The Link Below is a map of the Tectonic Plate Boundaries.

The Cascadia Subduction Zone is relatively small compared
to other Subduction Zones.

The Velocity of Movement is quite small - 26MM per year.

The Cascadia Subduction Zone is composed of (3) plates -
the Explorer Plate, the Juan de Fuca Plate and the
Gorda Plate.

The Pacific Plate Subducts under the Okhotsk Plate where
Japan Rests at a rate of 95MM per year.

Whereas Japan can expect large earthquakes periodically,
the Cascadia Subduction Zone is simply too small to
generate an Earthquake Larger than Magnitude 7 to 8.

[link to upload.wikimedia.org]