The Cascadia subduction zone, where the oceanic Juan de Fuca plate descends beneath the overlying North American plate, extends 1100 km from northern California to northern Vancouver Island.

The tiny Juan de Fuca plate is largely responsible for the volcanoes ... in a process called subduction, under a landmass. As the overlying plate lifts up, it also forms mountain ranges.

This earthquake is larger than much of the recent seismic activity around western Washington, per initial reports.