Cascadian Megaquake Resilience

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Cascadia, the bioregion defined by Pacific Ocean to the west + Cascade Mountains to the east, is long overdue for its next megaquake.

These earthquakes (megaquakes = > magnitude 8) have happened on average every ~250 years for at least 10,000 years.

The last Cascadian megaquake struck 323 years ago on January 26, 1700, with an estimated magnitude of 8.7–9.2.[1]

Projected Impact

Oregon state officials forecast an earthquake that would last 4 to 6 minutes, followed in as little as 15 minutes by a tsunami.

The earthquake would likely drop areas along the coast by several feet below sea level. The combination of lower elevations and the action of the earthquake could send tsunami surges of up to 80 feet above sea level into coastal areas and also flood the estuaries of the Columbia, Willamette and other rivers.

Fatalities could top 25,000 with tens of thousands of buildings destroyed, highways collapsed, electrical and other utilities broken and communications knocked out. The total economic loss would likely be more than $30 billion, making it the most expensive "natural disaster" in U.S. history.[2]

Some of the major cities along the length of the Cascade Volcanic Arc include Portland, Seattle, and Vancouver, and the population in the region exceeds 10 million. All could be potentially affected by volcanic activity and great subduction-zone earthquakes along the arc.[3]

Worst-Case Scenario

The more catastrophic scenario is for a 9.0 earthquake along the entire zone from Canada to the California border. That would destroy much of the infrastructure west of the Cascades, according to state and federal officials. In a 2015 New Yorker magazine article on a possible Cascadia earthquake, Kenneth Murphy, who then directed the Federal Emergency Management Agency Region 10, responsible for Oregon, Washington, Idaho and Alaska, said, “Our operating assumption is that everything west of Interstate 5 will be toast.”[4]

Urban Infrastructure

Seattle, Portland, and Vancouver B.C. all lie within the zones of elevated tremor and strong locking. In contrast, Eugene lies inland of the portion of the megathrust that is not strongly locked and that produces less tremor.[5]

Seattle

The city of Seattle, the nation’s fastest-growing city, lies in the Puget lowland on the shores of Puget Sound and Lake Washington. While the location creates an ideal trade gateway, it also means the city lies atop a deep basin. This has startling consequences for shaking, according to a recent study by scientists at the University of Washington, the USGS, and University of Southern California. They looked at how buildings ranging from 4-40 stories high would sway (engineers call this “drift”) in simulated earthquakes, comparing the ride in the Seattle basin, and outside it. They found that within the basin, buildings swayed at least three times more than outside of it because of stronger, slower shaking. Thus could result in much greater levels of damage throughout the city, and longer recovery times.[6]

Portland

In the event of a (M=9) Cascadia Subduction Zone earthquake, over 200,000 homes are likely to be damaged in the city of Portland, according to a 2018 study by the Oregon Department of Geology and Mineral Industries (DOGAMI).[7] By assessing the shaking throughout the metropolitan area, they found approximately 38-39% (235,000) of the city’s buildings would suffer some level of damage. This emphasizes that in the event of a Cascadia event, the impacts will not only be extremely severe, but extremely widespread.[8]

According to this study, in a worst-case scenario, the metro area could see more than $80 billion in building damage, tens of thousands of people wounded or killed and more than 250,000 people facing long-term displacement. The metro area is home to 44 percent of Oregon's total population and 50 percent of the state's jobs.

These estimates were higher than previous studies, because the DOGAMI 2018 study used better data about the buildings in the metro area, many of which were built using unreinforced masonry.[9]

As documented in the searchable OregonLive Earthquake Map:

"Structures predating 1974, when Oregon enacted its first statewide building code, would suffer the worst damage in a 9.0-magnitude earthquake. Buildings constructed after 1993, when western Oregon adopted its first seismic standards, are considered the most likely to withstand a severe earthquake."

The majority of the Portland Metropolitan Area's buildings were constructed before 1974, with the city home to over 1,600 unreinforced masonry buildings and an estimated 100,000 wood-framed homes in need of retrofitting.

Liquefaction

One of the advances in the most recent report, compared to previous studies, is the ability of researchers to detect which areas are prone to liquefaction, wherein saturated soil behaves more like a liquid than a solid. Vast swaths of Portland's west side, a large stretch of the south bank of the Columbia River and portions of the city's inner east side are all at high risk of liquefaction or landslides, especially if the quake strikes during the wet winter months, the report said.

Bridge Failures

Multnomah County EIS Report, 4/22/2022:

"Absent significant and targeted infrastructure resiliency improvements, the next CSZ earthquake is expected to render all of the downtown Portland Willamette River crossings unusable (either because of damage to each crossing’s bridge, its approaches, or both). This means that none of the designated lifeline routes or evacuation routes across the river will be available for emergency response, rescue, or evacuation immediately following the earthquake."[10]

Full Environmental Impact Statement

Vancouver

Volcanic Eruptions

The Cascade Mountains are volcanically active and it is possible the next megaquake will trigger a volcanic eruption.

As reported by the American Geophysical Union in its Fall 2018 Meeting[11]:

"Explosively erupting volcanoes and megathrust earthquakes (Mw 8+ magnitude) occur at subduction zones. At many locations and times volcanic eruptions occur just before or after megathrust earthquakes. Examples globally include the eruption of Cordon Caulle volcano following the 1960 Chilean Mw 9.0+ earthquake; the 1707 eruption of Mount Fuji after the Mw 8.6 Nankai megathrust earthquake; the 2004 Mw 9.1+ Sumatra-Andaman megathrust earthquake followed by the 2005 eruptions of Talang, Egon and Barren Island volcanoes. Timing linkages between megathrust earthquakes and volcanic eruptions might be found in Kamchatka, Alaska, and Central America. Some work has been conducted to explore possible triggering relationships in subduction systems, but the Cascadia Subduction Zone in the U.S. Pacific Northwest is missing this perspective."

