In the year 1700, there was a humongous earthquake on the subduction zone by Vancouver. This earthquake was so big that it sucked the life out of any subsequent earthquakes for 300 years. Now, the earthquakes have dared to nibble at the edges.
First, we had 3 earthquakes on the northern tip, and now we have this Oregon earthquake. A few more of these earthquakes, and we're looking at something serious. I suspect that this subduction zone is 'smooth', so that it really doesn't give a lot of 'warning' earthquakes, but responds to regional stress changes (earthquakes), with one big 'whoosh' every few hundred years.