A tiny seismic event that occurred in North Korea in 2010 — believed to be a nuclear explosion — may have been just an earthquake, a study found. The analysis published in the Bulletin of the Seismological Society of America debunks a 2015 report by Chinese seismologists, which concluded that the seismic activity was triggered by a nuclear explosion.

The new report by seismologists led by Paul G. Richards from Columbia University's Lamont-Doherty Earth Observatory concluded that the tremors were “much more like that of an earthquake than an explosion.”

The seismic signals produced in the May 12, 2010, event were nearly 3,000 times smaller than those that came from confirmed nuclear tests conducted in North Korea in 2013 and 2016.

“We can now monitor down to extraordinarily small sizes of seismic events with high confidence,” he said in a statement published Monday. “That wasn’t the case 30 or even 10 years ago.”

“We’ll never be able to do it perfectly, but we can do it down to such a low magnitude that for all practical purposes we can know whether a nuclear explosion has taken place in a nuclear weapon development program,” he added.

Stations in South Korea, Japan and Russia first picked up traces of radioactive isotopes indicating a nuclear explosion in the reclusive country in May 2010. Researchers suggested this may be evidence of a nuclear detonation in North Korea where such incidents have occurred in the past.

However, at first, researchers could not find concrete proof of seismic activity triggered by a low-yield nuclear detonation related to the unusual radionuclide release that was picked up by the seismic stations. In 2015, Chinese seismologists found evidence of a very small seismic event that occurred in the region on May 12, 2010, which they concluded was a result of a nuclear explosion.

“The event they found was so small that we did not detect it during our original study, but it all seemed to fit together with the radionuclide data,” Richards said.

Using newly available data from seismic stations in China, Richards and his colleagues reviewed the 2015 report but when comparing the data with seismic signatures from confirmed nuclear explosions, the researchers “realized that the problem event turns out to look more like an earthquake,” Richards said.

He added that the best way to confirm if a nuclear explosion had taken place is to conduct an on-site inspection by the United Nations but this is not a possibility as Pyongyang has not ratified the Comprehensive Test Ban Treaty (CTBT).

“You can imagine that every now and then you will get conflicting technical information on a seismic event when you only use data collected from a distance,” Richards said. “Under the CTBT, it would be possible to conduct an on-site inspection to resolve this sort of disagreement.”

Pyongyang has conducted several nuclear tests, all in violation of the U.N. Security Council regulations. The international body has imposed strong sanctions on the isolated nation following its nuclear weapons tests and other ballistic missile launches.