Ancient quakes signal future Northwest risk.

Geologists have discovered evidence that at least three large earthquakes have rocked the coast of northern California in the past two millennia, bolstering the theory that massive tremors may lurk somewhere in the near or distant future for a large stretch of the Pacific Northwest coast.

"Most people now acknowledge that a preponderance of the evidence supports the idea of huge earthquakes periodically affecting the Pacific Northwest," says Gary A. Carver of Humboldt State University in Arcata, Calif. He and Samuel H. Clarke Jr. of the U.S. Geological Survey in Menlo Park, Calif., present their findings in the Jan. 10 SCIENCE.

The coastline between northern California and southern British Columbia lies along a subduction zone -- a place where large pieces of the Pacific ocean floor have crashed against North America and are sliding into Earth's interior. At similar subduction zones elsewhere in the world, seafloor-continent collisions have spawned the largest known earthquakes: Alaska in 1964 and Chile in 1960.

Some scientists have argued that such superquakes might not occur along the Pacific Northwest's subduction zone. But four years ago, geologists uncovered evidence of extreme prehistoric shocks in Washington state (SN: 7/18/87, p.42; 2/17/90, p.104). Clarke and Carver have now extended the record by showing that large earthquakes have hit the southern end of the subduction zone.

Whereas critical faults in the northern end of the subduction zone lie far off-shore, the southern end curves toward shore, providing researchers with an opportunity to study such faults on land. Using the carbon-14 dating technique, Clarke and Carver found evidence of three earthquakes on such faults within the last 1,700 years.

In nearby locations, they discovered other signs that ancient earthquakes had altered the land surface. One beach had been pushed upward repeatedly, while another area had quickly dropped below sea level several times in the last few thousand years. The most recent quake occurred 300 years ago, they report.

The carbon-14 dates indicate that earthquakes may have rattled the north and the south at about the same time, suggesting that the entire subduction zone can slip at once, generating great quakes comparable to the magnitude 9.2 Alaskan shock of 1964, say the researchers. In another scenario, the southern end might move on its own, spawning a magnitude 8.4 quake followed by a similarly large quake in the north. In any case, says Carver, buildings in the Pacific Northwest were not built to survive a severe shaking, and the region is generally unprepared for a major quake.

Carver, Clarke and their colleagues who study other parts of the subduction zone say they have no idea when the next quake might strike. It could be hundreds of years from now, they note.