From just an earthquake? Probably no sudden mass migration. But if we're talking CSZ, a tsunami or series of tsunami could flood hundreds of square miles of coastline and push people inland in a hurry. Or a strong earthquake near Sacramento, CA could collapse the extensive levy system there, flooding much of the metro area and causing people to flee. And under the right conditions, a big, out of control wildfire could have a huge front that forces many thousands to flee in a hurry.

A slower unfolding but also plausible scenario is a shortage of water in Southern California if the water supplies imported from Northern California and Colorado are disrupted by a quake for an extended period of time. So, a strong quake near Sacramento or out in the desert along the San Andreas might not cause that much damage for the people in the LA/OC metro area, but severely disrupt water supplies and prompt SoCal residents to leave for greener pastures if most water imports are cut off. There are extensive local reservoirs but those won't last forever, nor are there enough ground water supplies to support everyone in the region.

Oh, and considering the ongoing troubles at the San Onofre nuclear power plant and the tragedy of 3/11/11 in Japan, a nuclear accident triggered by an earthquake could release radiation and cause a major migration. There are reports that the Japanese government considered evacuating up to 40 million people due to the Fukushima meltdown. Where do you send 40 million people on short notice?