Large Quake in Northwest Likely, Researchers Believe : Science: Evidence shows coastline racked by high-magnitude temblors during the past 7,000 years.

A tidal wave swamped an Indian fishing camp on Oregon’s coast 300 years ago, the first direct evidence early Northwest natives were dislocated and perhaps drowned during a huge earthquake, scientists said.

Researchers believe that a similar earthquake could happen again, and “people living on the Oregon and Washington coast would be subject to severe shaking and tsunamis causing more than a few deaths,” said Wendy Grant, a Seattle-based U.S. Geological Survey geophysicist.

Previous studies show that the Washington and Oregon coast may have been rocked by as many as 13 gargantuan quakes measuring 7 to 9 in magnitude during the past 7,000 years.

Some generated tsunamis, or big waves, as tall as 40 feet.

Scientists increasingly believe that such a disaster will occur again, but they don’t know if it will be soon or in hundreds of years. The quakes happened at highly irregular intervals.

Numerous studies indicate that the most recent huge quake had a magnitude of 8.5 to 9 or more and savagely shook the southern Washington and northern Oregon coast between 1695 and 1710, Grant said during the American Geophysical Union’s recent meeting.

Grant, archeologist Rick Minor and Geological Survey scientists Brian Atwater and Alan Nelson found evidence that Tillamook Indians established fishing camps near the ocean on the Salmon and Nehalem rivers, and that a quake-generated tsunami wiped out the Salmon River camp.

“They had a settlement there. It got inundated and they never went back,” Minor said by phone from Heritage Research Associates in Eugene, Ore.

The researchers don’t know whether the camp was occupied when the wave hit. Miner said 10 to 50 people usually lived at such camps. “If they were living there at the time of the earthquake, they were probably swept away by the tsunami,” Grant said.

Canoeing along the eroded river bank at low tide, the scientists discovered three fire pits at the Salmon River camp buried by 4 inches of sand deposited by the tsunami. A fire pit on the Nehalem was farther upstream from the ocean, and wasn’t buried by sand.

All along the Northwest coast, estuary sediments buried by sand have provided evidence of tsunamis generated by great quakes.

The scientists also found an 8-by-10-inch piece of a cedar bark basket on the Nehalem. It was covered by 1 or 2 inches of silt that Grant thinks was deposited by intense shaking.

Giant plates that make up the Earth’s crust collide along the Oregon-Washington coast. The plate under the ocean is diving beneath the mainland plate in a process called subduction. As stress builds toward a big quake, the coastline gradually rises upward. When a quake finally relieves the stress, the coast suddenly subsides three or more feet and a tsunami can rush inland.

The fire pits and basket fragment “provide the first direct evidence that earthquake-induced subsidence significantly affected . . . peoples living in coastal areas of the Pacific Northwest,” Grant and Minor’s study concluded.

The “evidence is overwhelming that sites Indians had been using were made unusable by tsunami and subsidence,” Atwater said. “You can deduce people might have been killed.”

Large areas of the Oregon, Washington and northernmost California coast are now rising, according to studies presented by scientists from the University of Oregon and Oregon State University.

Lawrence Young, of NASA’s Jet Propulsion Laboratory in Pasadena, said that U.S. and Canadian scientists are using Global Positioning System satellites to measure how quickly strain is building up as plates collide.