My question was closed as being "too story based," despite that it does not seem to fit this category. @Aify and @XandarTheZenon left a comment saying the question is "too broad/unclear and explained themselves satisfactorily. I have edited the question to help improve clarity. However, nobody has explained how it is too story based. Since the question was closed for that reason by some of our largest contributors and most respected members, I would like to request an explanation as to how it is "too story based." If it is too broad/unclear, should that not be the close reason? Thanks in advance.
I voted to close as story-based because you're asking us to help define your plot for you. I had no issues with its clarity; it made perfect sense to me and I knew exactly what you were asking for, so it wasn't unclear.
The problem I saw was that the answer to your question would lie in your work of fiction (well, hopefully fiction; a biography would be frightening). As such, it would be entirely up to you to decide whether characters would up and move because of whatever event. Constant earthquakes for the foreseeable future? Upswing in the purchase of houseboats and the population actually moves west to build a floating city (odd, but possible, and a premise for an amusing story). Outbreak of insect-borne disease? Screens, vaccines, and insect repellant. Politics? New elections.
So it is also fairly broad, but the issue, in my opinion, is rooted in what you decide your characters decide to do.
Your biggest problem is that all the "natural" reasons for moving, people already live in places which experience all of them, and have figured out workarounds for them.
Drought? Welcome to Nevada, baby! Or Arizona, or whatever area of semi-desert takes your fancy. All these states would dry up and blow away without federal funding to prop up their infrastructure (yay all those rugged individualists relying on communist principles to keep them alive! ;) and Californian cities are already unsustainable in terms of water supply anyway, so no real change there. Farming might be affected (at least they'd need different crops), but there's no chance of people leaving the cities.
Earthquakes? Never stopped them before. Nor Japan, nor all the other places built on fault lines. Some places will get toasted, and you may need to get rid of overpasses and redesign bridges, but most people will be OK.
Insect-borne diseases? The thing about insects is that they move around. There might be an initial panic, but ultimately people can't go anywhere because there's nowhere in the continental US where you can get away from them long-term. And the answer to major insect-borne diseases is straightforward: fuck the environment, bring the DDT. The environmentalists won't like it, but if it's a choice between losing some birds or losing the entire Western seaboard with a strong chance of losing the rest of North America too, then the birds are screwed.
Politics? People are still living in downtown Detroit. The thing is that when a city goes bad, you can't sell your house. Unless you want your family living in a tent for the next 20 years, you have no choice but to stay put.
Individual people will have their own reasons, sure. But if you want a complete dead zone, not just the semi-derelict state of downtown Detroit, then you need something major, on the scale of alien invasion or a nuclear/biological/chemical strike. And that's a major plot point.
You don't need the event to be the focus of the book - "The Passage" effectively has a jump-cut to something like 15-20 years later, "Station 11" runs linked pre-event and post-event storylines but skips the period in the middle, and "On the beach" is entirely post-war and has virtually no detail about what actually happened. But your world-altering event (and it will be world-altering - even Chernobyl and Fukushima didn't have this effect) will be the single biggest influence on your characters, so it's impossible to disassociate this from your story. It's not just detail.