Crustal uplift and sea level rise in northern Cascadia from GPS, absolute gravity, and tide gauge data

Abstract

We combine data from nine GPS, absolute gravity, and tide gauge stations to estimate the relation between sea‐level rise, vertical motion, and solid Earth processes in the Pacific Northwest. GPS vertical velocities (in ITRF2000) and absolute gravity rates are well correlated, with a gradient of 0.2 ± 0.1 μGal mm−1, but show a significant offset of 0.53 ± 0.30 μGal yr−1 (2.2 ± 1.3 mm yr−1) (95% confidence). Tide gauge and GPS data indicate a northeast Pacific regional sea‐level rise of 1.7 ± 0.5 mm yr−1, aligned to ITRF2000, or an unlikely regional sea‐level fall of −0.5 ± 0.5 mm yr−1, aligned to absolute gravity. Although we cannot rule out a bias in the GPS reference‐frame alignment, our results suggest a possible absolute gravity bias by a long‐period mass increase from an unknown near‐surface or deep‐seated source. The impact of such a mass increase on gravity, vertical motion, and sea level remains to be defined.