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Native tales tell of tremorous past
Cascadia up close

Native tales tell of tremorous past

A Native American tale tells of an epic battle between the supernatural Thunderbird and Whale in the waters off the coast of the Pacific Northwest. After the chivalrous Thunderbird finally succeeded in catching Whale in its talons, it returned to its nest, where another fight commenced. Then, “there were … shaking, jumping up and trembling of the earth beneath, and a rolling up of the great waters.”

A late 19th century interior ceremonial screen from Port Alberni on British Columbia’s Vancouver Island depicts Thunderbird carrying Whale in its talons. The scene is a common native symbol of seismic activity. Image courtesy of Edward Malin, Northwest Coast Painting — House Fronts and Interior Screens; copyright 1999, Timber Press.


This seemingly simple mythology piqued the interest of Ruth Ludwin, a research scientist at the University of Washington in Seattle, who has now published an analysis of it and other Native American folklore, linking the stories to actual seismic events in the Pacific Northwest. These stories passed down through the generations not only highlight past activity, but also serve as a reminder of the threat to the region, Ludwin says.

Variations of the Thunderbird and Whale story are found throughout the lore of Native Americans who lived along the Cascadia subduction zone, a long fault about 80 kilometers offshore that parallels the coast from Vancouver Island to Northern California. The stories help to confirm that a magnitude-9.0 earthquake struck the region in January 1700, Ludwin reported in the March/April Seismological Research Letters. “First I found the story of Thunderbird and Whale, and I just thought ‘wow, this is a great story,’” Ludwin says. “At some point, I realized that I had information that could be used to date the 1700 earthquake.”

Searching library archives, Ludwin turned up 39 other stories that involved earthquake- and tsunami-like details that she could attribute to a specific location along the West Coast. Of the 40 stories, nine provided information from which she could estimate a range of dates for the event. When Ludwin plotted the data together on a timeline, she found that they all overlapped the year 1700 — evidence that she says agrees with the estimated date of the earthquake and ensuing tsunami (see story, below).

Up until about 20 years ago, researchers were unaware of the hazards posed by the Cascadia subduction zone. Then in the 1980s, researchers started uncovering evidence from the geologic record — such as the unearthing of fire pits covered by tsunami deposits and the radiocarbon dating of drowned trees — and also from Japanese tsunami records that a rupture along the fault triggered a large event in 1700.

Ludwin’s recent work “complements what’s been done in the earth sciences,” says Brian Atwater, a geologist at the University of Washington who studies geologic evidence for historic earthquakes (see Geotimes, August 2005). Atwater says that the additional evidence posed by native lore “adds an intriguing human dimension” to earthquake research, and will prove valuable in educating the public about the potential risks associated with the fault.

In another set of Native American stories, Ludwin discovered an ancient awareness of another regional natural hazard: landslides. Seattle is prone to landslides because of the many layers of glacial deposits that cover the city. When groundwater percolates through the permeable layers and then gets stuck under a clay layer, it creates a situation where the land is likely to slide. Earthquakes also exacerbate the landslide risk.

In several stories, a supernatural shape-shifting serpent called a’yahos shook and churned waters. One story tells of a bluff in Seattle where “a great snake lived inside, shoving the sand down when people disturbed him.”

The locations of five of the a’yahos stories are concentrated around the Seattle fault, which runs across Lake Washington, Puget Sound and downtown Seattle. And four of those stories correspond to potential locations of old and massive landslides, some of which may have been caused by an estimated magnitude-7.3 earthquake in A.D. 900, Ludwin reported in the July/August Seismological Research Letters.

Ludwin first made the possible connection after reading a 1985 article in the Seattle Weekly that described a “spirit boulder” near Seattle’s Fauntleroy ferry dock, in close proximity to the fault. As the a’yahos stories also mentioned a boulder, Ludwin turned to U.S. Geological Survey (USGS) LIDAR images, which map aerial topography, for evidence of landslides in the area. She found that the appearance of the land above the boulder resembled features common to block landslides. Although not categorized as a landslide on USGS maps, the feature is “easy to see when you know it’s there,” Ludwin says.

