[Myrddin]

Your figure is correct. I still think the concern about the dam is not correct with respect to "wobble". That mass of water might have some significant influence with respect to depressing the land mass in the area. The weight wasn't there until the dam collected it in that location. A greater concern is what happens downstream when a significant earthquake breaches the dam.

[WVKayaker]

My thoughts were of “pushing on the side of a balloon and watching it expand or distort on the other side”. I’m sure I’m wrong, but not totally!