Seismologists monitor North Korea's nuclear blasts

— -- North Korea's nuclear test last week started one army on the march — seismologists bent on capturing the exact size of the blast.

"A nuclear test has a seismic signature completely different from an earthquake," says geologist Paul Richards of Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y. "And we have been steadily getting better at sizing them up."

During the 1980s, debate over the ability of seismologists to detect nuclear tests played a role in arguments over nuclear arms agreements between the United States and Soviet Union. A 1988 report from the Congressional Office of Technology Assessment, for example, suggested that underground nuclear tests smaller than 5 kilotons in size (a kiloton equals the explosive force of 1,000 tons of TNT) might be undetectable if conducted in cavities or if surrounded by porous rock.

Now two decades later, says Richards, much smaller explosions cannot escape detection. "We have had an explosion in the numbers of monitoring stations, more sensitive instruments and better analysis. Lots of boring things like that," he jokes.

North Korea's 2006 nuclear test, rapidly spotted by seismologists, might have resulted in a blast equal to only 500 tons of TNT. Some scientists such as George Smith of Global Security.Org suggested the test was a " fizzle", in which the plutonium in a bomb blasts itself apart faster than the nuclear chain reaction that causes a full explosion.

The recent test clocks in at 2.2 to 4 kilotons, based on the magnitude 4.5-4.7 tremors that it triggered, Richards says. "What we can say with confidence is that this most recent test was five times bigger than 2006."

Nuclear blasts feature a signature double bump of nearly instant tremors on earthquake monitoring records. "Some people see that double bump and think it was two tests, but that is quite wrong," Richards says. What actually happens is that sound waves from the blast travel faster through the Earth's viscous mantle, nearly 18,000 miles-per-hour, than through the Earth's solid crust, less than 14,000 mph, to monitoring stations. Real quakes feature a small run-up of tremors followed by the main shaking, a much different signature.

"I would not conclude this latest test was a fizzle," says physicist Edwin Lyman of the Union of Concerned Scientists. China reported that North Korea had hoped for a yield of 4 kilotons in its test two years ago, he says, suggesting they have reached their aim. Success or failure depends on the amount of plutonium in the test bomb and its design, something the North Koreans aren't revealing.

"I would assume their ultimate goal would be that they want the world to think they could deliver a device like this on a missile," Lyman says. North Korea also tested six short-range missiles last week. For that reason, Lyman thinks its unlikely that North Korea took steps to muffle their underground nuclear test blast, as some estimates of a higher-yield test suggest. "It's not in their interest to conceal what they actually achieved," he says.

Whatever the size of the North Korean test, even a small nuclear blast is a concern, says Richards. "An explosion like this in a downtown area would be a horrible event." The Oklahoma City bombing of 1996 was only an explosion equal to about two tons of TNT, for comparison.

More definitive analysis of the North Korean test awaits atmospheric analysis of radioactive isotopes blasted into the atmosphere by the explosion. The airborne concentration of these isotopes, gases such as Xenon created in the nuclear fission reaction that triggered the blast, should help reveal how much plutonium was involved.

Atmospheric radiation was the only way that nations learned of nuclear tests decades ago, such as the first Soviet nuclear test explosion in 1949. Three years ago, the federal Office of the Director of National Intelligence released a statement confirming such an atmospheric detection a week after the first North Korean test.

"I hope they will provide the public with more information this time," Richards says. "There is a real need to fill the information vacuum after a nuclear test."