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Pacific Northwest Geologic Mapping and Urban Hazards

Fault zones and basins in the Urban Corridor

Fault zones and basins in the Urban Corridor

Beneath the Puget-Willamette Lowland, a system of crustal faults and intervening basins extends from the Canadian border to Corvallis, Oregon. Some of the faults are active and present a hazard to the urban corridor and its critical lifelines. These faults are associated with historical seismicity and appear to accommodate northward migration of the Cascadia forearc at about 1 cm/yr with respect to North America. In cooperation with the Earthquake and Geologic Hazards Teams, we are doing geologic mapping and geophysical surveys to image these largely concealed faults and basins and determine their seismic potential.
Major faults in Pacific NW urban corridor Seattle area faults Portland area faults
Major faults in the Pacific Northwest urban corridor.

Puget Lowland

The lowland is subdivided by fault-bounded uplifts into several basins. Faults underlie the cities of Seattle, Tacoma, Olympia, and occur near by Everett, Victoria, and Bellingham. New geologic evidence indicates that some faults are active. The Seattle fault, an E-W-trending thrust fault crossing central Puget Sound beneath Seattle, has produced several large, ground-rupturing earthquakes in the last few thousand years.

Northwest Oregon

Between the Oregon Coast Range and the foothills of the Cascades, a system of NW-trending, late Cenozoic faults underlies the northern Willamette basin and the cities of the greater Portland metropolitan area. These faults are associated with diffuse seismicity and include among others, the Portland Hills fault zone beneath Portland, the Gales Creek fault forming eastern margin of the Coast Range, and the Mount Angel fault zone beneath the Northern Willamette Valley - possibly the source of the damaging M 5.6 Scott's Mills earthquake in 1993.

 

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