Migrations / self-evacuations after a quake depend entirely on the scenario - a 9.0M CSZ quake in November-February when the temps are 32-42 F and its wet out may cause a lot of displaced to try to get away. The focus of the initial response is to shelter folks so they don't die, then its like running a farm - feed them and water them and handle their sewage. You want your house to remain habitable, but if it doesn't crowding in with neighbors or in a nearby church, school or community center to get out of the rain will do, believe me. Ask yourself, if you're lucky and your house survives a 9.0M, are you really going to turn away the neighbor - or stranger - at your door? July when you can survive over night and not get rained on gives you more possibilities - as long as you can eat and have something to drink, you probably are going to stay close to home. You may send your family out to Aunt Martha's, and the kids could be expected to stay 4 counties over to go to school the next year.

During the Vancouver Olympics Washington EMD and various other agencies ran a drill simulating a rapid forced migration across the border of thousands of Olympic visitors, I think the identified threat was a terrorist attack, but the logistics are all the same - how many can cross the border per hour, how fast they can travel, how far before they need to stop for the night and bed down, etc. Dealing with any migration has a lot of similarities - how many are at risk from an event, how many are detected along major traffic routes out of the area, how far can they travel before they have to bed down. You open shelters and offer assistance along those routes, people will stop, or find their own shelters, food and water. I can believe that Bend OR doesn't have the resources to assist Western Oregon after a CSZ event, but then neither does anyone, not without a bit of planning.