GPS Vertical Land Motion Corrections to Sea‐Level Rise Estimates in the Pacific Northwest

Abstract We construct coastal Pacific Northwest profiles of vertical land motion (VLM) known to bias long‐term tide‐gauge measurements of sea‐level rise (SLR) and use them to estimate absolute sea‐level rise with respect to Earth's center of mass. Multidecade GPS measurements at 47 coastal stations along the Cascadia subduction zone show VLM varies regionally but smoothly along the Pacific coast and inland Puget Sound with rates ranging from + 4.9 to −1.2 mm/yr. Puget Sound VLM is characterized by uniform subsidence at relatively slow rates of −0.1 to −0.3 mm/yr. Uplift rates of 4.5 mm/yr persist along the western Olympic Peninsula of northwestern Washington State and decrease southward becoming nearly 0 mm/yr south of central coastal Washington through Cape Blanco, Oregon. South of Cape Blanco, uplift increases to 1 − 2 mm/yr, peaks at 4 mm/yr near Crescent City, California, and returns to zero at Cape Mendocino, California. Using various stochastic noise models, we estimate long‐term ( ∼ 50   − 100 yr) relative sea‐level rise rates at 18 coastal Cascadia tide gauges and correct them for VLM. Uncorrected SLR rates are scattered, ranging between −2 mm/yr and + 5 mm/yr with mean 0.52   ±   1.59 mm/yr, whereas correcting for VLM increases the mean value to 1.99 mm/yr and reduces the uncertainty to ±   1.18 mm/yr, commensurate with, but approximately 17% higher than, twentieth century global mean.