The most threatening earthquakes in Cascadia are those associated with the Cascadia subduction zone, which is the tectonic plate boundary between the subducting oceanic Juan de Fuca, Gorda, and Explorer Plates and the over-riding continental North America Plate.
The subducting plates do not slip continuously; instead, the plates are stuck together along the upper portion of the subduction fault, which is referred to as “locked zone.” Over time, the stress becomes too great and the locked zone ruptures. The accumulated vertical displacement (10 to 20 meters) occurs within seconds as the fault slips, causing a magnitude 8 to 9 earthquake and a series of tsunami waves. An earthquake of this magnitude is referred to as a “great earthquake.”
Unlike other subduction zones around the world, the interface fault in Cascadia appears to break almost exclusively only in great earthquakes and not also via moderate and small earthquakes. In the past few decades since dense earthquake monitoring has been in place, very few earthquakes have broken the plate interface, less than a half dozen with M4 to M5. These small earthquakes include M4.9 and 4.8 events off Newport, Oregon in 2004 and a M4.6 off Northern California in 2008.
The last great M9 event struck January 27th, 1700. There is geologic evidence of at least 13 events at intervals from about 300 to 900 years. Previous earthquakes are estimated to have occurred in 1700 AD, 1310 AD, 810 AD, 400 AD, 600 BC and 170 BC. Recent evidence suggests that the northern part of the Cascadia fault has a major earthquake every 525 years on average. The southern part, however, has one every 278 years on average.
A unique aspect of a great Cascadia subduction earthquake is the strong likelihood that the three greater metropolitan areas of Portland, Seattle, and Vancouver will simultaneously feel the effects of strong and sustained ground shaking. This wide-spread ground shaking combined with accompanying elevation changes and the likely generation of a tsunami along the Pacific coast, will cause loss of life, property damage, and business interruption in vulnerable locations throughout southwestern British Columbia, Washington, Oregon, and northwestern California. CREW has published a Cascadia subduction zone earthquake and tsunami scenario for emergency-planning purposes. 
Here you will see a plate tectonic map of the Pacific Northwest. Thick black lines indicate where the plate boundaries are at the Earth’s surface. The boundary with the arrowheads shows where the Juan de Fuca plate begins its descent beneath the North American plate. Thus, most of the plate interface is a plane that dips beneath the North American plate. With the exception of a few just offshore California, none of the earthquakes shown are on the interface between the Juan de Fuca and North American plate, but rather on other plate boundaries on within the plates themselves. Most of these had only moderate magnitudes (M~6.5-7.5) and none can be considered great earthquake (M>~8). The white arrows show how fast the plates are moving relative to the North American plate. Note that only earthquake ‘epicenters’ are shown (red dots), which indicate the points on the Earth’s surface where earthquakes begin. All earthquakes break planar faults and when M>6 the breaking faults have dimensions of a few kilometers to hundreds of kilometers.
This cartoon shows how subduction leads to earthquakes along the plate interface.
a) The oceanic plate is subducting beneath the continental plate. The two plates become stuck, or locked.
b) As the lower plate slowly descends, it pulls the upper plate down with it, deforming the land surface above and bulding up stress. The island close to the trench slowly subsides, while further inland the land slowly uplifts.
c) After hundreds of years when the stress buildup overwhelms the locking friction, the plates suddenly break free from each other and the upper plate slips back in a few seconds to minutes. This sudden slip generates an earthquake. The island pops back up, and the land to the right suddenly subsides. This quick displacement of the ocean floor can cause a tsunami. Courtesy James Jackson, from http://www.tectonics.caltech.edu/outreach/highlights/sumatra/why.html
A tsunami caused by an earthquake along a subduction zone happens when the leading edge of the overriding tectonic plate breaks free and springs seaward, displacing the sea floor and the water above it. From http://pubs.usgs.gov/fs/fs150-00/
A dead forest in coastal Washington records a great earthquake that occurred about A.D. 1700, a century before Lewis and Clark explored the mouth of the nearby Columbia River. Such forests are among the evidence that areas of the Pacific Northwest have experienced earthquakes of magnitude 8 to 9 in the past. From http://pubs.usgs.gov/fs/1995/fs111-95/