(Disclaimer: I am speculating on a monday morning on insufficient facts below, dont take it too serious.)
The yield (aka. explosion strength) on the North Korea nuklear test is suspiciously low. 0.55kt means either the test gave only a incomplete fission reaction (aka. partial failure) or the Kim-Yong-Il-devotees have access to advanced miniaturization technology to built a “tactical” nuke. Typical first-generation nuclear devices for new members of the nuclear club have been more in the 5-20 kiloton range.
Or, the third option, they “emulated” a nuclear test. You may remember on 9th of september 2004, a large explosion, followed by a mushroom cloud was detected inside North Korea. It was explained a few days later as “demolition of a mountain for a power plant”. Speculation was rife that this might be a nuclear test, but no radioactivity had been detected. The seismic patterns were apparently sufficiently confusing to not really rule out a nuclear test immediately. Maybe, just maybe, this mushroom-cloud explosion was a trial-run for sunday.
Underground nuclear tests of low yield weapons, if conducted properly, seem to result in very little detectable outside presence of the telltale isotopes that can be used to fingerprint a nuclear explosion. So in the fully closed environment of North Korea it would be rather difficult for everyones spy teams to get near enough to the test site to collect samples to verify or disprove the nuclear test claims.
So the main thing that would need to be simulated is the seismic pattern of a nuclear test. I have yet to read somewhere that this is impossible, given that the local geologic formation are known to them and the patterns that need to show up on seismic detectors to indicate a nuclear test are also no secret. North Korea would have all the motivation and ressources to do that.
Updated to add: There would be a very good reason for North Korea to limit the nuclear yield of a test intentionally. The country is rather small and densely populated. Nuclear underground tests tend to contaminate the subterranean waterflows with the radioactive gases and particles from the molten rock. A smaller yield gives a smaller “bubble” in the rock and a shorter radius of contamination. So it becomes possible to find a suitable test spot, where the waterflows can be either controlled or are not significant.
Another reason to aim for smaller weapons is of course the geostrategic situation on the korean peninsula. Too large weapons would create too large destructed areas and too much fallout, effectively making their use impossible.
Anyway, I still think they could have faked it.
Manfred Henger from the “Bundesanstalt fÃ¼r Geowissenschaften und Rohstoffe” thinks, this explosion might be caused by 0.5-1 kiloton TNT.