Geosciences: The Really Big One in the South Sound

Three UW Tacoma geoscientists assess the impact of a major Cascadia Subduction Zone earthquake on the South Sound, and highlight some controversial conclusions in a recent New Yorker story on “The Really Big One.”

It seems as if the entire Pacific Northwest has only one major topic of conversation: The Big One – the mammoth earthquake we are supposedly overdue to experience due to shifts of the Cascadia Subduction Zone fault.

The conversation has been provoked by a lengthy feature article in the July 20, 2015, issue of The New Yorker, “The Really Big One,” by Kathryn Schulz.

In the article, the author reports on the rapidly evolving understanding of how the structures of the earth come together off the coast of California, Oregon, Washington and British Columbia. Forty-five years ago, no one even knew the Cascadia Subduction Zone existed. Thirty years ago, no western scientist imagined that it could be the epicenter of big earthquakes, although indigenous residents preserved a long oral tradition of natural catastrophes that has proven invaluable in assembling a record of earthquakes and tsunamis in the region.

Scientific awareness has advanced faster than changes to the built environment. Schulz reports that the Federal Emergency Management Administration has planned for a quake that would kill 13,000, injure 27,000, displace 1,000,000 and assumes that “ ‘everything west of Interstate 5 will be toast.’ ”

The New Yorker piece, and a lot of the media coveage about it, talk about what will happen in Seattle or in Portland or in Vancouver. But what about the South Sound? And how certain is the science that says we are “overdue” for a big quake?

UW Tacoma geoscientists have been looking at these issues along with many other scientists across the region. Below is a point-by-point analysis of the New Yorker article and what we know about how a big Cascadia earthquake might affect the South Sound, written by three UW Tacoma faculty members.

Peter Selkin is an associate professor in UW Tacoma’s School of Interdisciplinary Arts & Sciences. He began his UW Tacoma career as a lecturer in 2006, and achieved his current rank in 2015. He is a geophysicist who studies the earth's magnetic field, and received his Ph.D. from University of California, San Diego.
Julie Masura is a lecturer and research affiliate faculty in UW Tacoma’s School of Interdisciplinary Arts & Sciences and the Center for Urban Waters. She is a sedimentologist and received her Master’s in Geology from Washington State University.
Cheryl Greengrove is an associate professor in, and currently the interim dean of, UW Tacoma’s School of Interdisciplinary Arts & Sciences. She is a physical oceanographer, and a founding faculty member of UW Tacoma’s environmental science program. She received her Ph.D. from Columbia University.

Cascadia Earthquake Effects - Notes in Response to “The Really Big One” by Kathryn Schulz

by Peter Selkin, Cheryl Greengrove, and Julie Masura

Great earthquakes have happened along the Cascadia subduction zone, and will likely continue to occur in the future. This is something geologists have known for a few decades, as stated in the article.

As with any earthquake, the direct effects of an earthquake along the Cascadia subduction zone will be greatest where tectonic plates meet at subduction zone (the fault). The Cascadia subduction zone runs along the Pacific coast, from Mendocino, Calif., to just north of Vancouver Island. Strong ground shaking will be greatest in places such as Seaside, Ore., or Neah Bay, Wash., mentioned in the article. Shaking in places such as Tacoma, as well as Seattle and Portland, will likely be much less. Suppose the epicenter of the earthquake was at the point on the plate boundary closest to Tacoma, about 120 miles away. At the same distance from the epicenter of the magnitude 9.0 Tohoku earthquake in Japan is the city of Yamagata, which has roughly the population of Tacoma. Yamagata experienced strong shaking but light to moderate damage, with 2 people killed and 9 injured out of roughly 200,000.

