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New maps reveal southern extension of southern Whidbey Island fault


September 10, 2009

OLYMPIA – A team lead by Washington State Department of Natural Resources (DNR) geologists has released maps showing that the southern Whidbey Island fault extends much farther south and east than once thought and connects to the Rattlesnake Mountain fault zone. Although not as well known or studied as the Seattle fault, the southern Whidbey Island fault is considered capable of generating equally large earthquakes. The fault represents a potential seismic hazard to residents of the Puget Lowland. Cities at greatest risk are Everett, Seattle, Port Townsend, and even Victoria.

“These maps bring to light more knowledge that will help scientists and planners better understand and prepare for our state’s earthquake hazards,” said Dave Norman, State Geologist and Manager, DNR Geology and Earth Resources Division.

The team determined that the Rattlesnake Mountain fault zone, which was originally mapped by DNR’s Tim Walsh in 1984, extends from near North Bend and through the Snoqualmie River Valley in the vicinity of Fall City and Carnation. The new mapping shows that it is the continuation of the south Whidbey Island fault. In the area of North Bend, the fault zone is 4 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.

Mapping along the Snoqualmie River Valley helped answer another important question: where does the Seattle fault lie east of its last mapped position near Issaquah? The DNR team found that the Seattle fault does not extend east of the Rattlesnake Mountain fault zone near Fall City. Studies by state and federal geologists in the next few years will help determine the frequency and severity of earthquakes along these fault zones.


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