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Detecting Underground Nuclear Blasts

Mike was the first "true" hydrogen bomb tested by the United States. This 10.4 megaton explosion obliterated Elugelab, the island it was detonated on in the Eniwetok Atoll, gouging out a crater more than 6,000 feet across, and sending a mushroom cloud more than 100,000 feet into the stratosphere. Today, nuclear tests are conducted underground. Corbis hide caption

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Earthquake monitors around the world 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. Here, a look at what scientists are able to detect about underground nuclear blasts:

Q: How do scientists estimate the size of the North Korea explosion?

A large underground explosion generates a shock wave that travels great distances through the Earth. Seismic detectors register the vibrations. The U.S. Geological Survey says the explosion they recorded Monday was the equivalent of a 4.2 magnitude earthquake. Assuming the test took place in rock, this would translate into an explosion equivalent to roughly one kiloton, or 1,000 tons of TNT. For comparison, the "Little Boy" bomb dropped on Hiroshima in 1945 exploded with an energy of about 13 kilotons. In 1952 "Mike" — the first hydrogen bomb — created a blast of more than 10 megatons, 1,000 times more powerful.

Q: Was this definitely a nuclear explosion?

Few doubt that North Korea tested some sort of a nuclear bomb. Detection of radioisotopes in the air would confirm this though, and could provide useful intelligence about the North Korean design. "Underground tests almost always leak," says Richard Garwin a physicist and longtime government adviser.

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Q: Was the test a failure if it only had a yield of a kiloton?

Many scientists suspect North Korea was hoping for something more. "Nobody would waste plutonium looking for a sub-kiloton explosion" says Richard Garwin. Raymond Jeanloz, a planetary scientist at the University of California, Berkeley, who also works on nuclear issues, says it would be wrong to call the test a dud. "A kiloton is nothing to sneeze at," he says, "It would sure ruin your day if it went off on the Mall" in Washington, D.C.

Q: Is it possible the actual explosion was larger, but muffled in some way?

Yes, but analysts seem to think this is unlikely. Raymond Jeanloz says to muffle an explosion, workers would have to dig a much bigger hole in the ground, something that would have been visible from spy satellites. And North Korea had no obvious need to hide the explosion. In fact, it told the world in advance the test was coming.

Q: What is required for an underground test?

Essentially a shovel and miles of cable. From 1951 to 1992, the United States performed 828 underground nuclear tests at the Nevada Test Site. The holes were sometimes 10 feet wide, over a mile in depth and could take two months to drill. Workers lowered the nuclear device into the hole, filled the hole with sand and gravel, and sealed it with a plug. The underground explosion generates an enormous shock wave and temperatures of sometimes 100 million degrees Celsius, enough to melt the surrounding rock. Electrical and fiber-optic cables carried the signals from sensors to a trailer, which would record data from the explosion.

Q: Is North Korea likely to conduct more tests?

Raymond Jeanloz points out that there are two kinds of testing. The first test "shakes the ground and shows you can do it." Subsequent tests are often aimed at testing and refining a design. The U.S. conducted more than a thousand tests. Pakistan and India only conducted a handful.