" Actually, in past massive earthquakes in the Pacific Northwest, I wonder what the border control situation was like? Would Canadian authorities be reluctant to let Americans cross over after, say, Seattle is devastated, fearing an influx of American refugees/evacuees who then might become a burden on the Canadian gov't, or vice versa? "

I don't think Americans need a passport to come into Canada; it's getting back into the U.S. that is increasingly difficult. (I don't believe Americans need more than a valid driver's license if going by car, but that will change in a year or two ... the rules are kind of a moving target.)

If Seattle is devastated, it's possible that Vancouver will have been hit hard as well. That might change the border situation.

Mostly, though, I think you'd find that Canadians will go out of their way to be good neighbours, and help people in need.