My 180g doesn't worry me at all. It's got a width of 24" and sits on a stand 27" wide. It's stable. Even with a good shaker, I don't see anything shifting. maybe some water splashing/sploshing, but nothing major. The tanks with the higher profile and less width could be a problem.
It's crazy to think about though. I remember back in either 92 or 94 a big one hit. My parents pool lost probably 1/2 - 1' of water. I suppose the dimensions and no top on the pool make it a completely different situation, but the power an earthquake unleashes is crazy.