New Washington quake faults found

SEATTLE – The U.S. Geological Survey’s new seismic hazard maps, released this week, show two more earthquake faults in Western Washington: one near the Canadian border, the other east of Port Angeles.

The new maps also contain some good news for Washington residents. Scientists now estimate that potential ground motion in the Western United States is 30 percent lower than they previously thought for the kind of quakes caused by long-period seismic waves that would affect taller, multistory buildings.

Scientists developed these new estimates by using new ground-motion predicting models created after looking at shaking records from 173 global shallow crustal earthquakes to better understand what is happening in the western U.S.

They believe one of the new faults, called the Boulder Creek fault and located near Bellingham and the Canadian border, is capable of a magnitude-6.8 earthquake and has been active over the past several thousand years. Residents of Canada are in more danger from this fault than people who live in Washington state.

The other new fault, the Lake Creek-Boundary Creek fault, is east of Port Angeles along the Strait of Juan de Fuca and Olympic National Park. It is capable of producing a magnitude-7.4 earthquake and has also been active over the past several thousand years.

Washington and Oregon have about 100 known faults. More than 1,000 earthquakes occur in Washington each year.

The February 2001 Nisqually earthquake, which had an epicenter northeast of Olympia, was a magnitude-6.8 quake.

On the Richter scale, every increase of one number means a tenfold increase in magnitude. Thus a reading of 7.5 reflects an earthquake 10 times stronger than one of 6.5.

The report also contains new information that a fault south of Whidbey Island is longer than previously thought, extending through Seattle’s northern suburbs at least as far as Woodinville and possibly to North Bend. The fault has the highest hazard level of any fault in Western Washington and could produce a magnitude-7.5 earthquake. There have been at least four quakes along this fault in the past 16,000 years.

Scientists now believe the Cascadia Subduction Zone, which runs off the shore of northern California, Oregon and Washington, is more likely to experience one great quake that will completely rupture the fault, rather than a series of smaller, but still major, quakes.