Emergency Preparedness

Oregon

The Cascadia Playbook is a 100-page outline of actions to be taken in the first hours and days after the disaster, issued by the State of Oregon in 2018. The playbook covers the first two weeks after an earthquake. Bend would be the center of the state’s emergency response in the worst-case scenario.

Oregon hopes to implement a $12 million statewide earthquake and tsunami early warning system by 2023. The plan is for it to link up with existing and planned warning systems in California and Washington to offer a regional alert.[12]

The Office of Emergency Management is also working with local governments, community groups and the American Red Cross on a $1.6 million plan to ensure that 250,000 homes in the earthquake zone have a two-week supply of water, food, medicine and other supplies in place by 2021.

The state also plans to spend $11.1 million to develop “more robust logistical staging bases,” according to the governor’s proposed budget.

“When the next Cascadia subduction zone earthquake strikes the Pacific Northwest, Oregon will face the greatest challenge of our lifetimes,” Gov. Kate Brown said in a statement in October. “Oregon’s buildings, transportation network, utilities, and population are not as ready as they should be and we must accelerate our preparations.”[13]

Oregon Coast

Beat the wave evacuation maps: https://www.oregon.gov/dogami/tsuclearinghouse/pages/beatthewave.aspx?utm_source=DOGAMI&utm_medium=egov_redirect&utm_campaign=https%3A%2F%2Fwww.oregongeology.org%2Ftsuclearinghouse%2Fbeatthewave.htm

Willamette Valley

The Willamette Valley is sitting on sediment, and if an earthquake hits, the highways may not be drivable. Because of this, the Redmond Municipal Airport may be the closest area not directly affected. The airport is being upgraded to be the command post for the state if the earthquake happens off the coast of Oregon.

Central Oregon will be an important aspect of supporting the emergency in the Valley, but "we get the majority of our commodities via the I-5 corridor," he said, which includes fuel. "Almost the entire state of Oregon gets all their fuel through a fuel hub in Portland that's not anticipated to survive the Cascadia earthquake, so getting things like fuel, supplies, groceries, power..." will not be possible for days, weeks or months.[14]

Just Transition

Historic Quakes

The most recent Cascadian megaquake struck on January 26, 1700, with an estimated magnitude of 8.7–9.2.[15]

Seismology

Cascadia Subduction Zone

It has only been in the past 25 years that scientists have begun paying close attention to the Cascadia Subduction Zone. It’s about 50 to 75 miles off the coast of Oregon where two of the earth’s geological plates meet — the Juan de Fuca plate is slowly folding under the Turtle Island plate, creating the Cascade Mountains and all the volcanoes within the mountain range.[16]

"The Cascades themselves are about 40 million years old, so this has been a volcanically active area for a long time," said Julie Donnelly-Nolan, a geologist with the United States Geological Survey. "Then if you just think really much more recently, the peaks that we see every day vary in age, but [are] somewhere from tens of thousands of years old to hundreds of thousands of years old."[17]

The zone stretches from the west coast of Vancouver Island in the north to just off Eureka, California, in the south.

Sources

  1. ↑ https://en.wikipedia.org/wiki/1700_Cascadia_earthquake
  2. ↑ https://www.bendbulletin.com/localstate/bend-is-worse-case-scenario-command-center-in-major-earthquake/article_0503ff13-835c-56af-99e2-db82e5f27f91.html
  3. ↑ https://en.wikipedia.org/wiki/Cascade_Volcanoes
  4. ↑ https://www.bendbulletin.com/localstate/bend-is-worse-case-scenario-command-center-in-major-earthquake/article_0503ff13-835c-56af-99e2-db82e5f27f91.html
  5. ↑ https://temblor.net/earthquake-insights/new-findings-clarify-the-seismic-risk-in-the-pacific-northwest-7443/
  6. ↑ https://temblor.net/earthquake-insights/new-findings-clarify-the-seismic-risk-in-the-pacific-northwest-7443/
  7. ↑ http://www.oregongeology.org/pubs/ofr/p-O-18-02.htm
  8. ↑ https://temblor.net/earthquake-insights/new-findings-clarify-the-seismic-risk-in-the-pacific-northwest-7443/
  9. ↑ https://www.oregonlive.com/pacific-northwest-news/2018/03/when_the_big_one_hits_portland.html
  10. ↑ https://wppo.blob.core.windows.net/ooh-burnside-eis-22/2022/04/SDEIS-Chapter-1-Purpose-and-Need-for-the-Project-1.pdf
  11. ↑ https://ui.adsabs.harvard.edu/abs/2018AGUFM.T13H0317G/abstract
  12. ↑ https://www.bendbulletin.com/localstate/bend-is-worse-case-scenario-command-center-in-major-earthquake/article_0503ff13-835c-56af-99e2-db82e5f27f91.html
  13. ↑ https://www.bendbulletin.com/localstate/bend-is-worse-case-scenario-command-center-in-major-earthquake/article_0503ff13-835c-56af-99e2-db82e5f27f91.html
  14. ↑ https://www.bendsource.com/news/apocalypse-when-16113015
  15. ↑ https://en.wikipedia.org/wiki/1700_Cascadia_earthquake
  16. ↑ https://www.bendbulletin.com/localstate/bend-is-worse-case-scenario-command-center-in-major-earthquake/article_0503ff13-835c-56af-99e2-db82e5f27f91.html
  17. ↑ https://www.bendsource.com/news/apocalypse-when-16113015
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