Seismic waves to understand iceberg formation
Scientists at the IPGP have analyzed the earthquakes caused when icebergs detach from a glacier. For the first time, they have identified the different sources of these earthquakes.
Assessing and characterising the forces involved in the detachment of icebergs from a glacier is crucial to understanding the dynamic processes at work and quantifying the loss of mass from polar ice cap glaciers. By combining the analysis of seismic data with filmed images of iceberg detachment, a team of researchers from IPGP, ENSAM (CNRS) and ETH Zurich are revealing the sources and mechanisms behind seismicity and demonstrating the potential of seismology for studying the dynamics of polar ice caps.
The increase in the Earth’s global temperature, and in particular that of the oceans, is a direct threat to the polar ice caps. This is demonstrated by the accelerated thinning and retreat of coastal glaciers, particularly in Greenland. This retreat is accompanied by the disintegration of floating ice tongues and the detachment (calving) of colossal icebergs, the rate of which has increased considerably over the last ten years. Knowledge of the processes and rate of calving is particularly important for quantifying their contribution to rising sea levels.
Iceberg calving generates so-called glacial earthquakes, with magnitudes of between 3 and 5. These icebergs have a considerable volume, up to several tens of millions of cubic metres. Because of their particular geometry, they are in gravitational imbalance and, once detached from the front of the glacier and floating in the surrounding water, they slowly turn over.
During this long process (5 to 10 minutes), they rub and eventually compress the terminus (the point where the glacier meets the sea). In their study, the scientists focus on the calving event of 21 August 2009. They calculate the induced force by inverting the long-period seismic signals from five stations in the Greenland GLISN network.
Variations in the amplitude of the source, its duration and its frequency distribution have made it possible to identify four forces at the origin of the seismic signal. The first is generated by an avalanche of ice debris along the calving front, triggered by the detachment of the first iceberg and the initiation of its rotation. The second and third parts of the seismic signal come from the contact forces applied to the terminus by the bottom-out calving (the roof of the iceberg moves towards the glacier) of the first iceberg, and by the top-out calving (the roof of the iceberg moves towards the sea) of a second iceberg, three times smaller. Finally, the researchers identified forces acting throughout the calving sequence, interpreted as friction and shear forces on the walls of the terminus and the fjord, induced by the acceleration of the floating mixture of ice debris initiated by the rotation of the icebergs.
This study reveals the complexity of the force behind glacial earthquakes induced by iceberg calving. These results will enable researchers to study similar events for which there are no visual observations. Obtaining a precise history of the force is an important advance that will enable us to constrain mechanical and dynamic models of the calving phenomenon, in order to characterise iceberg volumes and estimate fluctuations in the mass of the polar ice caps.
Ref: Sergeant, A., A. Mangeney, E. Stutzmann, J.-P. Montagner, F. Walter, L. Moretti, and O. Castelnau (2016),Complex force history of a calving-generated glacial earthquake derived from broadband seismic inversion,Geophys. Res. Lett. 43, doi:10.1002/2015GL066785.
