Scientists Scramble to Study North Korean Blast

Scientists are trying to determine whether the North Korean explosion on Monday was in fact caused by a nuclear bomb. The blast generated seismic signals heard around the world. But that alone does not prove the blast was nuclear.

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SUSAN STAMBERG, host:

Earthquake monitors around the world yesterday picked up vibrations from North Korea's nuclear test range, but those signals alone are not enough to prove that it was a nuclear blast.

NPR's Richard Harris reports on research that could ultimately provide critical details about the explosion and the device that caused it.

RICHARD HARRIS: Researchers have spent decades refining earthquake science to keep an ear to the ground for nuclear tests. Paul Richards at Columbia University's Lamont-Doherty Earth Observatory says the North Korean event has become instant fodder for study around the world.

Mr. PAUL RICHARDS (Lamont-Doherty Earth Observatory, Columbia University): The stations that are a long way away, such as Australia and the United States, one can tell that something happened; but for an event this small, it's hard to tell what it actually was.

HARRIS: It seems likely that the event was an explosion. North Korea reportedly gave China a ten-minute warning that a blast was coming, after all. And Richards says ultimately earthquake monitoring stations close to the event will have gathered enough detailed information about the various seismic waves to distinguish clearly between an earthquake and a blast.

Mr. RICHARDS: There are a lot of different signals that get put out that these types of seismic events.

HARRIS: Is it possible to distinguish between a small nuclear explosion and a large conventional explosion?

Mr. RICHARDS: Well, it's important to say that, from the seismology point of view, the answer is no. There have been a number of chemical explosions that have the characteristics of a nuclear explosion.

HARRIS: One way to tell the difference is by the size of the event, and seismic stations can help there. But it can take many weeks to get a definitive magnitude from any seismic event. This one has variously been estimated to have a preliminary magnitude of 3.6 or 4.2.

Mr. RICHARDS: If it's interpreted as a nuclear explosion in hard rock, it would be on the order of a kiloton.

HARRIS: That's equivalent to a thousand tons of TNT. That makes it a very small nuclear weapon or a very large conventional explosion.

For comparison, the atomic bomb dropped on Hiroshima was about 15 times bigger, and typical nuclear warheads in the U.S. arsenal are hundreds of times more powerful.

In terms of conventional explosives, 1,000 tons could fit into a short string of railroad cars. So it's awkward, but not impossible, for this to be a conventional blast.

Robert Norris at the Natural Resources Defense Council says earthquake analysis isn't the only line of intelligence we might have on this event.

Mr. ROBERT NORRIS (Senior Research Associate, Natural Resources Defense Council): Now if the blast was big enough, or if they didn't do a good enough job, there could be some leakage of radionuclides into the atmosphere.

HARRIS: Planes have been dispatched to look for those radionuclides. They would be a mix of gaseous radioactive bi-products of a blast. They could reveal a good deal about the nature of the explosion; for example, whether it was a plutonium bomb or one made of uranium.

But Norris says there are some things we may never know.

Mr. NORRIS: If it was intended to be a five kiloton explosion and it was only one, then it went off, but it didn't go off to the full design specification.

HARRIS: And one of the most critical questions is how big was the device itself.

Mr. NORRIS: Is it as big as an automobile, or is it as big as a chair or a suitcase? And the smaller it is, the more easily it would be able to be put on a missile. And we know that they're working busily on a missile program as well. And that's probably the way they want to weaponize this warhead.

HARRIS: Those are questions for intelligence agencies, who may or may not tell us what they discover in the course of their investigations.

Richard Harris, NPR News.

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