From Spiegel Online:

"Clearly something rattled the ground in North Korea. The US Geological Survey said it recorded a seismic tremor focused in northern Hamgyong with a preliminary magnitude of 4.2 -- which experts said would indicate an explosive strength of about 550 tons of TNT. That's half a kiloton, far smaller than the nuclear bombs dropped by America on Japan during World War II. The bomb dropped on Hiroshima had the destructive power of about 15 kilotons.

Some experts cautioned that North Korea could try to fake an explosion by setting off conventional weapons, and South Korea said it was still trying to detect escaped radiation near the site. A top Russian military officer, though, said Russian monitoring systems had 'detected the test of a nuclear weapon in North Korea ... It is 100 percent (certain) that it was an underground nuclear explosion.'"