5 kilotonnes, Russia's defense minister said there was "no doubt" that North Korea detonated a nuclear test and said the force of the underground blast was equivalent to 5,000 to 15,000 tonnes of TNT.
"People have different ways of cross-cutting the data and interpreting it," said Lassina Zerbo, director of the International Data Center at the CTBTO, which is based in Vienna, Austria.
The test ban treaty, which bans all types of nuclear explosions, whether tests or otherwise, will not enter into force until it has been ratified by 44 states that currently possess either nuclear weapons capability or nuclear research reactors.
So far 34 have ratified it. Nations that have not signed include the US, China, India, Pakistan, Israel and North Korea.
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