Which subduction zone is capable of producing a magnitude 9 earthquake in the Pacific Northwest?

The Cascadia Subduction Zone is capable of producing a magnitude 9 earthquake in the Pacific Northwest. It is a long offshore boundary where the oceanic Juan de Fuca Plate is forced beneath the North American Plate.

Subduction zones can generate exceptionally large earthquakes because a broad section of the plate boundary may lock together and then rupture at once. Geological evidence shows that Cascadia has produced repeated great earthquakes, including an event on January 26, 1700, estimated at magnitude 8.7–9.2.

Scientists reconstructed the 1700 earthquake from drowned coastal forests, layers of tsunami sand, and Japanese records describing an “orphan” tsunami that arrived without a locally felt earthquake. The event likely ruptured much of the margin from northern California to British Columbia.

The Cascadia threat is different from the San Andreas Fault, which is primarily a strike-slip fault and is not the source of the Pacific Northwest’s projected magnitude-9 megathrust scenario. A future Cascadia earthquake could also generate a major tsunami along the coast.