The Triple Threat of the Cascadia Subduction Zone

It was a cold, dark, wintery night in 1700 when the ground shook violently for several minutes during one of the largest earthquakes the world has ever experienced. This massive magnitude 9.0 earthquake ruptured the full length of the Cascadia subduction zone – a 1,000-kilometer (600-mile) long off-shore fault paralleling the west coast of North America, extending from mid-Vancouver Island, Canada to Humboldt County in Northern California.^1,2 Just minutes after the ground shaking subsided, a massive tsunami inundated coastal regions.

Ancient Native American and First Nations legend tells the story of the Thunderbird and the Whale, which has been passed down for three centuries, and details the account of this great earthquake.^3,4 While this oral history describes the event’s effects and date remarkably well, scientific evidence, including tsunami deposits along the coast, mud slides offshore, carbon dating of trees in drowned forests on the coasts of the Pacific Northwest and the Japanese record of an “orphan tsunami” reaching the coast of Japan, also confirm the occurrence of this massive Cascadia event on January 26, 1700. For centuries, the Japanese were unsure of the origin of this tsunami since it was neither generated from a local earthquake in Japan nor on any other known tsunamigenic subduction zone in the world.

It was Mount St. Helen’s volcanic eruption in 1980 that gave seismologists additional proof that the Cascadia subduction zone was still active. It was believed for years that the subduction zone was one of the few on Earth that did not produce massive earthquakes, but active volcanism is indicative of active subduction. In a subduction zone, an oceanic tectonic plate that is created at an oceanic ridge pushes beneath a continental plate. The descending plate will continue to be pulled, or subducted, under a continent until it reaches a depth at which it begins to melt. The melting of the plate at depth produces magma which then ascends through the continental plate where it is expelled by volcanoes.⁵

Research has shown that these megathrust, or giant, complete subduction-zone-rupturing earthquakes occur along the Cascadia subduction zone, on average, every 500 years.⁶ A full rupture of the fault from mid-Vancouver Island, Canada to Northern California would be similar to the 2011 magnitude 9.0 Tohoku-oki Japan earthquake.⁷ The disaster would be widespread, adding to response and recovery challenges as resources would need to be dispersed across the entire region.⁸

A 2012 study by Chris Goldfinger of Oregon State University suggests that the entire subduction zone may not always rupture, and that smaller – yet still catastrophic – earthquakes are more common along the southern end of the subduction zone with a recurrence interval of about 240 years.⁹ The 2014 U.S. Geological Survey National Seismic Hazard Maps¹⁰ incorporate this new research and highlight an important change because the result is a 40-percent increase in earthquake hazard along the southern end of the Cascadia subduction zone, which includes offshore Oregon and Northern California.

Understanding these megathrust earthquakes is critical for managing earthquake risk; however, it is important to recognize that there are three different types of earthquakes associated with the Cascadia subduction zone (Figure 1), producing a triple threat of earthquake risk in the region. While the massive megathrust earthquakes are the largest magnitude events that can occur, they are also very rare. Shallow earthquakes¹¹ – known in seismology as crustal earthquakes – and deep¹², or intraslab, earthquakes occur much more frequently. While the maximum magnitude of shallow and deep earthquakes in Cascadia is expected to only be approximately magnitude 7.5, if these events are located near an urban center like Portland or Seattle, they could be catastrophic. The deep earthquakes, mostly located in the Puget Sound region of Washington, are the most frequent of all earthquakes in Cascadia with large magnitude events recorded every few decades. The most recent was the 2001 magnitude 6.8 Nisqually earthquake south of Seattle.¹³ Given its depth of approximately 50 kilometers (30 miles), this earthquake caused relatively low levels of damage and loss. Deep earthquakes generally cause less damage than shallow earthquakes, but still have destructive potential.

Earthquakes around the world, including the 2011 earthquake in Christchurch, New Zealand¹⁴ as well as the 1989 Loma Prieta¹⁵ and 1994 Northridge¹⁶ earthquakes in California, are strong reminders of the threat that shallow crustal earthquakes pose. Among the largest crustal earthquakes the region has experienced were a magnitude 7.3 event located on a sparsely populated area of Vancouver Island, Canada in 1946¹⁷ and the 1872 Lake Chelan earthquakeeast of Seattle which had a magnitude estimated at 6.8.¹⁸ Both of these earthquakes caused considerable damage given the limited population and urban exposure at the time. If a similar event were to occur in today’s exposure, especially if located near one of the major cities in the Pacific Northwest, such as Portland or Seattle, the result could be catastrophic. One of the most significant known crustal faults in the Cascadia region is the Seattle fault, which traverses east-west through the Seattle metro area and is a direct threat to the Pacific Northwest’s largest city.

Fortunately, large damaging earthquakes are rare, but the infrequent occurrence of these events also leads many to forget that the risk exists. The triple threat of earthquake risk in the Pacific Northwest makes the region among the highest risk areas in the nation, following California – the highest risk in the country. It may be decades or centuries before the next megathrust earthquake occurs, but it also could happen much sooner, as could a shallow crustal or deep intraslab earthquake. Although much has been learned over the 30-year duration of the National Earthquake Hazard Reduction Program (NEHRP) regarding the seismic hazards of the Pacific Northwest, there is a continuing need to better understand and prepare for the risks that these earthquakes pose.

Sources:

1. https://earthquake.usgs.gov/earthquakes/states/events/1700_01_26.php
2. http://www.earthquakescanada.nrcan.gc.ca/historic-historique/events/17000126-en.php
3. https://www.pnsn.org/outreach/native-american-stories/thunderbird-and-whale/thunderbird-and-whale-stories/list-of-stories
4. https://www.hakaimagazine.com/article-long/great-quake-and-great-drowning
5. http://www.geology.sdsu.edu/how_volcanoes_work/subducvolc_page.html
6. http://www.amnh.org/explore/science-bulletins/earth/documentaries/tsunami-science-reducing-the-risk/ghosts-of-tsunamis-past
7. https://earthquake.usgs.gov/earthquakes/eqarchives/poster/2011/20110311.php
8. http://www.crew.org/sites/default/files/cascadia_subduction_scenario_2013.pdf
9. http://oregonstate.edu/ua/ncs/archives/2012/jul/13-year-cascadia-study-complete-%E2%80%93-and-earthquake-risk-looms-large
10. http://pubs.usgs.gov/of/2014/1091/
11. http://www.crew.org/sites/default/files/CREWshallowFinalSmall.pdf
12. http://www.crew.org/sites/default/files/CascDeepEQweb.pdf
13. http://earthquake.usgs.gov/earthquakes/eventpage/uw10530748#executive
14. http://www.teara.govt.nz/en/historic-earthquakes/page-13
15. https://pubs.usgs.gov/dds/dds-29/
16. http://scedc.caltech.edu/significant/northridge1994.html
17. http://www.earthquakescanada.nrcan.gc.ca/historic-historique/events/19460623-en.php
18. https://earthquake.usgs.gov/earthquakes/states/events/1872_12_15.php

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