Cathryn, The Pacific plate subducts beneath the Aleutians and a bit of mainland Alaska...but not beneath the Pacific NW or Canada. It is the Juan de Fuca (and smaller sub-plates that make it up) that is subducting. Then, there is a long transform zone, the Queen Charlotte fault etc, that is the plate boundary between Pacific and North American plates through the Canada west margin...is right-lateral like San Andreas. Since this is a separate plate boundary, there is no real reason for it to fail at same time as the subduction zone (unless by coincidence both are ready to fail and the shaking from one triggers the other). I'm not up in detail on the research on the Cascadia subduction zone (northern California, Oregon, Washington, SW-most Canada, but I recall they think the quake in 1700 was a M9...which means a large part of it went it one shot. That does not mean that it well all go in one shot next time. If it goes in pieces, it means more great quakes, but of smaller magnitude (still larger than M8)....with shaking of almost the same intensity, but that does not last as long, does not affect as large an area, and does not produce as large a Pacific-wide Tsunami..
Chris