Earthquakes in ice could be used to reduce uncertainty in sea-level rise projections.

Researchers at the Department of Earth Sciences, together with an international group of collaborators, have found a way to use earthquakes generated by the sudden release of strain at or near the base of an ice sheet to directly observe both friction and slip at the bed of an Antarctic glacier for the first time – here on named icequakes. Their results have been published in Nature Geoscience.

The world’s ice sheets contain enough ice to cause tens of metres of sea-level rise. One of the most important unknowns for these systems is how strong the friction is at the ice-bed interface, and how that controls the rate of slip of ice into the oceans, causing sea-level rise. Currently, the ice-bed conditions in numerical models used to calculate sea-level rise projections are informed by sparse observations. In order to improve the accuracy of sea-level rise projections going forward, these models require improved observational constraint of ice-bed friction to determine the speed at which ice can slide into the oceans.

Ice-bed interface for a glacier in Svalbard – Phot by Tom Hudson

Using seismic data from 100,000 icequakes, the study found that friction at the glacier bed varies by several orders of magnitude, both in space and time. The friction is controlled by the pressure at the bed, which can vary from 10 kPa up to a maximum of 10 MPa (this is equal to the weight of the entire ice column above it and is a natural limit). This is in contrast to the assumption commonly made by numerical models of sea-level rise that friction is typically constant and homogeneous. This variation is related to the presence of a more complex bed under the ice sheet and the presence of higher than expected stresses.

Lead author, Dr Tom Hudson said ‘A particularly surprising result from the study is that stresses at the bed can be so high that they suggest a total absence of water at certain parts of the bed. This is so surprising since the bed is > 1.5 km below sea-level and glaciologists current understanding of Antarctic ice streams assumes the presence of water which would result in lower stresses at the base of the ice sheet.’ Such high variation in stresses can result in significant variations in friction and slip at the bed, something that is currently not accounted for in the ice dynamics models used to estimate sea-level rise projections.

The results from this study have implications for how current observations are used to inform ice dynamics models. Tom says ‘Our new findings show that icequakes can provide the critical observations required to constrain the highly variable friction at the bed of an Antarctic ice stream. Applying such observational constraint to ice dynamics models would reduce uncertainty in corresponding sea-level rise projections.’ Going forward, icequakes will allow glacier bed properties to be measured elsewhere across the world’s ice sheets in Antarctica and Greenland, as most fast-moving glaciers likely generate quakes.

This study also involved researchers from the British Antarctic Survey, Karlsruhe Institute of Technology, Pennsylvania State University and the University of Swansea.

The study ‘Highly variable friction and slip observed at Antarctic ice stream bed’ is published in Nature Geoscience and the paper is available here.

A research briefing that makes the complexities of the research more accessible is available here: https://rdcu.be/deXaV

For media inquiries contact Dr Tom Hudson, Earth Sciences Department, University of Oxford: thomas.hudson@earth.ox.ac.uk.