The average time between magnitude 8 and larger Cascadia earthquakes is about 240 years … . The last megaquake, estimated as a magnitude 9, occurred in 1700 — that’s 311 years ago. In geologic terms, Cascadia is “9 months pregnant” and overdue.
Earthquake expert Yumei Wang quoted at Dot Earth, The New York Times.
The Cascadia Subduction Zone is off the northern California, Oregon, Washington and British Columbia coasts. One of the reasons we know of the great quake in 1700 is because they recorded the tsunami from it in the very parts of Japan hardest hit Friday.
There are a lot of variables, but a magnitude 9 earthquake is roughly half again as powerful as Friday’s 8.9.