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Geologic detectives uncover coast's violent past

The Oregonian - Science News

10/08/03

By RICHARD L. HILL of the Oregonian, reprinted with permission.

Geologic detectives uncover coast's violent past

A former University of Oregon graduate student's research team finds traces of huge quakes and tsunamis

The Southern Oregon coast has been jolted repeatedly by powerful tsunami-generating earthquakes, according to evidence three scientists have found buried in the Coquille River estuary.

Their study reveals that 12 offshore earthquakes have struck the coast in the past 6,700 years -- all but one generating tsunamis that raced at least six miles up the Coquille River at Bandon.

Most of the events not only affected Southern Oregon but also hammered the entire Northwest coast with huge magnitude 9 earthquakes, the most recent about 300 years ago.

"This study provides the longest onshore record of earthquakes in the Northwest," said Rob Witter, who led the research while working on his doctorate at the University of Oregon.

Another significant finding is that a few of the earthquakes might not have ruptured the entire fault that sits off the Northwest coast, said Witter, now a geologist with William Lettis & Associates in Walnut Creek, Calif. That suggests that smaller, but still powerful, magnitude 8 earthquakes occasionally strike only portions of the coast.

The research is reported in the October issue of the Geological Society of America Bulletin. The other scientists involved were Harvey Kelsey, an adjunct geology professor and researcher at Humboldt State University in Arcata, Calif., and Eileen Hemphill-Haley, a micropaleontologist with EHH Consulting Micropaleontology in McKinleyville, Calif.

Earthquakes centuries apart Radiocarbon dates of organic material from marsh deposits preserved beneath the Coquille River estuary found that the 12 earthquakes occurred on average every 570 to 590 years. However, the intervals between earthquakes were irregular -- as short as a few hundred years and as long as more than 1,000 years -- making it difficult to determine when a quake might occur.

The study supports previous research that earthquakes and tsunamis from the Cascadia Subduction Zone have repeatedly rocked the Northwest coast from Vancouver Island to Northern California. Scientists say such events undoubtedly will strike again, affecting some of the more than 11 million people living west of the Cascades.

For the past two decades, scientists have scrutinized the 600-mile-long subduction zone. Running roughly parallel to the coast, the zone is where the heavier Juan de Fuca Plate plunges beneath the North American Plate as they collide from 60 to 150 miles offshore. This plate boundary is one of North America's largest active faults.

Evidence indicates that the two tectonic plates repeatedly lock up as they grind past each other, resulting in tremendous strain that is unleashed in magnitude 8 and 9 earthquakes. Similar subduction zones have produced the planet's two largest recorded earthquakes: a magnitude 9.5 quake on the coast of Chile in 1960 and a magnitude 9.2 quake in southern Alaska in 1964.

As the plates shove against each other, the upper North American Plate near the coast slowly is being pushed upward and eastward. When the accumulated pressure is released in a powerful earthquake, the coastline snaps back -- plunging by as much as 6 feet.

The so-called "megathrust" subduction-zone quakes, which can last for three to five minutes, have not occurred in Oregon's brief recorded history. Evidence from other studies also has shown abruptly buried coastal marshes and forests along the coast, signs of such quakes. Tsunami records in Japan, tree-ring dating and other evidence indicate that the last such quake was a magnitude 9 in January 1700.

Evidence in estuary Witter and his colleagues found repeated evidence of subduction-zone quakes and tsunamis -- including the 1700 event -- in the Coquille River estuary.

"The estuary is a good geologic tape recorder of the vertical land-level changes that occur when earthquakes happen on the Cascadia Subduction Zone," Witter said. "Estuaries provide a protective environment that can preserve a sedimentary record of the earthquake, and it's also subject to the tsunami. The Coquille estuary seems to be in just the right spot for recording these events."

Each earthquake can be identified in layers of sediment that can be seen in core samples taken as deep as 23 feet below the surface.

When a subduction-zone earthquake occurs, the tidal marsh plunges below sea level and is instantly submerged by ocean water. Within minutes, a tsunami -- a series of quake-generated waves -- sweeps a layer of sand over the marsh. Afterward, fine-grained muds are deposited on top of the sand layer. Then another marsh slowly begins to develop in the decades before the next earthquake.

The scientists can see each earthquake cycle in a layer-cake sequence of buried soil from the core samples: a layer of dark peat, which once was the surface of a marsh; a layer of sand from the tsunami; and a layer of mud from tidal deposits.

"We see this sequence repeatedly -- 12 times -- over a span of 20 feet in the core," Witter said. "It's a wonderful geologic record."

Hemphill-Haley examined the fossil diatoms, microscopic aquatic algae, in the buried soil samples to evaluate sea-level and environmental changes that the earthquakes caused.

Radiocarbon dating of seeds, twigs and other organic fragments in the soil provide a time range for when the earthquakes occurred.

Similar studies along coast Several of the earthquake dates "reasonably correlate" with the age estimates of earthquakes from similar studies from Willapa Bay in southwest Washington to the Northern California coast, Witter said. "So that agrees with the idea that the entire subduction zone ruptures."

This isn't the first time the three scientists have found evidence of subduction-zone quakes on the Southern Oregon coast. Early last year, they reported in the Geological Society of America Bulletin they had evidence of 11 large tsunami-producing earthquakes in the past 5,500 years in the Sixes River estuary near Cape Blanco, about 20 miles south of Bandon. The study also found evidence that some earthquakes did not rupture the entire subduction-zone boundary.

Only one other study has produced a lengthier subduction-zone earthquake record. Marine geologists Chris Goldfinger of Oregon State University and Hans Nelson of Texas A&M University found that 18 earthquakes -- all estimated at magnitude 9 -- have ripped the Northwest coast in the past 10,000 years. They investigated turbidites, which are deposits of mud and sand from earthquake-generated landslides, on the ocean floor.

Brian Atwater, a geologist with the U.S. Geological Survey at the University of Washington, called the Coquille estuary study "a benchmark" that will add to the knowledge of discussions about the sizes and impacts of Cascadia earthquakes.

Richard L. Hill: 503-221-8238; richardhill@news.oregonian.com




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