Yeah John Vidale! John doesn't participate in our g+ seismic silliness because he is quoted in practically every news article on earthquakes and has to stay serious. :(
Anyway, double basins are fun as shown with the Mexico earthquake. You can have your soft sediments on top, which amplify PGV by a factor of ten. This is enough to destroy most cities. But a deep basin can jump things up by another factor of 10! Thus, you get hit with a double whammy of a factor of 100!
This is what will crumble Seattle when the 'Big One' hits, just like Mexico. I can't see Vancouver in this situation. A subduction earthquake can't put out more than 10 cm/s, like Japan, unless you are right on top of it, like Chile. I estimate this is what hit the coast of Mexico during that quake. The region around Mexico City only measured about 1 cm/s on firm ground. The basin knocked it up to 10 cm/s and the soil to 100 cm/s. Seattle will be much worse, but after 100 cm/s you can't measure anything! and it really doesn't matter.
Over 100 cm/s, eyewitness reports tell of visible standing waves that march across the basin like tornadoes, destroying everything. Things get thrown into the air. etc, etc.