Why? I live in Sacramento. We used to (historically) get floods that would make the valley into a lake. And every time it rains now people panic that all the levees are going to crumble and we'll all be underwater. Our levee systems themselves are probably going to do a lot to prevent anything like that kind of thing happening. Even IF one breaks here or there.

"The Big One" isn't something to be afraid of. If it happens, it happens, and you can't do anything about it. There's no reason for it to take up space in your head as something serious to worry about. And if it is something you can't get your head away from, then do what you can to move away so that "The Big One" most likely won't affect you.