Portland’s seismic hazards stem from subduction zone, local faults

Oregon is prone to shaking from both shallow crustal earthquakes and large subduction zone shocks. In either scenario, Portlanders would be in danger.

Written by Alice R. Turner, Ph.D., Science Writer (@SeismoAlice)

Citation: Turner, A., 2023, Portland Seismic Hazards Stem from Subduction Zone, Local Faults, Temblor, http://doi.org/10.32858/temblor.315

When people think of earthquakes in the continental United States, they often think of California. But Oregon — and especially the city of Portland — is also at the heart of earthquake country, with the seismic hazard coming from both the Cascadia subduction zone and shallow crustal faults in and around the city.

Portland, Oregon, faces seismic hazards from the offshore Cascadia Subduction and local faults. Credit: Amateria1121, CC BY-SA 3.0, via Wikimedia Commons

The biggest one

The Cascadia Subduction Zone is a 600-mile (1,000-kilometer) plate boundary where the ocean’s Juan de Fuca Plate sinks beneath the North American continent. This subduction zone has been relatively quiet and modest over the past three centuries, with few earthquakes recorded. But in the 1980s, scientists realized that this stillness could be a bad omen. Scientists have found multiple pieces of evidence confirming that the Cascadia subduction zone can host the most destructive type of earthquake – a so-called mega event.

This map shows the Oregon shakes a person is likely to experience in their lifetime, based on the Temblor PUSH model. Credit: Temblor, CC BY-NC-ND 4.0

Paleontologists carefully analyzed marine and land sediments, finding evidence of between 19 and 20 magnitude 9 earthquakes in the last 10,000 years. “Right now, we’re probably at the end of a set of five events that began 1,500 years ago and are about 350 years apart,” says Cascadia expert Chris Goldfinger, a seismologist at Oregon State University. “The last one we had was 1700.”

one close

Although the potentially massive Cascadia earthquake has been making headlines when it comes to coastal earthquake hazards in Oregon, it’s not the only fault in the area that could wreak havoc. Several active faults are dotted around and below Portland. These defects are also caused by plate boundaries. The Juan de Fuca Plate does not directly impinge on the North American continent. Instead, its motion results in a clockwise rotation as it covers, which is mostly internalized within the dominant North American plate by local shallow crustal faults.

Illustration of the subduction zone under Portland. Credit: USGS

For example, the Galis Creek fault has hosted earthquakes “for approximately 1,000 years, 4,000 years, and about 8,000 years,” says Ashley Strig, a professor at Portland State University and co-author of a recent study on this local structure. From the surface expression of the 45-mile (72-kilometer) fault, the research team estimated that the fault could produce earthquakes of magnitude 7.2. This active fault is located 22 miles (35 kilometers) west of Portland.

The offshore Cascadia will likely produce the largest shaking in Portland, but several local faults, including the Portland Hills fault that runs through downtown, can also produce shaking. Credit: Temblor, CC BY-NC-ND 4.0

Other drawbacks, like the one in Portland, are not as well thought out. For example, the Portland Hills Fault runs through downtown Portland. But its particularly dangerous location under the city isolates it from scientists keen to study it, which leads to a lack of information about past earthquakes. However, because this fault and other urban faults have the same northwest orientation as the well-studied faults outside the city, the scientists can infer that stresses from plate motion are distributed between them. As a result, Strig says, “other structures near the Portland metro area are also likely to present a seismic hazard.”

Different flaws, different feeling

Both how an earthquake feels to people and the damage it may cause depends on the distance from the fault that ruptured.

“When we talk about seismic hazard and the Pacific Northwest, particularly in Oregon, I notice that the focus is on ‘high risk,’” Streig says. Crust faults have earthquakes that are less frequent and smaller in magnitude than the subduction zone, and therefore less dangerous. But she says they are close, and “should not be ignored.”

If an earthquake erupts near or even just below a city, the ground shaking in even medium-sized events may be brief but intense, with little warning. This means little time — or no time — to take preventive action.

Portland has 12 bridges spanning the Willamette River, all of which are at earthquake risk. St. John’s Bridge was built in 1931 and a 2016 government report suggested that it would suffer major earthquake damage. Credit: Ian Sane, CC BY 2.0

“You won’t have time to react,” says Goldfinger. “It’s usually over by the time you find out what’s going on.”

The massive earthquake scenario presents different risks. The shaking will last between 5 and 7 minutes. However, such a rupture is likely to be located near the coast – more than 50 miles (80 kilometers) from Portland. These 50 miles will act as a buffer because the amplitude of seismic energy will decrease as seismic waves travel inland. A massive earthquake [Portland] It would be a little more cute [than a local event]says Streig.

