Western Washington fault may run through Cascades

An earthquake fault previously believed to be limited to an area south of Whidbey Island actually stretches 250 to 300 miles, from Victoria, B.C., to Yakima, crossing the Cascade Mountains and capable of producing a major earthquake, new research shows.

Many of the other faults in Western Washington could be connected to the South Whidbey Island Fault in a network similar to the San Andreas Fault system in California, Craig Weaver, the regional earthquake coordinator for the U.S. Geological Survey based in Seattle, said in an interview Wednesday.

Suzette Kimball, the USGS acting director, told Congress on Thursday that there was "strong evidence" other faults in Western Washington were connected to the South Whidbey fault. "It appears there is a very large (fault) system in the Cascade arc," she told the House interior appropriations subcommittee.

Weaver said scientists are trying to determine whether the South Whidbey Island Fault extends as far east as the Hanford Nuclear Reservation and if it could also be connected to the highly unstable Cascadia subduction zone off the coast.

"This is big stuff," said Weaver, adding that the South Whidbey fault was "most dangerous. A lot of people are looking over our shoulder."

The fault could be capable of producing a maximum earthquake registering 7.5 on the Richter scale, which is used to measure the strength of earthquakes, he said. An earthquake that size is capable of causing serious damage over large areas.

A 7.5 earthquake would be the largest in the state's recorded history.

In spring 1949, a 7.0 quake rocked the Olympia region, damaging nearly all the large buildings in the area and causing eight deaths. During a magnitude-7.3 earthquake in the Strait of Georgia off British Columbia in June 1946, the seabed sunk nearly 85 feet in some places.

Scientists believe a 9.0 quake rocked the Cascadia subduction zone, where two tectonic plates collide just off the West Coast, on Jan. 26, 1700, producing a giant tsunami that hit Japan. A similar quake, which could produce a tsunami of 60 to 90 feet that would hit the northwest coast in 15 minutes or less, is considered likely, though scientists aren't sure when.

The USGS, along with state and local agencies, is preparing a major earthquake preparedness drill in Pierce County in the coming months. Scientists are working to develop various disaster scenarios based on the latest discoveries, Kimball told the House panel.

In Eastern Washington, Kimball said, scientists would be looking for major faults, starting with studies of the greater Yakima area and the Hanford Nuclear Reservation.

"This is a very serious discovery," said Rep. Norm Dicks, D-Belfair, chairman of the subcommittee, which oversees funding for the USGS and other Interior Department agencies.

Scientists conducting aero-magnetic surveys discovered the South Whidbey Island Fault extended much further than thought. The equipment can detect magnetic fields in rocks. Eventually, scientists on the ground will dig trenches along the fault in an effort to learn more.

Weaver, in a telephone interview, said one of the most surprising things about the discovery is finding that a major fault crossed the Cascade Mountains in something now dubbed "trans-Cascade tectonics." Most faults run parallel to the Cascades, he said.

The South Whidbey fault could connect to offshore faults, Weaver said. In addition, the Seattle Fault, which is found mostly under Seattle, is now thought to hook around the south end of the Olympic Mountains and extend out to the coast, he said.

The goal now is to assemble a comprehensive map of the state's earthquake faults that could be better used to understand the hazards, Weaver said.

Over the past decade, Weaver said, scientists have discovered a dozen faults in Western Washington.

"We previously thought they were small, unconnected faults," he said. "Now we are sketching out connections."

Les Blumenthal: 202-383-0008

lblumenthal@mcclatchydc.com