Ancient Quakes Signal Future Northwest Risk
Monastersky, Richard, Science News
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. …