I'm positive that North Korea wants the world to believe that they're testing nuclear weapons, because that is their only bargaining card, but that doesn't mean they made a successful test.
NK stated they would perform a nuke test on this date and location.
Which do you think is more likely, a) the tremors are from a nuke test b) Kim Jong performed a mystical ancient Korean wardance that happened to summon an earthquake at the designated time and place?
If North Korea could trigger earthquakes at will, I think that would be even bigger news than a nuclear test (not to mention being a plot worthy of a James Bond movie).
