There is lots of data floating around: The CTBTO called it 4.0; The South Koreans report 3.58-3.7.
You're thinking, 3.6, 4.2, in that neighborhood. Seismic scales, like the Richter, are logarithmic, so that neighborhood can be pretty big.
But even at 4.2, the test was probablya dud.
Estimating the yield is tricky business, because it depends on the geology of the test site. The South Koreans called the yield half a kiloton (550 tons), which is more or less -- a factor of two -- consistent with the relationship for tests in that yield range at the Soviet Shagan test site:
Mb = 4.262 + .973LogW
Where Mb is the magnitude of the body wave, and W is the yield.
3.58-3.7 gives you a couple hundred tons (not kilotons), which is pretty close in this business unless you're really math positive. The same equation, given the US estimate of 4.2, yields (pun intended) around a kiloton.
Tuesday, October 10, 2006
Some more skeptical analysis for the North Korea nuke test earlier this week. Essentially the point is that this was either a small test (smallest ever...by a large factor), thick bedrock in NK, or something else. More coming later.