Monitoring shear-zone weakening in East Antarctic outlet glaciers through differential InSAR measurements

Abstract

The stability of the Antarctic Ice Sheet depends on ice flux into the ocean through major outlet glaciers, which is resisted by shear stresses in the lateral shear margins, both on grounded ice and on floating ice shelves. Within the tidal-flexure zone, where the ice sheet transitions from fully grounded to freely floating, ocean tides lead to a characteristic flexural pattern, which can be detected by radar satellites in differential interferograms. Here, we investigate how spatially heterogeneous elastic ice-shelf properties in the shear zones affect tidal flexure and whether a corresponding signature can be detected in satellite observations. We use the Young’s modulus (which, among others, depends on ice temperature and/or ice-crystal orientation fabric and damage) as a bulk tuning variable for changing ice stiffness across shear zones and show that this leads to centimeter-scale deviations in vertical displacement, compared with a homogeneous elastic flexure model. Using the tidal-flexure zone of Priestley Glacier as an example, we compare homogeneous and heterogeneous flexure-model predictions with observations from 31 differential interferograms. After adjusting the local tide model and validating it with in situ GPS data, we find that a 5-fold reduction of the Young’s modulus in the shear zone, i.e., an effective shear-zone weakening, reduces the root-mean-square error of predicted and observed vertical displacement by 33 % within the central part of the ice shelf. This suggests that satellite interferometry can detect changing ice stiffness across shear zones, with the potential to inform ice-flow models about the often unknown spatial variability in ice-shelf properties along the grounding zone.