2.12 Earthquakes in the Pacific Northwest: the Cascadia Subduction Zone and Juan de Fuca Plate

Seismic studies have shown that the Cascadia subduction zone exhibits less earthquake activity than any other subduction zone along the margin of the Pacific basin. Does this mean that earthquakes pose no major threat to the population centers of the Pacific Northwest? For some time, that was the conventional wisdom. However, that view changed with the discovery of buried marshes and coastal forests. These are best explained by the rapid subsidence that accompanies a large earthquake.

The Cascadia subduction zone is very similar to the convergent margin in central Chile, where the oceanic slab descends at a shallow angle of about 10 to 15 degrees. In Chile, the effects of large compressional forces are being felt regularly in the form of strong earthquakes. The strongest earthquake ever recorded occurred there in 1960, Mw 9.5.

Research predicts that subduction at shallow dip angles results in an environment that is conducive to great earthquakes, Mw 8.0 or greater. A partial explanation lies in the fact that in such settings, a large area of contact exists between the upper plate and the subducting slab.

Like the central Chilean subduction zone, the Cascadia boundary has a gently dipping plate and lacks a trench. This suggests that the Cascadia subduction zone is capable of great earthquakes. Evidence for past events of great magnitude include buried peat deposits found in some bay areas. These discoveries are consistent with episodes of rapid subsidence similar to what occurred during the 1964 Alaskan earthquake. In addition, a fault near Seattle apparently ruptured about 1100 years ago, producing a large tsunami.

However, evidence has also been uncovered suggesting that a great earthquake is not very likely, at least over the short-term. Geodetic studies conducted along coastal areas of the Pacific Northwest over the past few decades indicate that elastic strain is not accumulating to any great extent.

Which view is correct? Is a major earthquake in the Pacific Northwest imminent or unlikely? Hopefully future research will resolve this question. In the meantime, those living in the region bordering the Cascadia subduction zone should become aware of the precautions that can be taken to mitigate the effects of a great earthquake.

For further information on the Cascadia Subduction Zone and the Juan de Fuca plate, here is a link to a highly acclaimed modern documentary from GenSix Productions, see the trailer below.

Includes interviews with the most highly regarded seismologists and volcanologists from the USA, who give first hand knowledge of what would happen if and when one of the Cascade Volcanoes erupt.

Mega-Thrust earthquakes, lahar flows, volcanic explosions, a giant tsunami, economic calamity – is the USA ready for such a natural disaster? The Cascade Volcanoes and the Juan de Fuca Subduction Zone.

https://vimeo.com/394030083 Cascadia: The Big One from

https://vimeo.com/user42005132 GenSix Productions on Vimeo