Earth scientists don't have the ability to reliably predict when exactly a quake will happen, but they do have the ability to forecast the probabilities (percentage chance during say 30 or 50 years). The more they know about the history of the fault zone in question, the more accurate their forecast will be.
They have amassed much info on the Cascadia Subduction Zone near Oregon and Washington's coast through studying core samples from the ocean bottom to determine when undersea avalanches occurred. They have come up with a 10-15% probability (in 50 years) of the largest possible quake that can occur there (8.7-9.1 magnitude), when the whole 700 mile fault ruptures, and a 37% probability for a smaller quake (8.0-8.6 magnitude) on just the southern portion of the fault.
I have gone over the data, and in my opinion their estimation of the former is overstated (I would place it at more like 5%) and their estimation for the later is understated (I would place it at 50%). What this means, if I am right, is that there is a ten times greater risk of the southern fault rupturing before there is a rupture of the whole fault and the larger, more damaging quake probably won't happen till well after this smaller, but still dangerous quake occurs.
Unfortunately, everyone is paying attention (thanks in large part to our sensationalist media) to the worst possible case scenario and causing so much alarm that people in the potential danger zone want to go bury their head in the sand instead of prepare. Nobody is giving the most probable scenario the attention needed to prepare in the most likely geographic areas (the southern 2/3rds of the coastal areas of Oregon).
I believe that Portland, OR doesn't have to be as concerned with the most likely type of quake to come next, but if a quake of 8.7 magnitude or larger is next to occur, the epicenter will be around 100 miles due west of Portland and then, things would become much more dangerous for the area. Seismologists will say that one should prepare for the worst case scenario, but I say that if you can't afford to prepare for the worst case situation, prepare for the most likely. In my opinion it is best to look at things in the most realistic light rather than alarming everyone to the point that many hit the panic button or shrug their shoulders and give up (feel helpless and thus do nothing).