I will write something more substantive later; today was going to be busy anyway.
North Korea announced last night our time, this morning theirs, that they had tested “a miniaturized hydrogen bomb.” That is highly unlikely. Early estimates of the yield, from the earthquake magnitude, range from 10 to 30 kilotons. That is much too small for a thermonuclear device of the kind that nuclear weapons nations have in their arsenals.
It could be a fizzle of a thermonuclear design, or it could be a test of some of the materials and principles needed for a thermonuclear design. Or it could just be a fission design. North Korea lies a lot.
Here are some useful links:
Background, written last year by Sig Hecker. Watch for his comments on this latest test when they appear.
How to detect a secret nuclear test. This video tells you what will be going on for the next days and weeks to learn more about the test.
Nukemap 3D is a great way to visualize the effects of nuclear weapons, if you want to do that.
Analysis from the Institute for Science and International Security. This summary is similar to what a number of experts were saying on Twitter last night.
BBC (top graphic from here)
The North Korean statement. Seems to be longer than one I posted earlier on Twitter.