Plates
USGS Fact Sheet 2025-3050 describes the Cascadia Subduction Zone as the place where the Juan de Fuca plate is thrust beneath the North American plate. The same sheet says the Pacific Northwest can have three kinds of earthquakes: deep intraslab earthquakes, megathrust earthquakes on the plate interface, and shallow crustal earthquakes.
The 1700 earthquake
The fact sheet says the last great Cascadia earthquake occurred on 26 January 1700, with an estimated magnitude between M8.7 and M9.2. The dating uses tree rings from coastal forests that died when the land dropped and seawater flooded them, and written records of a tsunami that reached Japan without a local Japanese earthquake. USGS describes at least 19 great earthquakes (M8 or larger) that ruptured along most of the Cascadia coastline over about 10,000 years, from paleoseismic evidence. The sheet also notes that very few small-to-moderate earthquakes have been observed on the megathrust itself.
This site does not restate the 50-year occurrence probabilities in that fact sheet. Those figures are a hazard model. They are not a date, and this desk does not publish forecasts of when or where the next earthquake will happen.
Deep earthquakes under Puget Sound
The fact sheet says the most common moderate seismicity in the U.S. Pacific Northwest is deep earthquakes inside the subducting Juan de Fuca plate, typically at 25–40 miles (40–60 km). It names the 1949 M6.8 Olympia, 1965 M6.5 Seattle, and 2001 M6.8 Nisqually earthquakes. Those are historical events, not a template for the next one.
This site’s rectangle
The Cascadia / western North America coast rectangle is latitude 32 to 55 and longitude −135 to −114. It is wide on purpose. It is not the locked zone of the megathrust, and it also covers a lot of crust that is not the plate interface. Mount St. Helens and Mount Rainier fall inside it. The San Andreas fault is a different plate-boundary type farther south along California.
A scenario page, kept separate
If you want a teaching brief about shaking and coast effects under stated assumptions, use the scenario linked above the article. Read the banner. It is a scenario.