Click on map icons for earthquake information by region

The term "Cascadia" has been variously used to refer to a geographic region, a bio-geographic region, and a geologic province. It is this last definition which we evoke when we speak of the "Cascadia Region Earthquake Workgroup".

For the geologist (or seismologist), Cascadia is that portion of North America that lies between Cape Mendocino in northwestern California and the southernmost tip of the Queen Charlotte Islands just off Canada's west coast.

Cascadia's western boundary, in a geologic sense, lies approximately 50 to 70 miles off the Pacific coast, where two tectonic plates, the North America and Juan de Fuca plates, meet and collide at the breathtaking velocity of 1.5 inches per year. The Juan de Fuca plate, being denser than the North American plate, is preferentially forced beneath the North America plate as the two collide. This process of collision and underthrusting is called subduction.

Cascadia's eastern boundary is perhaps somewhat less well-defined. For our purposes, we define it as the crest of the Cascade range. It marks in an approximate way, the easternmost extent of the region that would be directly affected by a magnitude 8.0+ offshore earthquake (more about this earthquake below).