Here’s an article about the last time that fault generated a quake ... roughly 9 PM, 26 Jan 1700.
http://www.newyorker.com/magazine/2015/07/20/the-really-big-one
There is a problem with that article, but I suspect the writer could not help that: in the event of a destructive earthquake that destroys the cities located in the main valleys nestled against the Cascade Mountain ranges, so will the water reservoirs be destroyed.
That destruction will include the lowest two reservoirs on the Columbia River, and the eight that exist in the Cascades south of the Columbia River. So it will not just be the earthquakes that shake the Portland Metro area that is destructive, but flooding. That flooding will have little reason to rush the the sea, as it will be a tsunami flood racing upriver. The distance from the Columbia River bar at the mouth of the river to the juncture of the Columbia and Willamette Rivers is approximately one hundred miles.