Like a lot of other folks I hopped over to various news sources the second I heard NK had passed the point of no return.  However, it appears all the sources cited are simply quoting claims.  One anonymous source in SK is  quoted as citing seismic data showing an event registering 3.5 on the Richter scale.  That's a problem as you will see later.

Basically, we are looking for seismic data to show there was a nuclear blast.  That data is easy to get.

If that sucker is big enough to register a 4 on the Richter scale, I found a way we can all follow this story in real time from our own PCs!  (UPDATE: A seismic event in North Korea showed up on my PC around 04:30 UTC registering 4.2 on the Richter scale from  01:35 UTC).  That is about 10x what the SK source cited and makes it a little less than 1 kiloton.     HOWEVER I am not sure the real time telemetry backs up either claim.   I have uploaded telemetry from nearby Inchon Korea, as well as Beijing and the Solomon Islands.  In all cases, you see the Tonga quakes earlier in the day, but nothing around the time or after for the claimed NK blast.