Nuclear Mangos has a good summary and pointer set on North Korea and its nuke.
armscontrolwonk has a good series of informative posts. Including a seismograph image from IRIS which I shamelessly pass on:
IRIS says magnitude 4.2, presumably on the Richter scale.
I've heard ranges from 3.6 to 4.6 quoted (the scale is inherently ambiguous in absolute scaling).
It is a logarithmic energy scale, with magnitude 4.0 corresponding to roughly 1 kiloton of TNT equivalent (or 4.2 trillion Joules).
If the South Korean numbers, magnitude 3.6, are right, then it is a dud - only ~ 500 tons of TNT equivalent (the energy-magnitude scaling is not perfectly log-linear, especially at the low end).
At the high end, magnitude 4.2-4.6, it is a respectable few kilotons bang, but all news reports are now playing it to be at the low end.
Generally seismic estimates underestimate the yield, since, if anything, any use of cavity or presence of fluids will decouple the energy and give inefficient transfer. It is not clear to me what "impedance match" the magnitude-energy scaling is calibrated to.
If the yield is really under a kiloton, then the device probably failed.
Most likely the implosion was bad, their explosive implosion lens (65 year old technology!) was bad, either through bad construction (geometrically imperfect assembly) or through bad electronics (triggers didn't sync for a clean implosion), or even bad chemistry (some explosive segments burned slowly so the implosion was asymmetric).
It is also possibly their plutonium was dirty, leading to premature asymmetric detonation.
It is conceivable, but very unlikely, that the NKs actually have perfectly good working basic plutonium bombs, and that the test was for a trigger for a boosted device, deliberately sub-maximal in yield.
I seriously doubt that. They'd test the basics first, then try to get fancy. Plus they announced a yield of several kilotons.
It is also conceivable that their test site uses caverns designed to decouple the energy transfer to the rock, to mislead about the yield, but again I doubt it, they want to show a big bang, not play down the yield.
Bad news all around.
I expect we'll get some bluster, but no effective action.
I can't even see how the US would plan to blockade North Korea, there are two coastlines to guard, and stopping all cargo is a blockade, no matter what they call it.
They'd need at least 6 ships on station (three on each side) - fast frigates would probably be best.
That means 12 warships minimum, preferably 15-18 (for rotation and backup).
I don't think the US has that many to spare. There are 30 in total in the navy.
More like 50 destroyers, but putting many destroyers on blockade duty is expensive.
Maybe mix of 1:2 or 1:1 destroyers and frigates?
If it had the yield of 1000 tons of TNT, maybe it was 1000 tons of TNT.