Reaper's right, they noticed the seismic activity at the same time N.Korea was making the claim that it detonated an underground nuke. It was picked up on just like it was in 2006 when they did the same thing.

And the UN Security Council is pretty pissed off right now, so I'd say they've got all the evidence they need.

UN Security Council condemns NKorea nuke test

But, the question is, will they actually do anything about it?