The seismic energy from the quake will be detected by seismometers near the fault before it reaches Portland, meaning city residents may have up to a minute of warning to take precautionary measures before shaking begins.

The Meyer and Frank Building and Pioneer Courthouse in downtown Portland have been seismically modified. Credit: Steve Morgan, CC BY-SA 3.0, via Wikimedia Commons

Prepare for the next earthquake

Whether the next earthquake is a major earthquake or a near one, more than two million people live in the Portland metropolitan area, many of whom would be at risk in either scenario. There would be more than 18,000 deaths in Oregon from a tsunami caused by a subduction zone earthquake alone. The Cascadia Region Earthquake Working Group also expects direct and indirect economic losses to exceed $70 billion.

Yumi Wang, an engineer and geologist, has spent her career working tirelessly to influence policy to mitigate the damage that would come with the Cascadia earthquake.

Wang’s work began with schools. Children are required to go to school. They must go to school in a safe building and come home every day whether there is an earthquake or not. After 12 pieces of legislation over 10 years, in 2009 Oregon launched its first state-funded Seismic Rehabilitation Grant to help schools and emergency response facilities become earthquake-safe. In 2019 alone, $200 million in grants was provided through the Seismic Rehabilitation Grant Program to retrofit schools and emergency buildings.

Aloha High School in Beaverton, Oregon, was a top priority for the earthquake retrofit. Credit: MO Stevens, CC BY-SA 3.0, via Wikimedia Commons

Lifeline infrastructure came next. After an earthquake, survivors don’t have to worry about water – whether to drink, shower or flush toilets – yet safe water is often one of the biggest concerns after large earthquakes. After six years of advocacy, the Oregon Health Authority is now commissioning seismic vulnerability studies for public water systems.

The latest effort, which Wang is involved in, focuses on bulk fuel tanks located in liquefiable soil along the edge of the Willamette River that could cause a devastating spill and fire disaster in an earthquake. Just last year, a bill was passed that now requires studies of seismic vulnerabilities and mitigation.

The legislation is now institutionalized and self-sustaining. “No one needs to continue to fight for oversight and funding, legislative session after legislative session, to ensure progress on safety,” Wang says.

The Burnside Bridge, shown here, was built in 1926, and according to a government report likely sustained major earthquake damage. It is scheduled to undergo a seismic retrofit in 2025. Credit: dog97209, CC BY-NC-ND 2.0

But there is still work to be done. In 2016, the Bureau of Development Services reported that Portland had more than 1,600 unreinforced masonry buildings that are highly susceptible to collapse. It is estimated that less than 20 percent of these buildings have been demolished or modernized (either in whole or in part). Old bridges are also expected to suffer seismic damage in a major earthquake. Portland has 12 bridges that span the Willamette River, 10 of which were built before 1994, and have a higher risk of major or moderate earthquake damage. As of this writing, the Pioneer Courthouse is the only building in downtown Portland with basal isolation, an expensive foundation modification that allows the building to sway in an earthquake, not only preventing collapse, but also making the building safe immediately after an earthquake.

Unsupported masonry buildings in Portland, such as the Cadillac Hotel building in Seattle that was damaged during the 2001 Nisqually earthquake, shown here, are extremely vulnerable to earthquakes. Credit: Erik Stuhaug via Seattle Municipal Archives, Seattle, WA, CC BY 2.0, via Wikimedia Commons

Portland is getting more prepared for an earthquake, but it’s not there yet. “It really is possible,” says Goldfinger. “Spending a lot of money is what it takes.”

For more information on how to retrofit your single-family home, see this document from the Oregon Seismic Safety Policy Advisory Committee. And for more information about Oregon’s risks from earthquakes and tsunamis, whether you live in earthquake country or are just visiting, check out Oregon’s Department of Geology and Mineral Industries reports on individual counties and even communities here.

References

Horst AE, Streig AR, Wells RE, & Bershaw J. (2021). Multiple Holocene earthquakes on the Gales Creek Fault, NW Oregon Arc Front. Bulletin of the American Seismological Society, 111 (1), 476-489. https://doi.org/10.1785/0120190291

Walton, M.A.L., Staisch, L.M., Dura, T., Pearl, J.K., Sherrod, B., Gomberg, J., Engelhart, S., Tréhu, A., Watt, J., Perkins, J., Witter, R.C., Bartlow, N., Goldfinger, C., Kelsey, H., Morey, H.E., Sahakian, Wells, VJ, R. Towards an integrative geological and geophysical presentation of an earthquake Remove the Cascadia subduction zone. Annual Review of Earth and Planetary Sciences, 49(1), 367-398. https://doi.org/10.1146/annurev-earth-071620-065605

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