RUI: Collaborative Research: Quaternary Deformation in the Cascades Foothills and Southern Puget Lowland, Washington

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9627856 Umhoefer The Puget Lowland, Washington, is in the fore-arc region of the Cascadia convergent margin, where the Juanda Fuca plate is obliquely converging beneath the North American plate. Associated deformation occurs along the subducting plate boundary or Benioff zone and also in a wide, complex region of the upper (North American) plate extending across Northern California, Oregon, Washington and Southwestern British Columbia. Several models have been proposed to explain Quaternary deformation in the Cascadia margin, but present field data is inadequate to evaluate these models. This project will examine how slip on an active fault in the vicinity of Seattle continues to the South. The work is a pilot project to determine if surface exposures and seismic data will be adequate to test a model that suggests that the Seattle fault is part of an integrated, regional thrust system under the entire Puget-Lowland. If successful, the results will provide a much needed model framework for understanding this transtensional tectonic environment, which will also provide a focus for risk assessment.