Mainly by most of them dying...
Not the way it works. An earthquake would be least of our problems. Super volcanos don't ooze out lava and make mountains. The magma chamber is like a balloon of molten rock. Everything is fine as long as the chamber stays full and the walls and ceiling stay put. They are dangerous when the magma level goes down and/or the roof collapses. This releases all of the built up pressure and the gasses and water in the rock which explodes.
Doubtful, any more than a nuke could make California slide off into the Pacific ocean (but we can still hope right?).
It's still not something to worry about. If it happens it happens. It cannot be prevented. I will be prepared and I suggest others do too, but don't lose sleep over it.