Cascadia Earthquakes and Tsunamis Likely for Pacific Northwest America

Summary: Cascadia earthquakes and tsunamis are likely events for the Pacific Northwest, according to a study in Nature Geoscience Nov. 2, 2015.

A simulated tsunami reaches Japan 10 hours after its start along North America's Pacific coast; Brian Atwater et al., The Orphan Tsunami of 1700 (U.S. Geological Survey Professional Paper 1707; published 2005): Public Domain, via US Geological Survey

The Pacific Northwest awaits a mega-earthquake according to a study, published Nov. 2, 2015, in the journal Nature Geoscience, respecting the first thorough mapping of the mantle 160.93 kilometers (100 miles) beneath the Juan de Fuca underwater tectonic plate.

The study brings attention to coastal west North America’s other fault line, the 1,100-kilometer- (683.51-mile-) long Pacific fault running through the United States and Canada, from Cape Mendocino in southern California northward through Oregon and Washington to northern Vancouver Island in British Columbia.

Geologists call the lesser known equivalent of California’s 1,300-kilometer- (807.78-mile-) long San Andreas fault the Cascadia Subduction Zone. They describe 243-year recurrence intervals between Cascadia’s 41 magnitude-9, mega-thrust, strong-motion, subduction-zone earthquakes over the past 10,000 years.

Growth-rings dated 1699 in Copalis River’s saltwater-inundated western red cedar ghost forest, Huu-ay-aht First Nation traditions and a 965-kilometer- (600-mile-) long tsunami traveling 10 hours to Japan on the eighth day of the 12th month of the 12th year of the Genroku era enable seismologists to pinpoint the last mega-earthquake to 9:00 p.m. PST (5:00 a.m. UTC) Jan. 26, 1700 (Jan. 27, 1700), off coastal Seattle.

Eleven mappers since 2012 of Cascadia’s 1,126.54-kilometer- (700-mile-) long, 48.28 to 72.42-kilometer- (30 to 45-mile-) thick Juan de Fuca Plate by 10-month rotations of 70 seismometers along 120 seabed sites and 24 two-week ocean voyages furnish no specifics on upcoming mega-earthquake timings.

But three years of data give indications of the likeliest hotspots.

The Juan de Fuca Plate’s southerly portion, Gorda, has independent mantle motions that align with the Pacific Plate to the west. The Gorda Plate is in alignment with the Juan de Fuca Plate’s slippage under the westward-moving North American continental plate at 4.06-centimeter (1.6-inch) annual rates while its mantle aligns with the Pacific Plate’s mantle moving northwestward at 10.16-centimeter (4-inch) yearly rates.

As earth and planetary science departmental professor, Seismological Laboratory director, and senior author, Richard M. Allen of University of California-Berkeley judges full or partial rupture likelihoods: “When you look at earthquakes in Cascadia, they sometimes break just along the southern segment, sometimes on the southern two-thirds, and sometimes along the entire length of the place.”

Co-authors Richard Allen, Robert Martin-Short and Mark A. Richards of Berkeley, and Ian D. Barstow and Eoghan Totten of London’s Royal School of Mines know about Cascadia’s 315-year accumulated downward stresses. They look to the $20 million-dollar Cascadia Initiative’s data on the Pacific Ring of Fire’s American segments, sites of the magnitude-9.2 Great Alaskan and magnitude-9.5 Valdivia, Chile mega-earthquakes of March 7, 1964 and May 22, 1960.

Dr. Allen mentions that “Our goal is to understand large-scale plate tectonic processes and start to link them all the way down to the smallest scale, to specific earthquakes in the Pacific Northwest” since Cascadia earthquakes and tsunamis are likely mega-events for Pacific Northwest America early in the third millennium.

Cascadia Subduction Zone; Map of Juan de Fuca Triple Junctions and the Cascade Volcanic Arc by NASA World Wind/Black Tusk: Public Domain, via Wikimedia Commons

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

A simulated tsunami reaches Japan 10 hours after its start along North America's Pacific coast; Brian Atwater et al., The Orphan Tsunami of 1700 (U.S. Geological Survey Professional Paper 1707; published 2005): Public Domain, via US Geological Survey @ http://web.archive.org/web/20080509183901/http://pubs.usgs.gov/pp/pp1707/

Cascadia Subduction Zone; Map of Juan de Fuca Triple Junctions and the Cascade Volcanic Arc by NASA World Wind/Black Tusk: Public Domain, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Cascade_Volcanic_Arc.jpg

For further information:

Algar, Jim. 5 November 2015. "Pacific Northwest Earthquake Overdue,  Say Scientists Mapping The Region's Tectonic Plates." TechTimes > Science.
Available @ http://www.techtimes.com/articles/102778/20151105/pacific-northwest-earthquake-overdue-say-scientists-mapping-the-regions-tectonic-plates.htm

Martin-Short, Robert, et al. 2015. "Mantle flow geometry from ridge to trench beneath the Gorda-Juan de Fuca plate system." Nature Geoscience 8 (2015): 965-968. DOI: 10.1038/ngeo2569
Available @ http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2569.html

Sanders, Robert. 2 November 2015. "Scientists map source of Northwest's next big quake." Berkeley News > Research, Science & Environment.
Available @ http://news.berkeley.edu/2015/11/02/scientists-map-source-of-northwests-next-big-quake/

Richard Allen - Berkeley Seismo Lab. 28 October 2015. "Cascadia Initiative: Seismic stations across the Juan de Fuca Plate." YouTube.
Available @ https://www.youtube.com/watch?v=1624Am8Pb64

Schulz, Kathryn. 20 July 2015. "The Really Big One." The New Yorker > Magazine.
Available @ http://www.newyorker.com/magazine/2015/07/20/the-really-big-one