As can be seen from the pictures is very difficult to confirm with seismological data explosions below 2 kt even with close seismic stations. Seismology easily estimate the magnitude of the explosion/earthquake, but not its nature. Morever good results could take a couple of weeks to publish.
Keep in mind that North Korea has an tradition of announcing achievements nobody else managed to see.
(see http://en.wikipedia.org/wiki/Kwangmy%C5%8Fngs%C5%8Fng )
I believe that a small robotic plane, charged with chemical detectors, lunched by the US from S. Korea or from the sea will be able to detect nuclear explosions with less then ONE kt (even if underground).
Saying that I must add that, in my opinion
(a) any exposition below 2 kt (some will say 10kt) is probably a failure because large quantities of chemical explosives (in the range of 10 - 20 t) are typically used in crude devices to detonate the plutonium in the 2kt - 20kt range, except if highly sophisticated technology is available to detonate small amounts of plutonium with small amounts of explosives.
(b) using chemical explosives to simulate a SMALL nuclear explosion is insane, costly and dangerous (but possible). We are speaking of 1-2 kt of conventional explosives that could be coupled with a plutonium device, just in case it works or not, keeping always below seismic decrimination (i.e. 1 - 2kt)
(c) China in between a rock and a hard place. If it supports Pyongyang, as it usually does, Taiwan will have a REALLY nuclear bomb in less then 2 years (Japan, South Korea and the Republic of China could develop a fission bomb in less then 6 months, including lunching capacity).
(d) In 2 weeks those matters would be much more clear.
more links:
http://en.wikipedia.org/wiki/Nuclear_weapon
http://en.wikipedia.org/wiki/2006_North_Korean_nuclear_test
http://en.wikipedia.org/wiki/North_Korea_and_weapons_of_mass_destruction
http://en.wikipedia.org/wiki/2006_North_Korean_nuclear_test
