The melting and calving of glaciers generates inaudible noises. Using 18 years of infrasound measurements in Greenland, researchers from KNMI, TU Delft, and Japan discovered that glaciers have started to produce more noise in recent years.
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Infrasound
Infrasound is inaudible to humans. It is a type of air pressure with a vibration frequency lower than that of sound that humans can perceive.
Large moving volumes of air or moving surfaces are needed to produce infrasound. Think of exploding volcanoes, large explosions, violent storms, and aircraft breaching the sound barrier (sonic boom). Above-ground nuclear bomb tests are also known to produce infrasound.
Infrasound measurements are used worldwide to verify if countries adhere to the so-called 'Comprehensive Nuclear-Test-Ban Treaty.' Although the Greenland monitoring station near Qaanaaq is installed to monitor nuclear bomb tests, this study (recently published in Geophysical Research Letters) demonstrates the broader application of such measurements. Existing solutions for worldwide monitoring of nuclear bomb tests can be utilized to listen to glaciers.
Research into infrasound measurements indicates that glaciers around the aforementioned Qaanaaq station generate infrasound during the summer months, while the glaciers are quiet during the winter.
Monitoring station I18DK in northwestern Greenland. I18DK consists of 8 microbarometers to measure infrasound and determine its direction. The red lines point to the glaciers. - (Image Credit: KNMI / AGU)
By determining the direction from which the infrasound originates, its source can be traced. Every summer, there appeared to be glaciers that generate infrasound originating from a specific direction. Three glaciers some 30 kilometers away have been identified in addition to one near the station just 2 km away.
Differences between types of glaciers
The three glaciers south of the station terminate in the sea (calving glaciers), while the nearest glacier, immediately northeast of the station, has its tongue on land (terrestrial glacier). As a result, the noise peaks at different times in the melting season. For the glacier on land, this is mid-July. The more distant and calving glaciers make the most infrasound at the end of August. It is plausible that the peak in the first case is related to the maximum in the atmospheric temperature. The seawater temperature seems to be the controlling factor in the second case.
Infrasound due to calving ice and meltwater
The infrasound analysis was performed with a continuous measurement series of 18 years. During this period, the glaciers have started to generate more noise. The differences between years are significant, but an increase in the number of infrasound events can be seen. For the terrestrial glacier, this was compared with a model and measurements of meltwater.
The number of infrasound events for the Qaanaaq glacier, located 2 kilometers northeast of infrasound station I18DK. In blue the modeled discharge, in red the measured values. - (Image Credit: KNMI / AGU)
The model shows similarities with years of low and high infrasound events during the melting seasons.
Infrasound is generated by calving ice and meltwater that flows through all kinds of channels and cavities. The analysis can contribute to insight into the behavior of glaciers.
Sources and further reading:
Long-Term Infrasonic Monitoring of Land and Marine-Terminating Glaciers in Greenland (Geophysical Research Letters)
Glaciers in Greenland are starting to produce more noise (Universal-Sci)
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