MOAB?
My guess would be either an "unscheduled detonation" or a demonstration of a small hydrogen bomb above ground.
Emphasis on GUESS.
These repeated underground nuclear tests, while limiting a considerable amount of above-ground radioactive aerosols, also open fissures beneath the earth’s crust. It would be some kind of cold and implacable cosmic justice, if these fissures should open up a large caldera of molten lava that would burst forth, with the power of Krakatoa (the Korean Peninsula is relatively close to the Pacific Ring of Fire, you know), and make all this talk of global warming, imminent war with Kim Jong Un, and the economics of widespread migration entirely moot.
Kim Jong Un plays with these nuclear devices as if they were ladyfinger firecrackers.
And these devices are anything but.
Not that deep.
This is a magma chamber collapsing.
[MOAB?]
Rosie O’Donnell landed in North Korea?