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Present-day vertical deformation of the Cascadia Margin, Pacific Northwest, United States


  • Clifton E. Mitchell,

  • Paul Vincent,

  • Ray J. Weldon II,

  • Mark A. Richards


We estimate present-day uplift rates along the Cascadia Subduction Zone in California, Oregon, and Washington in the Pacific Northwest, United States, by utilizing repeated leveling surveys and tide gauge records. These two independent data sets give similar profiles for latitudinal variation of contemporary uplift rates along the coast. Uplift rates are extended inland through east-west leveling lines that connect the north-south line along the coast to the north-south line along the inland valleys just west of the Cascades. The results are summarized as a contour map of present-day uplift rates for the western Pacific Northwest. We find that rates of present-day uplift vary latitudinally along the coast and inland valleys, as well as longitudinally along transects connecting the coast to the inland valleys. Long-term tidal records of Neah Bay, Astoria, and Crescent City indicate uplift of land relative to sea level of 1.6±0.2, 0.0±0.2, and 0.9±0.2 mm/yr, respectively (±1 standard error). Unlike previous estimates of relative sea level change at Astoria, we adjust for discharge effects of the Columbia River, including human management influences. After approximating an absolute framework by using 1.8±0.1 mm/yr to compensate for global sea level rise, results indicate that much of the western Pacific Northwest is rising at rates between 0 and 5 mm/yr. The most rapid uplift rates are near the coast, particularly near the Olympic Peninsula, the mouth of the Columbia River, Cape Blanco, and Cape Mendocino. Two axes of uplift are identified: one trends northeast from the southwest Oregon coast, and the other trends south-southeasterly from the Olympic Peninsula to the Columbia River. The Puget Sound vicinity and a small east-west region from the north central Oregon coast to the inland Willamette Valley are subsiding at rates up to 1 mm/yr. We interpret the overall pattern of rapid present-day uplift to be generated by interseismic strain accumulation in the subduction zone. This interseismic elastic strain accumulation implies significant seismic hazard.

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