North Korea sets off an earthshaking explosion — and claims 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 on 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, with its constant seismic activity of tectonic plates grinding together, complicates the task of trying to determine whether a smaller blast was caused by conventional explosives or a nuclear device, Xavier Clement of France’s Atomic Energy Commission said. 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 1 kiloton or less — equivalent to the explosive force of 1,000 tons 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 the North Korean reported test.

Mr. Clement said it could take days before scientists can declare with certainty whether the explosion was nuclear. And when blasts are very weak, “we could be in a situation where we cannot tell the difference between the two,” he said.

America, Russia, China, Japan, and South Korea are among the countries with equipment strong enough and close enough to monitor a North Korean test, a Russian nuclear physicist, Vladimir Orlov of the Moscow-based PIR Center, a nonproliferation think-tank, said.

“It takes days, dozens of lab hours, to evaluate results. Now, we can have only a rough estimate,” he said.

The Comprehensive Nuclear Test Ban Treaty Organization, or CTBTO, has about 200 stations worldwide designed for monitoring nuclear tests as part of what it hopes will become the world’s most reliable source for such tests. But until the treaty comes into force, the data are 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 and graphs mean. With the North Korean blast, there were wide variations. While the French atomic agency estimated around 1 kiloton and South Korea’s geological institute half of that, Russia’s defense minister expressed “no doubt” that North Korea detonated a nuclear test and said the force of the underground blast was equivalent to between 5,000 and 15,000 tons of TNT.

“People have different way of crosscutting the data and interpreting them,” Lassina Zerbo, who is the director of the International Data Center at the CTBTO, which is based in Vienna, Austria, said.

The test ban treaty, which bans all nuclear explosions, will not enter into force until it has been ratified by 44 states who possess either nuclear power or research reactors. So far 34 have ratified it. Holdouts include America, China, India, Pakistan, Israel, and North Korea.

The CTBTO’s stations are more extensive than those used by most countries. They monitor seismic events but also underwater data, radioactive particles in the air, and radiowaves.

“Within 72 hours, we will have full data. Then, all this will be available to member states,” Ms. Zerbo said.