Ludwin has not been able to confirm the hypothesis, as that would require excavation in a developed area. However, she hopes that the a’yahos stories raise awareness among residents that many of the bluffs and hillsides of Seattle exist on landslide-prone geology. “These ancient, place-specific stories have a powerful effect on the human imagination,” Ludwin concluded in her second paper. “The profound respect in which a’yahos was held by the natives of Puget Sound for perhaps a thousand years may help contemporary Puget Sound residents grasp the severity of the earthquake effects experienced by A.D. 900 Puget Sound residents, and grapple with the hazard issues that the Seattle Fault continues to present.”

Kathryn Hansen

Link:

"Brian Atwater: Earthquake hunter in the field," Geotimes, August 2005

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Cascadia up close

Ever since geologists in the late 1980s discovered that the Cascadia subduction zone in the Pacific Northwest could generate magnitude-9-plus earthquakes, they have sought a better understanding of the history and mechanics of the 1,200-kilometer-long fault. Now, two new studies suggest that Cascadia is more complex, but also perhaps has a pattern of movement that could help planners and emergency management personnel better prepare for the next large earthquake.

A low-budget coring platform (plywood bridging two canoes with additional buoyancy provided by 55-gallon drums) was the setting for retrieval of multiple 6-meter-long cores from the bottom of Bradley Lake on the southern Oregon coastal plain. The cores revealed a sedimentary record of multiple tsunamis that have have hit the Oregon coast over the last 4,600 years, generated by earthquakes on the Cascadia subduction zone. Courtesy of Alan R. Nelson.

When studying Cascadia, Harvey Kelsey, a geologist at Humboldt State University in California, and colleagues began with Bradley Lake, a small lake about 500 meters inland on the southern Oregon coast. They chose Bradley, after a year-long reconnaissance of lakes from Washington to California, because for the past 7,000 years, it has recorded Cascadia-generated tsunamis, but not farther-traveling ones, as the lake is too high and too far inland. What they found there were two major observations: “One is that the entire margin of Cascadia does not move during an earthquake, and second is that tsunamis occur in a cluster-gap-cluster pattern,” Kelsey says.

Reporting in the July/August Geological Society of America Bulletin, they determined that tsunamis entered the lake on average once every 390 years, whereas thelocal record of Cascadia earthquakes near Bradley shows an interval of one every 500 years. “If the entire Cascadia margin moved, then we would see a one-to-one match,” Kelsey says.

The research team also assembled a detailed record of when tsunamis hit Bradley. The data reveal that tsunamis occur in 250- to 400-year clusters followed by gaps of 700 to 1,300 years, with the earthquake in 1700 possibly beginning a new cluster of quakes. On Jan. 26, 1700, a magnitude-9.0 earthquake ruptured the entire length of the subduction zone and generated tsunami deposits from California to Canada (see story, above). This “megathrust” event was the last major movement on the Cascadia fault system.

“Their work is meticulous. They have put together an excellent standard for dating along the Oregon coast,” says Brian Atwater, a U.S. Geological Survey (USGS) geologist at the University of Washington who specializes in Cascadia tsunami research. Previously, geologists thought that the 1700 event, which broke along the entire margin, represented the “norm,” Atwater says, but Kelsey’s team’s work “brings back to light earlier observations that Cascadia has a variable rupture mode. This is a really important finding.”

In another study in the July/August Geological Society of America Bulletin, a team led by Andrea Hawkes, a graduate student at University of Pennsylvania, proposed a potential method to give advance warning of large earthquakes in the Cascadia zone. This study is the first to look at two widely spaced megathrust events.

The team analyzed fossils of microorganisms that are highly sensitive to changes in water chemistry and thus record minor changes in land level. These fossils are proxies for small-scale land sinking or uplift produced by smaller earthquakes that appear to precede large-scale movements, such as on the Cascadia fault. Although the two sites differ in sedimentary records, both clearly record a distinct sequence of precursor events — subsidence, deposition of tsunami-generated sediments and uplift — that occur three to 10 years prior to a megathrust movement along the Cascadia fault. “The most beneficial aspect of our study is that it gives modelers better variables with fewer errors,” Hawkes says. She is now looking at additional marshes along the Oregon coast to better refine her data.

“These precursors are interesting observations, but there is still a lot of uncertainty,” says Tom Brocher, co-project chief of the Pacific Northwest Earthquake Hazard Investigations for USGS. “They may be good alerts for long-range concerns, such as building safety, but not necessarily for a megathrust movement,” he says. “Our records are too short in time to know exactly what this data means.”

David B. Williams
Geotimes contributing writer

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