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California's shift north producing string of faults in Washington state

WASHINGTON—Over the years Californians have flocked to Washington state, driving up property prices and adding to sprawl and traffic congestion. Now, geologists say the entire state of California is moving north—at a quarter-inch a year.

The result is a series of earthquake faults in western Washington that scientists are increasingly starting to understand. Aiding that understanding is the use of sophisticated laser range finders, which map the ground by shooting 70,000 pulses a second from a small plane equipped with a global positioning system.

Among the latest discoveries is a fault near Bellingham, near the Canadian border, that may have been more active than originally thought. At least three shallow earthquakes along the Boulder Creek Fault, 20 miles northeast of Bellingham near the community of Kendall, apparently broke the surface, producing a 10-foot high, three-mile-long ridge known as a scarp.

Rather than being inactive for 30 million years, as had been previously thought, scientists now believe that the fault produced the three earthquakes roughly 7,700, 3,000 and 1,000 years ago. No one knows how large the quakes were or when there might be another.

But the fault could produce an earthquake between 6.5 and 7.0 on the Richter scale, which could cause structural damage in Bellingham and Vancouver, B.C., and be felt as far away as Seattle, said Brian Sherrod, a geologist with the U.S. Geological Survey in Seattle.

The Boulder Creek Fault is the farthest north of the handful of "crustal" faults discovered over recent years. It suggests that Washington state is being squeezed as California grinds north. The compression extends almost to the Canadian border, said Ralph Haugerud, another USGS geologist in Seattle.

"Our perception over the past 10 years has changed, and we believe northwest Washington is a more dangerous place for earthquakes than we thought," Haugerud said.

Geologically, Washington state is being buffeted by a number of forces, including the movement of tectonic plates, on which the Earth's crust rides.

Off the Washington coast, the Juan de Fuca Plate is colliding with the North American Plate, gradually pushing Neah Bay near the tip of the Olympic Peninsula northeast toward Vancouver, B.C., Haugerud said. The collision of these plates off the coast eventually could produce a major earthquake and a tsunami that could inundate coastal Washington and Oregon.

"We can see it being spring-loaded," Haugerud said of the growing pressure building up along the Juan de Fuca and North American plates. He said it's impossible to tell when that pressure might release.

Off California, the Pacific Plate is colliding with the North American Plate, pushing the state of California north. As California moves north, Oregon apparently is along for the ride. There are no signs that Oregon is being compressed by the movement like Washington state.

Haugerud said Vancouver, Wash., along the Oregon-Washington border near Portland, is moving north toward Vancouver, B.C., at the rate of 2 { inches a decade.

The compression of Washington state has produced a string of earthquake faults, including ones near Tacoma, Olympia, Seattle, Whidbey Island, Mount Vernon and now as far north as Bellingham.

"The hazards have not changed in the past 100 years—our perception has," said Haugerud. "This is earthquake country."

Sherrod and Haugerud are among the dozen or so USGS geologists studying the earthquake faults of western Washington.

The newly named Kendall Scarp over the Boulder Creek Fault near Bellingham was first noticed when the Nooksack Tribe used a laser range finder to search for abandoned river channels that could provide salmon spawning beds.

Previously, aerial photography had been used as the foundation for the detailed topographic maps that a geologist might use to detect scarps. But cameras can't see through trees, and the use of the laser range finders has made it possible to better detect geographic features.

Prior to 1996, there was only one known earthquake fault scarp in western Washington, near Lake Cushman. Now, using the laser range finder, more than a dozen have been detected.

"It's revolutionized everything we have done," Sherrod said. "We wouldn't have discovered the Kendall Scarp without the lasers."

Scarps are usually associated with shallow earthquakes, about five miles or so underground.

"Even a medium-sized quake near the surface can be dangerous," Haugerud said.

Scarps also can be indications of recent earthquake activity, at least in geologic time.

The glaciers that covered the Northwest 15,000 years ago wiped the landscape smooth, so the existence of a scarp would indicate earthquake activity since then, said Elizabeth Barnett, a geologist who works with the USGS and has studied the Kendall Scarp.

A swarm of small earthquakes near Kendall several years ago and in the early 1990s intrigued scientists, but it wasn't until they saw the mapping from the laser range finder that they suspected that the Boulder Creek Fault was active, Sherrod said.

Barnett said it's possible that such faults as Boulder Creek may have been reactivated as a result of the pressure the northern movement of California is putting on Washington state.

Sherrod said geologists are still in what he called the "reconnaissance stage," and it may be too early to draw conclusions.

"That's one hypothesis," he said. "It seems to work well, but it may not be as simple as that."

Barnett said that by studying the soils in a scarp, geologists can estimate when earthquakes occurred. Though the Kendall Scarp is the only one found so far in far northwest Washington state, Barnett said, "it would seem strange if it was there all by its lonesome."

Geologists also are looking at the possibility of faults north of Bellingham along the coast, Sherrod said.

Haugerud and Barnett will present results of their research on the Boulder Creek Fault and the Kendall Scarp at the annual meeting of the Cordilleran Section of the Geological Society of America at Western Washington University in Bellingham on May 5.


(c) 2007, McClatchy-Tribune Information Services.

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