What I Need
I need to bring about a scenario where most, if not all, of the west coast has been abandoned before the alien-robot-zombie apocalypse wreaks havoc on the world. Sweet, sweet havoc! Chiefly, I want a few minor populations of a million people in California to produce food, and I want the currently existing naval and military bases to continue to exist and be fully manned. I need this between 2015 and 2025.
How I have considered getting this
The first thought that came to mind was to simply continue the recent super-drought after El Niño wears off. The water supplies are dangerously low even after a crap-load of rain and will likely remain like that for quite a while. Humans (along with all other life) need water, and without it we have to choose whether to either stay and die or move somewhere where we can get water. This also means things such as power-plants can be safely shut down, which is s plus.
San Andreas makes big boom!
If the San Andreas fault went active, it could in theory cause mass destruction. Combine this with some other tremendous environmental disaster (such as a 5 year-long drought) and you have the recipe for a mass migration. If my family was under the nonstop threat of dehydrating and getting crushed under a fallen roof, I know what I would do.
That is pretty much everything...
It is hard to make ~40 million people move across the country. I cannot see many other ways to make this happen. Even if either or both of the above things happen, people will come back when conditions improve. The anarchists and the criminals (who suddenly have more than enough water if the powers-that-be leave the water running) would likely stay behind and take advantage of companies that did not loaded up their stock quickly enough.
The military would stay behind because they have invested massive quantities of money building and maintaining military bases. That, and relocating the Pacific fleet is not something that will be cheap or easy. Besides, with people gone, the military can test all of their experimental weaponry in a real urban environment. Furthermore, I am not entirely sure this will cause the mass-migration I want. People might need water, but those same people can be very stubborn, and the current drought has not caused an evacuation.
Apparently what I want is not clear and my title contradicts my body. This has made my question potentially unclear and too broad, so I have edited to clarify. Given modern conditions and technology, how can I make the movement of between 30 and 39 million (a vast majority) of the ~40 million residents of California decide to pick up stake and move somewhere else (I really don't care where) in 19 years or less? Would either of or both of the suggested catastrophe's be enough to cause this? If not, is there any realistic way to accomplish this, be it political or natural disaster or some combination?