But this is at best a strong indication of a test and not an absolute confirmation.
An earthquake expert at the state-run Korea Meteorological Administration said his office aims to find out the magnitude of the tremor, the time it started and the exact location on the map within 10 minutes of the explosion. He spoke on condition of anonymity because he wasn't authorized to speak to the media.
Experts also note that artificial earthquakes, such as those created by nuclear explosions, rarely trigger the same wave patterns as natural quakes.
North Korea could also try to deceive and give the impression that it exploded a nuclear device by simply exploding sophisticated conventional weapons that would trigger the same seismic waves produced by a nuclear test, said Chi Heoncheol, an earthquake specialist at the government-funded Korea Institute of Geoscience and Mineral Resources.
By raising tensions this way, North Korea may hope to wrest concessions or aid in return for promises to scale back its unproven nuclear capability.
"Even if they bring truckloads of high-powered conventional explosives, put them (into an underground tunnel) and explode them, they will generate the same seismic wave and sound wave," Chi said. The only difference is no radioactivity would be detected from the explosion of conventional weapons, he said.
The best course for scientists would be to collect air samples to look for increased radiation but the process could take days. Even if the wind is favorable — and assuming North Korea conducts the test at Punggye-ri in the country's northeastern corner — it will take more than one day for airborne radioactive isotopes like xenon to reach South Korea, according to an official at the government-run Nuclear Safety and Security Commission.
The official, who requested anonymity citing the sensitive nature of the subject, acknowledged it may be impossible to confirm a test if the wind doesn't blow southward or if North Korea plugs the underground tunnel so tightly that no radioactive gas escapes.