Characterizing Active Crustal Deformation in the Pacific Northwest: Implications for Seismic Hazards and Landscape Evolution

Scott Bennett


Date & Time
Building 3, Rambo Auditorium
Tom Brocher

Convergent tectonic plate boundaries host elevated seismic hazard and spectacular topographic relief. Detailed characterization of earthquake sources (e.g. faults) provide critical insight into the spatial variability of contemporary seismic hazards, while mapping their long-term (i.e. geologic) history can improve our understanding of how topography is generated and evolves over time.

In the Pacific Northwest of the United States, active crustal deformation (faulting, folding, block rotation) above the Cascadia subduction zone is driven by oblique plate convergence (Juan de Fuca-North America) and clockwise rotation of the overriding North American plate. Upper-plate rotation results in distributed transtension in western Idaho and eastern Oregon, which transitions to distributed transpression in northwest Oregon and throughout much of Washington State and southwest British Columbia (Canada). In Washington, transpressional deformation occurs from the Cascadia fore arc to its back arc, at least 500 km from the trench. Unresolved questions persist about how regional strain is distributed over, and transmitted across, a network of low strain rate structures in the upper plate.

To improve our understanding of structural linkages across the Pacific Northwest, we interrogate the latest Pleistocene to Holocene record of crustal deformation along several faults and folds in the Olympic Mountains, Puget Lowland, and Yakima folds province. We employ paleoseismologic, neotectonic, and marine geophysical techniques. Our observations allow us to model the timing of surface-rupturing Holocene paleo-earthquakes, quantify co-seismic folding and uplift, and estimate fault slip rates. We also examine the tectonic geomorphology of structures to characterize their recent growth and understand how local topographic relief is generated. Collectively, these results will aid seismic hazard analyses related to major population centers and critical infrastructure and improve our understanding of the evolving Pacific Northwest landscape over space and time.

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