http://asia.news.yahoo.com/061009/ap/d8kl919g0.html

Scientists Probe North Korea Nuke Test

North Korea sets off an earthshaking explosion _ and claims it was
nuclear. Was it? For scientists, that was not a quick and easy question
to answer.

Like earthquakes, large explosions send out shockwaves that can be
detected on seismographs. Big nuclear bombs make big waves, with clear
signatures that make them fairly easy to detect, analyze and confirm
that they were caused by splitting atoms. But smaller blasts _ as North
Korea's appears to have been _ are trickier to break down.

The natural sound of the Earth, with its constant seismic activity of
tectonic plates grinding together, complicates the task of trying to
determine whether a smaller blast was caused by conventional explosives
or a nuclear device, said Xavier Clement of France's Atomic Energy
Commission.

He likened the problem to trying to "detect the violins or a flute in a
symphony orchestra when you are playing the cymbals."

His agency estimated the North Korean blast at around 1 kiloton or less
_ equivalent to the explosive force of 1,000 tons of TNT. For a nuclear
device, that would be so weak that the French defense minister
suggested that "there could have been a failure" with the North Korean
reported test.

Clement said it could take days before scientists can declare with
certainty whether the explosion was nuclear. And when blasts are very
weak, "we could be in a situation where we cannot tell the difference
between the two," he said.


The United States, Russia, China, Japan and South Korea are among the
countries with equipment strong enough and close enough to monitor a
North Korean test, said Russian nuclear physicist Vladimir Orlov of the
Moscow-based PIR Center, a nonproliferation think-tank.

"It takes days, dozens of lab hours, to evaluate results. Now we can
have only a rough estimate," he said.

The Comprehensive Nuclear Test Ban Treaty Organization, or CTBTO, has
about 200 stations worldwide designed for monitoring nuclear tests as
part of what it hopes will become the world's most reliable source for
such tests. But until the treaty comes into force, the data are not
made public, only released to governments and vetted partners.

Seismic data comes in almost immediately, and is usually passed to
governments within an hour or so. Their scientists must decide what the
numbers and graphs mean.

With the North Korean blast, there were wide variations. While the
French atomic agency estimated around 1 kiloton and South Korea's
geological institute half of that, Russia's defense minister expressed
"no doubt" that North Korea detonated a nuclear test and said the force
of the underground blast was equivalent to 5,000 to 15,000 tons of TNT.

"People have different way of cross cutting the data and interpreting
them," said Lassina Zerbo, director of the International Data Center at
the CTBTO, which is based in Vienna, Austria.

The test ban treaty, which bans all nuclear explosions, will not enter
into force until it has been ratified by 44 states who possess either
nuclear power or research reactors. So far 34 have ratified it.
Holdouts include the United States, China, India, Pakistan, Israel and
North Korea.

The CTBTO's stations are more extensive than those used by most
countries. They monitor seismic events but also underwater data,
radioactive particles in the air and radiowaves.

"Within 72 hours we will have full data. Then all this will be
available to member states," said Zerbo.

While the North Korean explosion was small, potentially complicating
monitoring efforts, sensors in South Korea were likely close enough to
categorize it as nuclear, if that is what is was, said Friedrich
Steinhaeusler, professor of physics at Salzburg University.

A nuclear blast also gives off a clear signature _ a clear graph of
peaks and curves _ that differentiates it from other kinds of shocks,
he added.

"We'll have the confirmation soon," he said.