Discussion of MOUNTAIN collapsing was that horizontal tunnel was dug for all 6 nuclear tests. Too hard to dig down for tests. But this also was reason MOUNTAIN might collapse.
Should be radio-active fallout detected first in Russia, then China, and finally South Korea. Japan is farthest away, so they might detect radiation next.
The deepest mine is in South Africa (AngloGold Ashanti’s Mponeng gold mine) at a depth of 3.9 km. The get frequent rock bursts and it is very hot and difficult. Fat Boy hasn’t the ability to go anywhere near this deep. Now, the mountain could well be collapsed or a deeper fault could be moving. We’ll have to see if the volcano is a possibility, it doesn’t seem so now.