Using Seismic Waves to Measure Ice Melt? Sounds Good

A recent study in the journal Science Advances proposes a novel methodology to track melting ice sheets and the glaciers associated with them: rather than viewing the ice from above with airplanes and satellites, a team from MIT and Princeton is monitoring it from below. The new technique makes it possible to gather information about ice melt in real time by listening to the seismic activity of the Earth’s crust. Due to the continuous sound of ocean waves crashing on Greenland’s shore, there are near-constant seismic vibrations in the bedrock that can reveal a great deal of information about the overlaying ice. This method, which was originally developed to track volcanic and fault line activity, may be able to provide more  accurate data on exactly where and when melting is occurring, the authors report.

The Incorporated Research Institutions for Seismology IRIS) install a seismic station in Southwestern Greenland. Photo provided by Dr. Chris Harig
The Incorporated Research Institutions for Seismology IRIS) install a seismic station in Southwestern Greenland. Photo provided by Dr. Chris Harig

How can seismic data communicate information about glaciers? The researchers predicted that the great mass of the ice weighing down on the rock below would compress the Earth’s crust and change its density—possibly enough to have a measurable effect on the seismic waves passing through it. By listening to the speed of the seismic waves moving through the ground, the team was able to determine the density of the rock and calculate the amount of ice lying above. According to the study, the speed of the seismic waves depends on the crust’s porosity, or the amount of small spaces and cracks that are not solid rock. When the crust is compressed by heavy ice masses, the area of open spaces in the rock decrease and waves travel more quickly through the material.

However, when ice melts and there is less weight on the bedrock, more spaces in the crust open up and the velocity of the seismic waves is significantly slower. This newly tested method shows immense promise, and incorporating seismic data from other Greenland stations is the next step.

We think if the seismic station density were increased we would be able to observe these changes in greater spatial detail, and be able to make a map of the changes instead of averaging them over a large region,” study author Dr. Chris Harig of Princeton University explained in an email to GlacierHub.

In addition to calculating the amount of ice melt, this new method may be able to pinpoint the location of the melting. While findings are preliminary, the study indicates that the seismic data from 2013 picked up differences in melting between the main Greenland ice sheet and the Jakobshavn Glacier, widely considered the fastest moving glacier in the world.

Satellite image of the Jakobshavn Glacier. NASA/USGS image courtesy of the Science Visualization Studio, at Goddard Space Flight Center
Satellite image of the Jakobshavn Glacier. NASA/USGS image courtesy of the Science Visualization Studio, at Goddard Space Flight Center

If you do look at the station pairs individually, the stations near Jakobshavn Glacier show a bit more signal in 2013 than the rest. This could be due to the fact that Jakobshavn is a place of massive changes, and still had large changes in 2013,” Harig commented in an email.

This outlet glacier has shown significant melting since the 2013 data was collected—in 2015, a massive 12.4 square kilometer area of ice calved into the ocean, possibly the largest calving event in the glacier’s history. If seismic activity can pick up on the different rates of melting between Greenland’s glaciers and the main ice sheet, it may be possible to predict which glaciers are most fragile and likely to have calving events.

While testing is needed at more seismic stations, Harig seems optimistic about the potential applications of the new method.

I was surprised how well the results turned out in the end. We are measuring very small changes in the seismic velocity to compare them to the ice sheet mass. So it attests to the high data quality from these stations, how well the processing techniques worked, and the very large signal we have from the Greenland ice sheet as it gains and losses ice,” he said.

The new technique may be able to fill the gap that remote monitoring methods cannot: measuring ice melt on small, short-term time scales. The researchers state that monitoring methods like NASA’S Gravity Recovery and Climate Experiment satellites (GRACE) have collected valuable data on the long-term changes in ice sheet and glacier changes, but the resolution is not high enough to pick up on shorter inter-seasonal shifts in ice melt. On a seasonal scale, seismic waves may be better equipped to measure the melting ice, and the method introduced the exciting possibility of measuring melt in real time— impossible with the current monitoring mechanisms used today. By combining the wide scope of satellite data with the precision of this new seismic methodology, sea level rise projections will be more accurate and allow the global community to better adapt to the impacts of a warming climate.

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