I'm not sure who "they" are, but "they" all have different theories.
Check out HAARP on the net. I've even seen some say that's what's really happening.
The latest I heard, and this at least sounds plausible, the ground water table is so low that the rocks that it normally flows through are shifting as it empties. This would at least explain while it's so localized. Plate shifting I would think would cover a larger area.