We cannot predict earthquakes, though we have an idea how likely they are in a particular span of time. For example, the article discusses great earthquakes that occur on the Cascadia subduction zone about every 250 years on average. This means that a great earthquake has a 1 in 250 chance of occurring every year, or a one-in-four chance of occuring in an average person's lifetime, along that 850-mile-long zone. It does not mean that great earthquakes occur precisely every 250 years, or even close to every 250 years. This likelihood is based on observations of sediments believed to be deposited in landslides due to great earthquakes. The degree to which these sediment deposits represent great earthquakes is not something on which all geologists agree (though many do). Furthermore, the 1 in 250 chance of a great earthquake is for earthquakes of magnitude 9; smaller earthquakes occur frequently in the Pacific Northwest, though they occur very rarely along the “locked zone” close to the edge of the boundary between the tectonic plates.

Earthquake effects will be biggest in areas built on loose sediment or steep slopes. Places built on artificial fill or loose ground<—>such as the Tacoma tide flats and Pioneer Square in Seattle<—>will experience comparatively more shaking than places built on firmer ground such as the hard rocks of the Chuckanut Sandstone (Bellingham). For comparison, the rocks underneath Yamagata in Japan were hard. Most of Seattle and Tacoma are built on glacial sediment that is only moderately compacted. The amount of shaking here, then, is likely to be somewhat stronger than Yamagata. Earthquakes can also cause steep slopes to shake loose - a problem particularly on the sides of glacial hills (some of which are near the UW Tacoma campus and along Tacoma’s Schuster Parkway) or in places where soil has been cut and filled in.

An earthquake early warning system is being evaluated for the Pacific Northwest, despite what the article says. It is still in its prototype phase, and is being tested by the Pacific Northwest Seismic Network, under the auspices of the University of Washington.

A tsunami is likely to occur with a great Cascadia earthquake, but its effects will likely be focused along the Pacific Coast. The statement in the article that everything west of I-5 is at risk of tsunami flooding is a mistaken generalization. In some places with low topography—near the mouth of the Columbia River, for example, where I-5 runs closest to the coast—tsunami flooding may reach the freeway. Places on the Pacific coast like Long Beach, Wash., and Seaside, Ore., are at substantial risk. Our understanding is that most of Seattle, Tacoma, and Olympia are not at substantial risk from a Cascadia tsunami. However, the effects of such a tsunami on the South and Central Puget Sound are difficult to determine. An open-ocean tsunami may affect places in North Puget Sound and the Strait of Juan de Fuca: National Oceanic and Atmospheric Administration models indicate that low-lying ground near Bellingham may experience a tsunami as large as a few meters (maximum 9 feet) from a magnitude 9.1 earthquake on the Cascadia subduction zone. It seems unlikely that such a tsunami will reach the South or Central Puget Sound without losing a lot of energy (and height) as it moves around the islands - though this will probably cause the wave to vary in height around the region in unpredictable ways. Of greater concern is a tsunami due to an earthquake on the Seattle or Tacoma faults, which tend to produce smaller, more frequent earthquakes and small, local tsunamis that could nonetheless still damage low-lying areas. These are unrelated to earthquakes along the Cascadia subduction zone.

The risk of secondary effects (e.g. loss of power) and cascading failures (e.g. an earthquake causing a landslide causing a tsunami) are significant and require careful planning. We define earthquake risk as the combination of earthquake likelihood with the potential to lose the things we value. Here, our risk of losing life and property in an earthquake depends on geological, geographic, engineering, economic, social, historical, and political factors, all of which are interconnected in complex ways. Related to risk is the idea of resilience, the ability of a community to recover from disaster. Like risk, resilience requires thinking about more than geology. For example, Los Angeles recently finished a wide-ranging, multidisciplinary effort to plan for earthquake risk and resiliency called “Resilience by Design.” The USGS leads an annual international earthquake preparedness drill called the “Great Shakeout” to test emergency response systems and to simulate earthquake risk. Students engage in earthquake planning and risk assessment in our introductory geology courses, and recognize the multitude of factors that play a role in seismic risk. At the individual level, we citizens in the Pacific Northwest need, at a minimum, to understand our level of earthquake risk and the level of risk we find acceptable. As communities, we need to work toward implementing systems that will promote resilience.

Earthquake Awareness and Resiliency Resources

Written by: 
John Burkhardt / July 21, 2015
Media contact: 

John Burkhardt, UW Tacoma Communications, 253-692-4536 or