Originally Posted by Jan-Poo
All Izu islands are volcanic so Rokkenjima should be no different. But as for digging foundations with explosives... that's something I've never heard off, maybe? Is it something you speculate or you know it is done?
I do speculate, but googling around suggests this method does indeed exist
and is "necessary in all but the weakest rocks such as weathered mudstones, chalk and shales". Whether the method is permitted in this particular case can only be determined with certainty if we look up the Japanese construction codes for the period, which is not easy.
The foundation does need to be rather deep though, because it's a quake area, and so base-isolating foundations would have to be used.
When possible, I expect jackhammers would be used for solid rock instead, but on a small island there may well be issues with doing that.
That's merely the most recent time any explosives at all could have ended up at Rokkenjima for a legitimate reason of any kind. I'm not sure how it could be reconciled with the fact that we know all the buildings apart from Kuwadorian were destroyed, because yes, a bomb would need to wipe all those out to fit the other constraints.
But this construction work is otherwise fishy - it's also the perfect moment to calculate a potential landslide, which would need far less explosives, that could be secreted away or even installed during the construction. Also, didn't Krauss plan to turn the island into a resort? That involves clearing out a lot more land and digging more foundations, for which explosives could well have been stocked and abandoned when the guesthouse construction was completed because Krauss postponed further development due to lack of funds. Then, the supply blowing up could be an 'accident'.