Earthquakes in the Cascadia Subduction Zone

Author’s Note: This part of a weekly series on geology for a class I am taking this semester at BYU-Idaho. This week’s prompt required students to find a news article, post it on our blogs and explain how plate tectonics played a role in the situation. Other posts from this assignment can be found under the “Geology 111” category.

Featured image: Damage and flooding in Sendai, Japan as a result of the 2011 Japan earthquake. (Source: U.S. Navy)

The article I’ve chosen for this week’s assignment is from The New Yorker entitled “The Really Big One.” This article (located at this link / archive) is one of my favorites. It details the possibility of an earthquake similar to the one that struck Japan in 2011 striking the Pacific Northwest.

It’s no secret that the American West Coast is prone to earthquakes. In 2001, I lived on Fort Lewis near Tacoma, Washington. On February 28, right after I got home from half-day kindergarten, went upstairs, and the world promptly became completely unglued. I had never experienced an earthquake before, so this 6.8 earthquake, named the Nisqually earthquake, was an unexpected and new adventure for me.

It turns out the Nisqually Earthquake was centered less than seven miles from our home. The Puget Sound Region took some damage, with bricks falling from older structures near downtown Seattle and some highways around the area needing repair.

This earthquake was big, but its nothing compared to what the Cascadia Subduction Zone has to offer when it unleashes its big one. As Schulz mentions in her article, the zone has the capability of unleashing an earthquake somewhere between a magnitude of 8.7 and 9.2.

As the Juan de Fuca Plate subducts under (or gets pushed beneath) the North American Plate, several things occur. The more obvious impact this subduction has is the creation of volcanoes in the Cascade Mountains. Arguably, the most famous of these volcanoes is Mount St. Helens. Of course, St. Helens blew its top in its infamous 1980 eruption. My parents, who were four and five years old, lived in the Tri-Cities in eastern Washington at the time and can remember the sky turning so dark at noon that street lamps turned on. I was able to climb to the summit of St. Helens in the summer of 2009, which was an awesome experience (its hard to believe that was almost ten years ago).

Less obvious to visitors of the Pacific Northwest is the earthquake threat. As the Juan de Fuca Plate subducts below the region, it will snag and get caught as its rocky surface slides against the bottom of the North American Plate. When enough pressure builds, an earthquake will happen. Some of these (like the Nisqually Earthquake) will have epicenters almost directly below the I-5 corridor and will be pretty deep. These can reach magnitudes above 7.0 in some instances.

Others will have epicenters offshore, shallower and more massive. These earthquakes have the potential to exceed a magnitude of 9.0, making them similar to the 2011 Japan earthquake. With the epicenter being offshore, these earthquakes also have the capability to create massive tsunamis like what was seen in Japan which was measured to be in excess of 100 feet in some areas.

The 2011 tsunami didn’t just cause damage in Japan, though. The wave traveled across the ocean to the United States, where a man was swept out to sea in California (archive). In 1700, the Cascadia Subduction Zone generated a large earthquake that sent a similar wave across the sea. Because of this tsunami’s impact in Japan (archive), geologists are able to pin down the specific date and an approximate time of the earthquake.

It is important that areas affected by this subduction zone be prepared for the earthquake and tsunami. The USGS expects the earthquake itself to cause significant damage to metro areas along the I-5 corridor from Vancouver, British Columbia clear south to Redding. On top of this, light damage is expected south to San Francisco and as far east as Reno and the Tri-Cities.

While California may have more earthquakes, the Cascadia Subduction Zone has the potential of creating much bigger ones offshore Washington, Oregon and British Columbia (though I should point out that the subduction zone could generate an earthquake of this size as far south of Eureka, California). The famous 1906 San Francisco earthquake, for example, registered at 7.8. This means that the subduction zone earthquake will likely release 63 times more energy than happened in the Bay Area (according to this calculator / archive). Also using that calculator, it looks like the Cascadia earthquake is anticipated to release nearly 2,000 times (not percent, but times) more energy than the Nisqually earthquake which I was in in 2001.

Of course, this doesn’t mean you should fear going to the Pacific Northwest (or any subduction zone, for that matter). That said, we should all be prepared. If you even have a basic plan in place, you will be much better off when the earthquake eventually strikes. On top of that, you will also be in a better position to help those around you. Rational preparation is key.

Oh, and if there’s an earthquake while you’re at the beach, get as far from the water (and as high above it) as you can. Don’t wait for someone to tell you to leave.