Hi Beth, The rate of subduction is much higher in Japan than in Cascadia. I'm not sure of the numbers, but I think it is about 8 cm/yr at northern Honshu and may be 2 or 3 cm/yr in Cascadia. I guess I could find the numbers if I did a Google search but might take a little while. The area of Japan that just had the M9 has fairly frequent subduction quakes, but they tend to be much smaller...I think the last 100 or 200 years more like M7.5 to 8. I don't see why you could not have the same thing in Cascadia: while it is capable of a M9, it may instead break in a series of M8s.
Chris