What Just Happened in North Korea?

North Korea detonated some sort of bomb last night. Pyongyang says that it was a hydrogen bomb, although, as Barbara Demick notes, the seismological readings don't really support that claim. The tremor from the explosion seems to have had a magnitude of 5.1, indicating an explosion estimated at anywhere between six and fifteen kilotons, and even a partially successful hydrogen-bomb explosion (driven by nuclear fusion, rather than fission, as in a classic atomic bomb) would have been bigger. This...