Glacial earthquakes in Greenland trace the history of polar cap mass loss
Rising temperatures in recent decades have accelerated the melting of the polar ice caps. By discharging ice in the form of meltwater or icebergs into the ocean, coastal glaciers in the Arctic and Antarctic are contributing to rising sea levels.
Quantifying the distribution of mass loss from polar glaciers is necessary in order to understand whether the decrease in ice volume is the result of changes in atmospheric or oceanic conditions and how these impact glacier dynamics. By combining seismological observations and mechanical analyses, an international team of professors and researchers from laboratories at the IPGP, INSIS (Centre des Matériaux and PIMM laboratory), ETH Zurich and Swansea University has been able to estimate the loss of ice from the Greenland ice sheet linked to the detachment of icebergs of several million cubic metres and associated with the production of teleseisms.
Since the 2000s, Greenland has seen an acceleration in the rate of flow, thinning and retreat of its coastal glaciers, accompanied by numerous calvings of icebergs of up to several billion cubic metres. In certain circumstances, the glacier may release gravitationally unstable icebergs into the sea. The imbalance between the forces of gravity and Archimedes causes these icebergs to tip slowly under the influence of hydrodynamic forces.
The action of the iceberg turning against the front of the glacier generates a horizontal contact force on the glacier for several minutes, responsible for seismic waves propagating through the solid Earth. These iceberg calvings produce magnitude 5 earthquakes that are detected by global seismological networks.
The inversion of seismic waveforms recorded at stations in Greenland (www.glisn.info) makes it possible to trace the history of the force at source and capture the tilting dynamics. In their study, the researchers used a mechanical iceberg calving model to calculate this contact force, which they then compared with the seismic observations in order to constrain the volume of ice associated with each earthquake.
Between 1993 and 2013, 400 to 500 earthquakes originating from around ten Greenland glaciers were detected. Although satellite images can be used to measure glacier migration, it is difficult to separate the different components of mass loss (melting of the ice due to higher ocean temperatures or production of individual icebergs). Seismograms can be used to quantify the volume of icebergs, revealing that these earthquakes were produced by the calving of icebergs with volumes of between 0.2 and 2.8 km³. In total, such events were responsible for discharging at least 370 Gt of water into the Arctic and Atlantic oceans. The study reveals major contributors such as the Kangerdluqssuaq and Helheim glaciers in the south-east and the Jakobshavn Isbrae in the west. Between them, they produce 70% of the volume of ice lost over a 20-year period, the dynamics of which are tracked using seismic wave records. The study shows that the amount of discharge appears to be declining in East Greenland, while it has accelerated sharply and is spreading towards the glaciers of West Greenland which, prior to the 2010s, produced few earthquakes. Data obtained from seismic recordings have made it possible to attribute between 8% and 21% of the mass loss measured to the calving of such icebergs. While this percentage seems to be holding steady or even decreasing in the stabilised glaciers of eastern Greenland, it is exploding in western Greenland, at a time when these same glaciers began a retreat cycle in the 2000s that continues to this day. The researchers show that, in general, the western glaciers have tended to release larger icebergs since 2010, notably in connection with the acceleration of flow speeds and the specific positions of the ice fronts in their fjords.
Ref:
- news on the CNRS website
- Sergeant A., Mangeney A., Yastrebov V., Walter F. Montagner J.-P., Stutzmann E., Castelnau O., Bonnet P., Ralaiasoroa J.-L., Bevan S. and Luckman A., 2019: Monitoring Greenland ice-sheet buoyancy-driven calving discharge using glacial earthquakes. Annals of Glaciology: Progress in Cryoseismology – doi.org/10.1017/aog.2019.7
