I came across this article about how our neighboring state of Arizona is preparing for an influx of 400,000+ refugees from California in the event of a major earthquake in that state. Personally I just don't see how that would work due to extreme heat (should the earthquake happen during the summer), the lack of water (it isn't like the state has a lot of water to start with), the very small roads (it's mostly all four lane highways all the way to Phoenix, other areas only have two lane highways), and the mostly tiny towns (aside from Phoenix) that can't support such a large population even if only temporarily.
But it does give one something to think about. I have no doubt that if something major happens to make a large swath of California unlivable the people would no doubt migrate east. Already the traffic is astoundingly heavy on an average Friday heading towards Las Vegas and on Sunday evenings heading back to California. Multiply that times 100s and there will be miles of gridlock and desperation.
Then once they get to the city (Las Vegas or Phoenix), or I should say "if" they make it to the city as gas stations are few and far between between LA and Vegas/Phoenix and sitting in miles and miles of traffic in the searing heat won't bode well for them, then what will happen? Fortunately Las Vegas has the ability to house hundreds of thousands of people in its hotels, Phoenix not so much. And even if taking care of these people for a weekend is easy for a city Las Vegas, having hundreds of thousands of displaced people for months on end is a whole other issue.
So the bottom line is to give some thought to either being the refugee (what situation in your area would make you flee and how would you do this) and/or being the person in the city/town that could become overwhelmed by people fleeing a disaster (there's no doubt that 400,000 people suddenly showing up in Phoenix would have a huge impact on the population that already lives there).