Newly Discovered Fault Could Shake Washington with 7.5 Quake
Laura Geggel
September 23, 2009
By Laura Geggel
SnoValley Star
Geologists are hoping a newly discovered fault line between North Bend and Whidbey Island doesn’t shake things up any time soon.
Geologists have known about the Rattlesnake Mountain fault zone since 1984, but they did not know it extended west, connecting with the southern Whidbey Island fault.
A team led by Washington’s Department of Natural Resources geologists recently published maps showing the newly surveyed fault, which is capable of producing earthquakes as high as 7 to 7.5 in magnitude, DNR geologist Tim Walsh said. Such an earthquake could affect cities as far away as Everett, Seattle, Port Townsend and Victoria.
The fault is also relatively shallow, meaning earthquakes could cause more damage. The 2001 Nisqually earthquake measured a 6.8 on the Richter magnitude scale and happened 36 miles below the earth’s surface. The southern Whidbey Island fault is only about 12 miles beneath the surface.
Information about fault lines affects building codes, especially for hospitals and schools. Walsh predicted the new information about the southern Whidbey Island fault would be incorporated into building codes in 2012 or 2013.
“It’s expensive to prepare a building for the increased seismic hazards because you have to make the buildings stronger and more flexible,” Walsh said. “It’s costly. It’s worth it, but you want to be careful to do it right.”
Until then, the geologists will map north of Rattlesnake Mountain, into Carnation and Duvall.
Walsh first mapped the Rattlesnake Mountain fault zone by accident in 1984. While working on a coal exploration project, he decided to survey land near the mountain because he had heard nearby Brewster Lake used to be called Coal Lake.
“It turned out it was just a log that was caught in a volcanic deposit, it wasn’t coal at all,” Walsh said. “In the process of doing that, I found there were faults cutting through Rattlesnake Mountain.”
He estimated the fault had moved sometime within the last 30 million years, but didn’t have time to study it more, given his boss’ orders.
“He gave me a stern lecture and said, ‘five minutes only for rocks that don’t burn,’” Walsh said.
The current effort is a more detailed mapping of the fault. A team from DNR’s Division of Geology and Earth Resources, headed by Joe Dragovich and assisted by geologists from King County, Pacific Northwest Seismic Network, Colorado College and Washington State University, spent three years mapping the area’s geologic features.
The group received funding from the federal U.S. Geological Survey and DNR.
Geologists found the Rattlesnake Mountain fault zone is four miles wide and consists of a series of parallel faults. These fault strands follow the valley edges and control the location of the Snoqualmie River along parts of the valley.
By studying the rock around the fault, geologists determined the fault is less than 13,000 years old, much younger than Walsh’s original estimation.
Age is an important factor — older fault lines are usually considered inactive. Geologists can tell if a fault line is younger than 13,000 years by determining if the line disrupts post-glacial deposits.
“If a fault cuts through a layer, it means it has to be younger than that layer,” Walsh said.
Radio carbon dating helps geologists nail down more precise dates for fault lines’ formation.
Evidence shows four large earthquakes have rocked the Pacific Northwest since the last glaciation, occurring at intervals of every 2,000 to 4,000 years.
“While that sounds like a long time, and it certainly is, the last one was about 2,300 years ago,” Walsh said.
Earthquake Safety
Several fault lines run throughout the Pacific Northwest, and Snoqualmie Fire Chief Bob Rowe follows their tremors at the Pacific Northwest Seismic Network’s Web site at www.pnsn.org.
People should have an emergency container full of supplies, such as non-perishable food, water, batteries, a radio and more, not just to prepare for earthquakes but also winter storms and volcano eruptions, Rowe said.
“In 2006, we went through that week-long power outage,” he said. “A lot of (people) dipped into those emergency containers because it was the wintertime.”
To learn more, go to www.ci.snoqualmie.wa.us and select “Emergency Management” under the “Departments” link, or call the Snoqualmie Fire Department at 888-1551.
Those who want to take a proactive step toward emergency preparedness can sign up for Community Emergency Response Team classes offered by the cities of Snoqualmie and North Bend. The classes run every Saturday from 9 a.m.-3 p.m. Oct. 17 to Nov. 14.
“They teach citizens how to care for their families and neighbors and how to be a leader in their community,” Rowe said.
The classes cost $25, but are refundable if the person passes the class. To register, e-mail eoc@ci.snoqualmie.wa.us.
Although earthquakes do have warning signs, Rowe said, people who take the right precautions should be a step ahead of the rest.
“As long as you make sure you have a good response plan and good prep, you should be able to survive it just fine,” Rowe said.
http://snovalleystar.com/2009/09/23/earthquake-fault-connects-north-bend-to-whidbey-island
www.standeyo.com/NEWS/09_Earth_Changes/090924.S.Whidbey.Fault.html