Using the Earth’s noise to see beneath the Greenland ice sheet

The team studied Rayleigh waves -- seismic waves generated by movements such as earthquakes -- to produce high-resolution images of the rocks underneath the ice sheet, helping to identify which areas are most susceptible to faster ice flow.

The Greenland Ice Sheet is the second largest reservoir of freshwater on Earth. However, the rate of loss of ice mass from the ice sheet has increased six-fold since 1991, which accounts for around 10% of the recent rise in global sea levels.

The geological conditions in the ground beneath an ice sheet or glacier play a key role in determining ice flow. Key factors include: the make-up of the layers of rock; the temperature of the earth's crust beneath; and the amount of water present in liquid form between rock and ice as this acts as a lubricant, causing so-called "basal slip" and speeding up the flow of ice.

The problem, however, is how to assess what is happening deep underground, given the remoteness of Greenland and the fact that the ground is covered with ice approximately 2.5 kilometres thick.

In 2009, a permanent network of seismic monitoring stations was installed across Greenland, which have been used in previous research. However, these studies have offered limited insight on the geological controls on the ice sheet.

This is where the new research comes in. The team were able to map out what is happening down as far as 5 kilometres by measuring Rayleigh waves extracted from the Earth's noise. These seismic waves travel along the Earth's surface and are sensitive to variations in Earth's properties. .

By measuring the speed, shape and duration of the waves, researchers are able to work out what material they are travelling through: the mechanical properties of the rocks, such as rigidity and density; the layering of the rocks and the physical properties of the surface soil.

Rayleigh waves travel in an elliptical pattern and the specific feature that the researchers assessed was the horizontal to vertical ratio of particle motion within the waves

They found:

• Regions of high geothermal heat concurrent with the proposed historical location of the Iceland hotspot track
• Soft sedimentary substrates beneath major fast flowing outlet glaciers, revealed by lower wave speeds
• Some outlet glaciers are particularly susceptible to basal slip, including Jakobshavn, Helheim and Kangerdlussuaq
• Geothermal warming and softening of basal ice may affect the onset of faster ice flow at Petermann Glacier and the Northeast Greenland Ice Stream.