November 25, 1995 in City

Two New Studies Support Theories On Past, Future Quakes Research Focuses On 750 Miles Of Pacific Northwest Coast

Associated Press

Two new studies lend further support to the belief that the coast of the Pacific Northwest has been hit by major earthquakes in the past and faces more of the same in the future.

A study in Thursday’s issue of Nature suggests that a quake as big as magnitude 9 - or a series of magnitude 8 quakes - probably occurred early in the 1700s and affected nearly 600 miles of coastline between British Columbia and Northern California.

The other study, in the November issue of The Journal of Geophysical Research, indicates that a geologic “on-off switch” is at work off Oregon and Washington and could result in a devastating quake.These two reports build on the work of previous studies on the Cascadia Subduction Zone.

The zone is where the eastward-moving Juan de Fuca Plate is slowly moving beneath the North American Plate from about 60 to 150 miles offshore. The zone runs about 750 miles from Vancouver Island in British Columbia to Eureka, Calif.

If the plates stick as they slide past each other, stresses build up that eventually are released by an earthquake.

In the Nature article, Brian F. Atwater and Alan R. Nelson of the U.S. Geological Survey and their colleagues used radiocarbon-dating techniques to determine when 85 samples of trees and small plants were killed, apparently when earthquakes caused coastal marshes to drop and be flooded with sea water.

The vegetation samples were taken from nine sites along the Northwest coast, including four in Oregon and three in Washington.”Our dating strengthens the possibility that one plate-boundary earthquake ruptured most of the length of the Cascadia Subduction Zone in the early 1700s,” the scientists wrote in their Nature article. “Such a rupture would have had an area larger than 50,000 square kilometers (more than 19,000 square miles).”

The new research is consistent with previous studies that say the last major quake happened about 300 years ago, Nelson said. But, he added, the dating was more accurate in this study.

Chris Goldfinger, a marine geologist at Oregon State University, remained skeptical that Cascadia zone quakes could reach magnitude 9. He and another researcher published a study earlier this year saying a series of faults off the Oregon coast could act as a shock absorber, limiting the largest quakes to about magnitude 8.

In addition, Goldfinger said some geophysical evidence, based on known earthquakes in other locations around the Pacific, indicates the Cascadia zone is too narrow to rupture all at once along a length as great as the Nature article implies. Limiting the length of a rupture limits the size of the quake.

And, he noted, some of the vegetation samples were from areas that have fault structures closer to the earth’s surface. Evidence shows quakes in such shallow structures also can cause submerged marshes, he said.

In the study published in The Journal of Geophysical Research, scientists said they had developed a theory about why the world’s largest earthquakes occur in some places where the Earth’s crustal plates collide but not in others.

They found that when an oceanic plate - such as the Juan de Fuca Plate - meets another plate and bends down into the relatively soft mantle below, it does so at a shallow angle if the two plates are moving toward each other. If the plates are moving in the same direction, however, the oceanic plate will dive below the continental plate at a steep angle.

Shallow plate dips, such as in the Cascadia Subduction Zone, intensify earthquake-causing friction where the plates rub together. Deep dips produce much less friction and quakes do not occur, according to Christopher H. Scholz, a geophysicist at Lamont-Doherty, and Jaime Campos, a seismologist at the University of Chile.

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