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.