By Rachel Berkowitz
As a graduate student at Oregon State University in the 1960s, Gary Griggs helped to collect 48 core samples from the deep-sea channel Cascadia, off the coast of Washington and Oregon. The samples, like the one shown here, contained clays that alternated regularly with turbidity current deposits, or turbidites. Turbidites are graded beds of sand and silt that were transported by underwater avalanches.
At the time, the regular recurrence of turbidity currents in the Cascadia Abyssal Plain perplexed Griggs and his colleagues. Carbon dating of the samples—together with the appearance of an ash layer from Mount Mazama's eruption 7700 years ago and calculations of the time needed for enough material to be deposited to flow the entire 2200-km channel length—led the team to conclude that large turbidity currents flowed along the continental slope of the Pacific Northwest every 400–600 years.
What caused that regular pattern? Griggs speculated that a large volume of sediment from the Columbia River had accumulated on the outer continental shelf and was later released by periodic earthquakes or severe storms. It took nearly 30 years for the earthquake idea to be verified and accepted.
The Oregon State team had decided to look at the Cascadia channel because it was one of the few deep-sea channels that are close enough to the coastline for convenient study. They weren't looking for seismic events.
By the 1960s, evidence in support of Alfred Wegener's 1912 theory of plate tectonics had been growing. In particular, the Juan de Fuca plate's collision with the North America plate became known for its role in shaping the Washington–Oregon coastline.
John Adams of the Geological Survey of Canada reanalyzed the turbidites in light of plate boundaries. Displacement of the sea floor can trigger turbidity currents, submarine landslides, and tsunamis. In 1990 Adams published the proposal that the 13 deposits above the precisely timed Mazama ash layer can best be explained by powerful earthquakes on the Cascadia subduction zone.
Had the turbidites been closer to shore, they could conceivably have been caused by sands flowing from beaches into canyons. “But by the time we get 200 miles offshore . . . it's only really large events, hundreds of thousands of cubic yards of stuff, that will flow these kinds of distances,” says Griggs. That includes magnitude-9 earthquakes.
Buried marshes and dead trees
Buried tidal marshes and dead trees submerged in Cascadia coastal bays provide more evidence of rapid subsidence. The submerged Sitka spruce found off the Washington coast suggest a widespread event some 300 years ago. Written records from Japan tell of a tsunami striking that country's south coast on 27 January 1700. Both the buried spruce and the tsunami point to the possibility of a massive earthquake along the Pacific Northwest margin.
If the turbidite record is reliable, it suggests that the area could be at risk for another big event sometime this century.[1] The world learned just how devastating such a big event can be when the magnitude-9 earthquake and tsunami hit the shores of Japan on 11 March 2011.
Cascadia has shown that a very long record can reveal patterns that might not be apparent on a shorter, more recent time scale. Oregon State University geologist Chris Goldfinger focuses on extending such paleoseismic records by examining what the sediment history reveals about the extent, magnitude, and timing of prehistoric earthquakes.[2]
“If there's a good enough record, we can then look at the size of the events,” says Goldfinger. With the newly emerging field of paleoseismology comes the goal of identifying the worst-case scenario and dermining the likelihood of a given event. And once long paleoseismic records exist, it could be possible to look at interactions between faults and relate stress transfers from one to another.
Griggs comments that paleontology used to be the only 'paleo' study. Then paleohydrology developed, and then paleofloods. All of those look at terrestrial pieces of the geological puzzle. Paleoseismology, the newest study of the old, can only broaden our perspective by taking the discipline to the sea.
References
- 1. J. Adams, Tectonics 9, 569 (1990).
- 2. C. Goldfinger, Annu. Rev. Marine Sci. 3, 35 (2011).
