Dr. More information, Or: How this blogger learned to stop jumping to conclusions

China heard about this blast before it's execution (interesting in itself) and it was supposed to be a 4 kiloton bomb. I believe that about 1 kiloton worth of explosives detonated still, based on the earthquake. CNN:

NEW YORK (CNN) -- The United States believes North Korea attempted to detonate a nuclear device and that "something went wrong," a U.S. government official told CNN Tuesday.

The official confirmed North Korea informed the Chinese government prior to the test that it would be a 4 kiloton nuclear device.

The official added that the unexpectedly small blast, of a half kiloton or less, indicated "something went wrong."

Nuclear tests in 1998 by India and Pakistan were about 24 to 50 times as powerful - or in the 15 kiloton range, according to the Federation of American Scientists.

The United States still cannot say for sure the underground detonation was a nuclear blast; the working assumption is that it was, but not very successful, the official said.