In the first Austin Powers movie, Dr. Evil steals a 50 kiloton nuclear weapon, straps it to a drill and is going to fire it into the mantle to trigger volcanic eruptions world wide.
My instinct is that, compared to a lot of nuclear weapons in the megaton range, 50 kilotons is not really a lot, and using it to try to move rock (even molten rock) is not going to accomplish his goal. Would it even cause a measurable change in the pressure that far down?
But, these fissures at faults are already under stress. Just fracking is enough to lube them up to go off frequently, mostly smaller ones, but there have been a few big ones, including one in Youngstown Ohio that was pretty big, almost certainly from deep injection wells to dispose of fracking waste, (that have a 15 percent failure rate.)
Drilling waste is class 1 waste because the shit they put in the fracking fluid, and what it picks up underground (radium, arsenic, etc.) and flowsback to the surface with the gas, but they’ve an exemption from dick cheney’s energy bill where bush let the oil majors and their ilk write the energy policies. But that’s another story.