In the Pacific Northwest, the Juan De Fuca plate is one of the smallest tectonic plates but plays a major role in developing our coastlines and geography. The Juan De Fuca plate is diving under the western side of the North American plate at the Cascadia subduction zone, a fault line 50 miles west of the Washington-Oregon coastline creating hundreds of small earthquakes each year, most of which cannot be felt.
In a 2015 article, entitled The Really Big One, Katheryn Shultz of the New Yorker, discusses the Cascadia subduction zone, which runs over 600 miles north to south, from northern Vancouver Island to Cape Mendocino in northern California. Her article has made many in our region aware of the extent to which we are in a seismically active zone, and led to a significant increase in the demand for seismic retrofit work.
Earthquakes that occur along subduction zones have been the most destructive in recent history, with many exceeding a magnitude of 9.0. In March 2011, the Tokuku earthquake and tsunami killed nearly 16,000 people, with a magnitude of 9.1. In 2010, the Chile earthquake had a magnitude of 8.8 with a death toll of 525.
The last Cascadia subduction zone earthquake of a significant magnitude occurred in 1700. Our history has shown that these earthquakes occur every 300-600 years.
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As previously mentioned, the Plate of Juan De Fuca is constantly subducting beneath the North American plate. In our region, deep slab earthquakes occur several miles below the Puget Sound, within the Plate of Juan De Fuca. As the subduction plate bends to dive more steeply beneath the Puget Sound, the upper part stretches and fractures in normal faults. Because the hypocenter of these types of earthquakes is so deep, typically miles below the surface, the forces of these earthquakes are dissipated and spread out across a wide area.
The Nisqually earthquake was an intraplate, or deep slab earthquake, which had anepicenter 30 miles deep near Olympia. Thankfully, this earthquake only claimed one life and very few were injured, but there was still plenty of monetary damage done; it was estimated that the Nisqually earthquake caused between $2-4 billion dollars in damage.
Most of us experienced the 2001 Nisqually earthquake, and very few rode through the 1965 and 1949 earthquakes without serious injury or damage. Most people do not realize that these earthquakes we experienced were relatively small or moderate in comparison to the more catastrophic events our region could be hit with. Washingtonian’s who experienced any of the three deep slab earthquakes, experienced the mildest of the three that could occur at any time in our region of the Pacific Northwest.
These types of earthquakes are the most frequent in our region and typically occur every 30-50 years.
Shallow crustal earthquakes differ from deep slap earthquakes in several important ways. First, shallow crustal earthquakes originate where the crustal plates of the earth’s surface move against one other, as opposed to the areas where tectonic plates move under one another, like deep slab earthquakes. Second, their focus is much nearer to the surface of the Earth, therefore, they generally cause greater damage at the earth’s surface than deep slab earthquakes. Though shallow crustal earthquakes tend to be of smaller magnitudes, the maximum of which is 7.5, and are rarely felt, about 75% of the world’s energy that is released during earthquakes is from shallow crustal earthquakes.
The constant load and release of energy from subduction zone earthquakes, occurring every 300-600 years for the last 16 million years, has created a permanent deformation of the North American plate. There are several fractures within the North American plate, just below the surface of the Puget Sound and cities and towns, to include South Whidbey and Tacoma. Seattle has the misfortune of sitting directly on top of one such fracture.
The 43-mile Seattle fault runs west from the Hood Canal through the Puget Sound and south Seattle, parallel
beneath Interstate 90 and east through Bellevue, eventually ending near Fall City. This is a fault where shallow crustal earthquakes have the potential to occur. It is important to note that shallow crustal earthquakes, while are often of a lower magnitude than deep slab earthquakes, have the potential to cause greater damage as they occur much closer to the earth’s surface. In comparison, deep slab earthquakes, such as the Nisqually earthquake, have a much deeper epicenter and, therefore, less concentrated damage.
It is estimated that the last earthquake in the Seattle fault occurred around 1,100 years ago, between 900-930 AD. These types of earthquakes are the most unpredictable to scientists We are due for another one, but we don’t know when the next one might hit.