New Earthquake Maps Show Two More Faults in Western Washington

April 25, 2008

Scientists are discussing two more ways Western Washington could shake, rattle and roll.

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 newly added 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 newly added 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 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.

An earthquake of 3.5 on the Richter scale can cause slight damage in a local area, 4 can cause moderate damage, 5 considerable damage, and 6 can be severe. A 7 reading is a “major” quake, capable of widespread heavy damage, and 8 is a “great” quake, capable of tremendous damage.

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 Woodinville and possibly southeast 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 the other possibility of a series of smaller, but still major, quakes.

“The primary constraint on our Cascadia earthquake model is that great earthquakes occur on average once every 500 years,” cautioned Arthur D. Frankel and Mark D. Petersen, the scientists who wrote that section of the report. That makes it difficult to make predictions concerning quakes in that zone.

The last great Cascadia rupture is believed to have occurred in January 1700, based on analysis of tsunami records in Japan, trees along the Pacific coast and a study of tsunami deposits and other geophysical data, the report said.

USGS last published nationwide hazard maps in 2002 and 1996. The new maps are being released at the same time as a national revision of the model building codes used by state and local government so that buildings, bridges, highways and utilities can resist earthquake damage.

The National Seismic Hazard Maps are a series of maps and data sets that project the ground shaking that might happen at points throughout the country.

Insurance companies use the data to set rates in some places. Engineers use the maps to forecast landslides and the stability of hillsides. Federal environmental regulators use the maps to ensure waste-treatment facilities will hold up. Emergency planners use the information to decide how to allocate money for education and preparedness.

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