This question already has an answer here:
I am evil and wish to destroy a town near a volcano and kill all of its citizens.
The volcano is small - about 15 square kilometers in area. The town is even smaller.
The thing is that I don't live inside the volcano like a bona fide supervillain, but I live near it, a mere 25 metres underground. I'm tired of this glorified basement, so I'm going to fill it with explosives and move out.
My question is this: How strong must this underground explosion be, and where must it and the town be, in order to trigger a pyroclastic flow destroying homes, and ash clouds thick enough to obscure the sun for days?
To be clear, the lair and town can both be any reasonable distance from the volcano and to each other, unless the explosion itself would wreck the town. I presume that the lair would be closer to the volcano than the town.
Edit: Unlike Yellowstone, the volcano is active and it is assumed that less energy is needed to trigger an eruption. I am also specifically asking about the use of an explosive charge. The goal is to avoid giving the townsfolk enough notice to evacuate.
The lair can be deeper underground (e.g., 40m) if truly needed.
Mount St. Helens erupted due to an earthquake exposing a hole in the mountain, so surely blowing a hole in a volcano has the potential to have similar effects.