There is still so much we don't understand about earthquake zones in general, and Cascadia in particular. When I was an undergrad in the early 70's, the concept of Plate Tectonics was still relatively new, but geos had recognized that the west coast of OR, WA, and BC were a subduction zone. Even back then people were wondering why this subduction zone was seismically quiet. Only much later did research demonstrate that Cascadia did in fact have a record of major earthquakes.

In the past, the only available seismic array's were onshore. Since the portion of the subduction zone generating earthquakes is mostly offshore, these arrays could only detect quakes down to about M 3. We've known for decades that there was very little seismic activity down to that level. What is new is the ability to put sensitive Ocean Bottom Seismic (OBS) arrays right on the sea floor which can record for long periods of time and detect very small quakes. We now know that Cascadia is even quieter than we thought.

For a bit more general background, see Is 'eerie quiet' at Cascadia earthquake fault related to Nevada's swarm of tremors? (Note that the article says it probably isn't closely related to Nevada quakes.)

For more detailed into see the Cascadia Initiative website, and The Cascadia Initiative: A Sea Change In Seismological Studies of Subduction Zones (PDF).