South Korea quake center watches North

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DAEJEON, South Korea -- A normally unburdened South Korean earthquake center has become the front line of the world's vigil for North Korea's first nuclear test, monitoring the peninsula's geological pulse with seismic sensors and sound detectors.

The Korea Earthquake Research Center recently was put on 24-hour watch as fears grew about a test. The Defense Ministry sent soldiers to scrutinize the center's wall of video screens, which display skittering graphs of data from some 90 seismic stations across South Korea. Alert systems have been installed to link government institutions with the earthquake center 100 miles south of Seoul.

U.S. and Japanese press this month reported that intelligence agencies were monitoring suspicious activity at a suspected North Korean underground nuclear-test site, detecting moving vehicles and cables that could be used to connect the test site to aboveground diagnostic equipment.

Many specialists think the North has enough radioactive material to make at least a half-dozen bombs. The North has claimed it has nuclear weapons, but no test proving it has been detected.

On Monday, the head of South Korea's intelligence agency, Kim Seung-kyu, said the North was ready to test a nuclear device at any time, but there was no direct sign that the communist nation was preparing a detonation.

Still, the South Korean center remains on the lookout using two methods: infrasound and seismic waves.

The most reliable means of confirming a nuclear test would be from infrasound -- ultralow noises from an explosion below the range of human hearing.

Such infrasound can travel up to 620 miles, depending on wind direction, and would provide a relatively quick indication of a nuclear test, said Chi Heon-cheol, director of the earthquake center at the Korea Institute of Geoscience and Mineral Resources in Daejeon.

In the demilitarized zone dividing the Koreas, the center has deployed four infrasound arrays with tubes that detect minute changes in air pressure.

The network was completed several years ago and can also be used to detect other North Korean activity, such as the movement of tanks, Mr. Chi noted.

But even a blast as powerful as a nuclear explosion could be difficult to immediately confirm if the North thoroughly seals the underground site, he said.

So the center can also measure seismic tremors, although they would be less conclusive, he said. A North Korean nuclear explosion would likely generate the force of an earthquake measuring at least magnitude 4.2 and as strong as magnitude 5.0, he said.

The United States also is expected to be watching for any signs of a test with satellites and reconnaissance aircraft, such as high-flying U-2 spy planes that regularly draw the North's ire for flying near its territory.

The United States, China, Japan, Russia and South Korea have tried to persuade the North to abandon its nuclear program at six-party negotiations that have been on hold since November. The issue has taken on new urgency after Pyongyang raised tensions in early July by test-firing seven missiles over international objections.