Defense tech declares it a dud
Captain Ed has more that points to the idea that this was a big conventional boom disguised to look like a nuclear one.
Chester reminds us that the DPRK has tried this one before. I've been thinking that if they hadn't announced it as nuclear test in advance, an explosion of that size would have been construed as a mining blast (albiet a big one), or possibly an accident. Announcing it in advance elminates the second possibility before it can be considered.
The links above do mention that you don't typically detonate weapons at full strength as that's a costly endevor especially if you're new to the nuclear club. Rather, you use enough to make things go boom and get a good reading on how things went.
I'm undecided at this point but I'm leaning toward the fake nuke game right now. Simply because that would be par for the course where Kim is concerned. The upside to this is the rift developing between China and the DPRK. Exploiting that wedge is the key to bringing the DPRK to heel.