550 tons TNT-equivalent?
That's this morning's report, and it strikes me as a bit odd. Isn't there a lower limit to the size of a fission explosion?
The report also showed video of the blast site surface, and the shock wave that pulverized a small crater. It was very small.
Is it possible that the North Koreans faked the blast with conventional explosives, to yank everybody's chain? Possibly clever: We get the seismic signature, but it's too deep underground to check for radioactivity.
I think there's reasonable doubt about whether North Korea actually tested a nuclear device.