OREGON STATE UNIVERSITY

DROWNED RIVER VALLEY GIVES CLUES TO OFFSHORE QUAKES

05/08/1996

CORVALLIS - A recently discovered fault near the Oregon coast southwest of Newport could produce an earthquake comparable in size to the magnitude 6.7 quake that hit Northridge, Calif., in 1994, scientists say.

The discovery of this fault, which also is a "blind thrust" fault like the one that caused major damage in the 1994 California earthquake, illustrates the variety of seismic risks Oregon may face - quite separate from the major "subduction zone" earthquake that occurs every 300 years or so.

An analysis of the fault was given recently by Oregon State University geologists at the 92nd annual meeting of the Cordilleran Section of the Geological Society of America in Portland, Ore.

Advances in the combined use of sonar and submarine research are providing a new and improved picture of offshore Oregon, researchers say.

"Only a small fraction of the Oregon offshore has been mapped using sonar," said Robert Yeats, a professor of geology at OSU. "In the last few years we have found really exciting landscapes - sea cliffs, bays, sand bars, all covered by hundreds of feet of water.

"Each time we go out, we view terrain never before seen by human beings," Yeats said.

The discovery of this new fault was made on the Stonewall Bank, an important fishing ground 18 miles southwest of Newport. As scientists were scanning the sea floor with sonar, a river channel suddenly moved across the screen - a channel now covered with more than 200 feet of water.

"It looked so much like a river channel on land that we had the feeling of being up in a balloon in eastern Oregon," Yeats said. "But it was night and we were on board an oceanographic vessel off the coast of central Oregon."

The following day, researchers visited the stream channel with a two-person submersible, the Delta, mapping the channel banks and crawling across the floor of the channel itself. They found it completely covered with fine-grained mud deposited in the past 12,000 years.

The channel had been carved during an Ice Age when the Oregon coastline was about 25 miles west of its present position near Newport.

The biggest surprise, however, came when the researchers further examined their data at OSU laboratories, Yeats said. They found that the river channel was actually sloping backwards towards its ancestral source, the Yaquina River.

"Water runs downhill, so you would expect the channel to slope to the west," Yeats said. "The backward tilt means the river channel has been warped by a buried earthquake fault on the continental shelf."

Faults such as this, because they can be so much closed to shore than the subduction zone, pose a special earthquake threat to coastal communities, Yeats said.

If the Stonewall Bank fault, which scientists determined to be about 15 miles long, were to rupture all at once it could produce a quake comparable to the Northridge earthquake, Yeats said. Damage would include ground shaking and a tsunami threat.

The National Oceanic and Atmospheric Administration, which funded this research, is committed to using this type of specialized undersea technology to better assess earthquake risks in the Pacific Northwest, Yeats said.

"Sonar mapping, followed by detailed examination from a submarine, can put the geologist in the field where answers can be obtained," Yeats said, "even if in this case, the field is hundreds of feet below sea level."

Offshore research will continue this July, he said.