See Inside January 2007

Seismic Sentries

Why underground nuclear tests are so hard to hide

On October 9, 2006, seismometers around the world picked up a magnitude 4.2 event located in northeastern North Korea. Considering that an average of 20 earthquakes that size or larger occur every day, seismologists might not have immediately singled out this incident had not North Korea's leader, Kim Jong Il, promised an underground nuclear test.

Although seismometers could not conclusively confirm the nature or size of the explosion, they did indicate that the bomb was more fizzle than blast. Perhaps more important, they showed that as long as the geology of the region is well known, local sensors can accurately distinguish even small detonations from earthquakes.

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