How we know North Korea didn't detonate a hydrogen bomb

StartsWithABang writes: The news has been aflame with reports that North Korea detonated a hydrogen bomb on January 6th, greatly expanding its nuclear capabilities with their fourth nuclear test and the potential to carry out a devastating strike against either South Korea or, if they’re more ambitious, the United States. The physics of what a nuclear explosion actually does and how that signal propagates through the air, oceans and ground, however, can tell us whether this was truly a nuclear detonation at all, and if so, whether it was fusion or fission. From all the data we’ve collected, this appears to be nothing new: just a run-of-the-mill fission bomb, with the rest being a sensationalized claim.