The ground in North Korea shook ever so slightly on May 12, 2010. The tremor was so small that it initially went undetected by scientists outside those closed borders.
But when scientists scrutinized the data, it became clear that something had happened. Still, the question of just what the seismic event might have been remained. Was this shaking natural, or was the seismic event evidence of a nuclear test?
After some debate among scientists, a team of seismologists now say the event was just a minor earthquake of a magnitude of about 1.5 and did not have the signature of an explosion. But just being able to discern and scrutinize those characteristics of such a tiny seismic event from outside North Korea could have significant implications for policymakers and scientists keeping an eye on the nuclear capabilities of the enigmatic country.
"You couldn't really develop a nuclear weapon in secret with this kind of monitoring," Jeffrey Lewis, the director of the East Asia Nonproliferation program at the Middlebury Institute of International Studies, who was not involved in the research, tells The Christian Science Monitor.
Scientists have long been monitoring data picked up by seismometers, sensors that detect motion in the Earth's surface, to look for signs that the regime might be testing nuclear weapons. Those explosions are typically quite clear to detect, but this event was hundreds of times smaller than normally thought to be associated with an explosion of any kind, be it nuclear or otherwise, study author Paul Richards, a seismologist at Lamont-Doherty Earth Observatory at Columbia University, explains to the Monitor.
The scrutiny of the 2010 event didn't begin with anything Earth-shaking. It began when scientists noticed radioactive isotopes characteristic of a nuclear explosion floating around the region. That's when seismologists went back to take a closer look at the data coming out of seismometers installed near North Korea.
Initially, Dr. Richards and his colleagues didn't find anything. But in 2015, another team of seismologists published a paper saying that there was indeed a seismic event in May 2010 – and, they said, it bore a signature characteristic of a nuclear explosion.
So Richards' team tried again to look for evidence of such an event.
"The question is what seismographic stations does one have access to," Richards explains. The 2015 paper used data from seismometers in mainland China that wasn't available to scientists across the globe.
So Richards looked for seismological stations he might not have thought to scrutinize before, close to the North Korean border. And lo and behold, his team also found evidence of an event.
But why do they say it was an earthquake, not an explosion? It comes down to the seismic waves that the event produced.
There are two types of waves propagated by a seismic event, P waves and S waves, Richards explains. "Explosions are particularly good at making P waves, and relatively poor at making S waves. Earthquakes are the other way around," he says. "Earthquakes are quite good at making S waves, and relatively poor at putting out P waves."
And when Richards and his team examined the fresh data coming out of that other network of seismometers, they determined that the event "was definitely earthquake-like."
They detailed their findings in a paper published Monday in the Bulletin of the Seismological Society of America.
Although this research is "topnotch" and "very detailed," says Keith Koper, the director of the University of Utah seismograph stations, who was not part of the research, this paper does not settle the question of whether the event was an explosion or earthquake.
That's not because this data doesn't "lean earthquake," Dr. Koper tells the Monitor. It does, he says. But the fact that both teams that have tackled this question have looked at different datasets leaves some ambiguity.
And the size of the event certainly didn't help, Koper says. "With this particular event, the main reason why there's this ambiguity and uncertainty is because the event was so tiny. With other events that North Korea has done, their other tests, it's very clear within hours. And seismologists are able to confirm that there was an explosion, estimate the yield, and get a very precise location, because they were all much bigger than this."
But the larger picture, both Koper and Richards say, is that this research shows that seismologists are able to scrutinize incredibly small events. And that means, Richards points out, that this could be enough evidence for policymakers to request an on-site inspection to resolve the question.
"We live in this really transparent world today," Dr. Lewis says. "We live in this incredible era where we can find these incredibly small events and have these debates about whether they were explosions or earthquakes."
And furthermore, he says, "North Korea can shut off the border, but we're still having a debate about an earthquake in the middle of North Korea."
So, Lewis says, "if you're a country that's thinking about cheating on the comprehensive nuclear test ban treaty, what chance do you have of getting away with it?"
No chance, Koper says. "The fact that this tiny, tiny event can be picked up, scrutinized, taken apart, that's amazing. Any nuclear test one would do would be much, much larger than that;" any nuclear-related seismic event of that size, he says, would probably just amount to a trivial science experiment.