A GNSS velocity field for crustal deformation studies: The influence of glacial isostatic adjustment on plate motion models

Abstract

The two main causes of global-scale secular deformation of the Earth are tectonic plate motion and Glacial Isostatic Adjustment (GIA). We create a bespoke global 3D GNSS surface velocity field “NCL20” to investigate tectonic plate motion and the effect of GIA on plate motion models (PMMs), drawing on a set of 1D and 3D GIA model predictions. The main motivation for creating NCL20 is to include a larger number of GNSS sites in the most GIA-affected areas of investigation, namely North America, Europe, and Antarctica. We do this using the IGS repro2 data and other similarly processed GNSS datasets. Our final GNSS velocity field has horizontal uncertainties mostly within ±0.5 mm/yr and vertical uncertainties mostly within ±1 mm/yr (at 95% confidence), which make it suitable for testing GIA models. We generate a suite of 117 global GIA model predictions by combining three different ice history models (ICE-5G, ICE-6G and W12) with a range of 1D and 3D Earth models. By subtracting this ensemble from the GNSS velocity field, we identify and compare a range of PMMs which are expected to be unaffected by GIA. Our method allows us to include GNSS sites that are typically excluded from PMM estimations due to their location in GIA-affected regions. We demonstrate that significant GIA-related horizontal motion outside of the rapidly-uplifting regions may bias PMMs if left uncorrected. Based on their ability to explain the observed surface velocity field, a group of best-performing GIA models is selected for three regions of interest: North America, Europe, and Antarctica. The range of GIA predictions in each best-performing group is assumed to represent the uncertainty in regional GIA modelling insofar as it can be constrained by present-day geodetic velocities. In the horizontal component, we note that 3D GIA models show more variation in the direction of predicted velocities than 1D GIA models, confirming that horizontal velocities are strongly sensitive to lateral variations in Earth structure. Furthermore, for Antarctica the variation in predicted GIA vertical velocities suggests that the total GIA contribution to annual gravimetric mass change ranges from -3 Gt/yr to 22 Gt/yr depending on which of the best-performing GIA models is used.