Apparently the study did not include the Willamette valley rift. The rift took place about 10,000 years ago and then stopped.

Only very few earthquakes in the region nowadays. I would look towards the volcanic Cascades to see if there has been any activity before any quakes took place. The only one that has been active is mount St. Helens.

If you look at the plates themselves you would see that very little of the plate off the shore of Oregon is subduction. Most of the movement and the bigger quakes are from slip faults and they are mostly in California.