Forget the quake. The tsunami will wipe out the northwest.
No worries, Bill Gates will rebuild the region.
They cover that in the National Geo clip upthread. For this type of situation (subduction), what happens is the plate which is trying to slip under the other, gets stuck, and bulge (almost shaped like a wrinkle in the crust) forms in the upper plate; and the earthquake happens when they suddenly come *un*stuck and slide past each other.
The magnitude of the quake is proportional to the surface area involved: and in this case, the area stuck is about the size of Ireland.
Up until a decade or so ago, the region was considered very safe in terms of earthquake, based on the historical lack of any incidence (since the European settlement). Unknown until then that a fault was capable of storing all the accumulated stress of many century as uplift strain, without any release between major quakes.
California’s experience with structural damage at that time wasn’t considered applicable to points north. Now, only Japan’s engineering experience with severe events comes close to the expertise appropriate to building codes in the northwest.