According to wikipedia:

The Cascadia subduction zone is a 960 km (600 mi) fault at a convergent plate boundary, about 112-160 km (70-100 mi) off the Pacific Shore, that stretches from northern Vancouver Island in Canada to Northern California in the United States. It is capable of producing 9.0+ magnitude earthquakes and tsunamis that could reach 30m (100 ft).

Most recently, the 1700 Cascadia earthquake occurred along the Cascadia subduction zone on January 26, 1700, with an estimated moment magnitude of 8.7–9.2. Some geologists have predicted a 10% to 14% probability that the Cascadia Subduction Zone will produce an event of magnitude 9.0 or higher in the next 50 years, and other have stated the risk could be as high as 37% for earthquakes of magnitude 8.0 or higher in the next 50 years.

See also Kathryn Schulz' Pulitzer Prize winning piece The Really Big One