The Cascadia Subduction Zone (CSZ) “mega-thrust” fault is a 1,000 Km long dipping fault that stretches from Northern Vancouver Island to Cape Mendocino California. It separates the Juan de Fuca and North America plates. The Juan de Fuca plate is created offshore along the Juan de Fuca ridge. The Juan de Fuca plate moves toward, and eventually is shoved beneath, the continent (North American plate).

The last known mega-thrust earthquake in the Northwest was in January, 1700, just over 300 years ago. Geological evidence indicates that such great earthquakes have occurred at least seven times in the last 3,500 years, a return interval of 400 to 600 years.

To learn more about the history of the Cascadia Subduction Zone and the science that led to the discovery of it, delve into land level changes and turbidites created by the CSZ earthquakes. The CSZ may be unique among the worlds subduction zones in that it produces very few (if any) earthquakes unambiguously on the plate interface. Coupled with evident occurrence of great megathrust earthquakes, the CSZ must be much more strongly locked than other subduction faults.

The geological evidence has led to different interpretrations, moreover, about whether the entire CSZ always ruptures in great M9 earthquakes, or whether smaller M8 or M8.5-sized events also can break parts of the zone in between the full rupture events.

Deep Earthquakes

The most common source of damaging earthquakes in Washington and Oregon, are deep earthquakes that rupture faults within the subducting Juan de Fuca plate, “intraplate” earthquakes. These occur beneath Puget Sound at depths from 30 km to 70 km.

Although plate interface earthquakes can be much larger, and crustal earthquakes and can be much closer to our cities, damaging deep earthquakes are the most frequent. They have historically occurred about every 30 years. The USGS estimates there is an 84% chance of another deep earthquake, of Magnitude 6.5 or greater, striking the region sometime in the next 50 years.

Deep intraplate earthquakes usually are caused by normal faulting, and reliably have very few aftershocks. These earthquakes occur where the plate is bending as it plunges beneath North America, and may also be associated with mineral changes as the plate encounter increased temperature and pressure. Stresses cause faults in the downgoing crustal part of the plate to rupture. While many subduction zones feature deep interplate earthquakes extending to depths exceeding 600 km and with different characteristics, Cascadia’s deepest earthquakes are about 100 km deep.

What makes subduction zones like Cascadia so especially dangerous is how the plates are configured. In contrast to strike-slip faults like the San Andreas, where one tectonic plate slides and/or grinds past the other, subduction zones are created when oceanic plates dive beneath continents. See, oceanic crust is much heavier than continental crust, but at the same time much less flexible. The result is a fault that is just as long as the San Andreas at the very least, but many times deeper, with about 30 times the surface area that can slip. That’s why Cascadia is the only fault in the entire CONUS with a magnitude potential above 9.0.

And as both Banda Aceh and Tohoku proved, when there’s a quake on one of those kinds of faults, there’s a good chance a tsunami will soon follow. When the continental plate finally springs back, it takes the ocean floor with it, causing vertical displacement of the ocean floor in excess of 30 feet, which then goes on to send the whole ocean column propagating as a hundred-plus-mile-long, 2-foot-high wave in the open ocean that can travel at speeds of 500 miles per hour, only to rise up 50 feet or more while retaining that extraordinary length when their motion is suddenly impeded by a coastline. See, tsunamis aren’t just waves, they’re minutes-long torrents of seawater that can take the destruction far away from the parent quake. That is why Cascadia, by far, is the most dangerous fault in the entire United States outside of Alaska.

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