Vertical Velocities, Glacial Isostatic Adjustment, and Earth Structure of Northern and Western Alaska Based on Repeat GPS Measurements

Abstract

Glacial isostatic adjustment (GIA), resulting from the Pleistocene loading of the Laurentide and Cordilleran ice sheets, is frequently associated with positive vertical velocities, or uplift. In Northern and Western Alaska, thousands of kilometers from the center of these ice sheets, vertical motion is primarily negative, or subsidence. Previously, no regional Earth structure model has been estimated for these areas using GIA modeling techniques, and the contribution of GIA processes to the observed subsidence signal has not been studied. We compare the vertical motion rates from 54 campaign and continuous GPS sites in Northern and Western Alaska to the predictions of the ICE-5G and ICE-6G GIA models, and to a suite of models that vary with four adjustable parameters defining the lithospheric thickness, asthenospheric thickness and viscosity, and upper mantle thickness and viscosity with the ICE-3G loading model of Tushingham and Peltier (1991, https://doi.org/10.1029/90JB01583). The best overall fit with the ICE-3G loading model and Earth model parameters were 120-km lithosphere over a 100-km asthenosphere with a viscosity of 2.5 × 1019 Pa/s, overlying a 450-km-thick upper mantle with a viscosity of 1.5 × 1021 Pa/s. These values are for a fixed lower mantle viscosity of 3 × 1021 Pa/s in a one-dimensional Earth model that uses a linear Maxwell rheology. The GIA estimates are found to fit the GPS observations well and can be used to more accurately interpolate between measurement sites in a region where there is sparse spatial and temporal coverage of tectonic vertical velocities.