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Evidence Grows for Major Quake in Northwest

December 07, 1989|LEE DYE | TIMES SCIENCE WRITER

SAN FRANCISCO — New evidence reinforces a growing concern that the Pacific Northwest is likely to be hit by an earthquake that will be far more powerful than the region is prepared for, scientists reported Wednesday.

Teams of scientists from a wide range of institutions have collected evidence that indicates that major quakes killed giant trees along the Washington coast several hundred years ago and plunged some areas below sea level. Other research shows that strain accumulating along the Oregon coast is likely to lead to a major earthquake.

For the Record
Los Angeles Times Friday December 8, 1989 Home Edition Part A Page 3 Column 1 Metro Desk 2 inches; 40 words Type of Material: Correction
Northwest quake--A story Thursday on earthquake hazards in the Pacific Northwest incorrectly reported that the Pacific Plate is sliding beneath the state of Oregon. In fact, it is the smaller Juan De Fuca Plate that is subsiding beneath the North American Plate along the coast of Oregon.

But there have been so few quakes there in recent years that many people have been lulled into a false sense of security, scientists say, despite the fact that new evidence indicates that the Pacific Northwest may be in for a megaquake like the one that struck the coast of Chile in 1960. The Chilean temblor, with a magnitude of 9.5, was the most powerful in recorded history.

Yet "the state of preparedness is abysmal" in Oregon and Washington, said Craig Weaver of the U.S. Geological Survey.

Washington, which has experienced significant quakes in recent decades, is a little better prepared than Oregon, scientists told the American Geophysical Union here Wednesday, but both states have many buildings that scientists say would not fare well in a major earthquake.

Oregon has not had a major quake in more than 250 years, according to a study released Wednesday by UC Berkeley and the University of Oregon. Yet there is a "tremendous buildup of elastic strain" in the geological formations along the Oregon coast, the study says.

The report warned that cities such as Portland and Salem, Ore., are "severely unprepared" for the great quake that many scientists now believe has struck that area in the past and should be expected in the future.

The Pacific Northwest is a "subduction zone," meaning one of the giant tectonic plates that make up the Earth's outer crust is diving under another. As the Pacific Plate inches under the North American Plate, it creates the volcanoes that dot the region, including Mt. St. Helens, which erupted nearly a decade ago.

Until recently, scientists had thought that the Pacific Plate slid so smoothly beneath the North American Plate that the process did not produce giant earthquakes.

But scientists have discovered layered geological formations that suggest violent earthquakes in the past. The evidence consists mainly of layers of saltwater fossils interspersed with layers of plant fossils that could only have existed above sea level.

That suggests that the coast has dropped and then been pushed back up several feet over and over again.

The most plausible explanation for that, insisted U.S. Geological Survey geologist Brian Atwater, is a series of "very great earthquakes."

Other scientists reported additional evidence Wednesday.

David Yamaguchi, a forester with the University of Colorado's Institute of Arctic and Alpine Research, said he has found "strong evidence" of a sudden drop in a 60-mile-long stretch of coastline in southern Washington.

Yamaguchi said tree-ring samples from dozens of dead western red cedars show that the trees died suddenly in the late 1600s, most likely after their roots were submerged in saltwater when the land suddenly dropped.

In subsequent years, the land on which the dead trees stand was slowly pushed back up, and today, the trees dot the coastline like dead sentries from a violent epoch.

Other evidence gathered by the UC Berkeley-University of Oregon team points to "a tremendous buildup of elastic strain" along the coast of Oregon.

That strain "most likely gets released as a major earthquake off the Oregon coast every 300 to 400 years," the scientists reported.

They based that conclusion on the fact that coastal regions north and south of the town of Tillamook have risen an average of about a quarter-inch a year more than the town itself over the last 50 years. That tenfold difference means that the crust in the area is being deformed severely, and the most likely result will be a major earthquake in which the strain is released, according to the researchers.

"The chances are very high that a great earthquake will occur" in that area, said geologist Paul Vincent of the University of Oregon. "As to when, that's hard to predict."

But not everyone is convinced.

Lynn Sykes of the Lamont-Doherty Geological Observatory said he fears that his colleagues "have jumped the gun" in claiming that the evidence shows a history of giant quakes.

Sykes said the Pacific Plate is very young where it subducts beneath the North American Plate and it "deforms plastically rather than snagging and breaking" as it slips beneath the North American Plate, suggesting that the buildup of strain in that region is not sufficient to trigger a major quake.

Other scientists, however, insist that the similarities between the Pacific Northwest and other violent subduction zones are quite strong and thus the prospects for major quakes should be regarded as equally strong.

The Geological Survey's Atwater recently spent four months in Chile, studying the scars left by the 1960 quake, and he returned more convinced than ever that great quakes should be expected in the Pacific Northwest.

He said he found many remnants of the Chilean quake that are similar to what he has seen in Washington and Oregon, including layered beds that prove the area has repeatedly plunged downward and then been uplifted.

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