As climate change raises temperatures across the Arctic, the seismic waves traveling through permafrost in Svalbard, Norway, are slowing down.
By Lauren Koenig, Ph.D. Candidate, Michigan State University (@Lauren_A_Koenig)
Citation: Koenig, L., 2021, Seismic waves reveal the Arctic is on shaky ground, Temblor, http://doi.org/10.32858/temblor.192
Halfway between Norway and the North Pole, the archipelago of Svalbard oscillates each year between the midnight sun and the polar night, when the darkness of winter is broken only by the moon and the Northern Lights. On the surface, these extreme cycles are tracked by reindeer, polar bears, and about 2,400 fulltime residents. But just outside the town of Longyearbyen, the northernmost settlement in the world, the seasonal rhythms that play out underground are recorded by a cluster of machines tuned to vibrations in the Earth.
These seismographs were deployed by scientists at NORSAR (or NORwegian Seismic Array), an independent research foundation that listens for earthquakes and possible nuclear explosions as part of the Comprehensive Nuclear Test Ban Treaty. Now, a study published in Seismological Research Letters shows that seismic monitoring can also be used to observe stark warming trends in the Arctic. The study’s lead author, Julie Albaric, an environmental seismologist currently affiliated with the University Bourgogne Franche-Comté, and her team at NORSAR found that seismic waves beneath Svalbard appear to be slowing down, likely due to permafrost thawing.
Permafrost instability is a major threat to the Arctic ecosystem, but it also has enormous consequences for climate change worldwide. Coming on the heels of a series of extreme weather events like the catastrophic flooding in Europe and the deadly heat wave in the U.S. Pacific Northwest, this study shows that seismic monitoring is another method that may improve current perceptions of how global warming will continue to affect the Earth.
Probing the state of permafrost
Permafrost is ground that remains frozen for at least two years, although the uppermost layer often thaws temporarily when summer brings in warmer air. Albaric and her colleagues found that seasonal freezes in shallow permafrost are reflected in the ebb and flow of seismic waves. The waves travel quickly through ice and harder substances and slow down as they pass through liquid and softer materials.
This makes seismology “a very useful tool for probing the state of permafrost,” says Steven Gibbons, a geoscientist who studies natural hazards at the Norwegian Geotechnical Institute and who formerly worked with Albaric at NORSAR. “The speed at which seismic waves travel through permafrost is far more sensitive to temperature than it is in solid rocks.”
Seismic arrays in the Adventdalen Valley
Eavesdropping on the North Pole’s seismic pulse is logistically challenging and expensive. Fortunately, the NORSAR team was able to take advantage of two networks of seismometers: the SEISVAL and SPITS seismic arrays. The temporary SEISVAL array was deployed by Albaric’s team in 2014; SPITS was built in 1992 as the most northerly array for nuclear explosion test ban monitoring. Both arrays are installed in the Adventdalen Valley, a wide-open moonscape rimmed by glaciers and rugged mountains.
According to Gibbons, the same features designed to fulfill SPITS’ original purpose also enable it to be valuable for monitoring the permafrost. High-tech seismometers like these are often placed far apart to get largescale coverage of earthquakes. But in this case, each array involves a close configuration of seismometers designed to improve the signal-to-noise ratio. Multiple seismometers are tuned to the same frequency so that scientists can measure how high-frequency seismic waves are moving relative to one another and detect from which direction they come. The absence of noise from wind or people allows the sensors to pick out weak, regional seismic signals from the general thrum of activity beneath the Earth.
Concerning seismic signals
Albaric and her colleagues were able to describe the ambient seismic wavefield in the region by analyzing both tectonic and cryogenic waves — the latter of which is seismic activity from sudden cracking in frozen soil or rock caused by freezing water, often referred to as “ice quakes” — recorded from 2007 to 2014. These recordings picked up more than 350,000 microseismic events, most of which seem caused by ice quake activity and glacial movement. During summers, high-frequency (greater than 2 Hertz) waves rise throughout Svalbard as glaciers crack, causing meltwater to accumulate and basal sliding to occur. During the winters, lower-frequency waves predominate as groundwater remains frozen, and seismic noise from the northern Atlantic Ocean rings out instead.
More concerning were data from 2009 through 2011 collected in a borehole near SPITS showing that these relative seismic velocity changes are decreasing. At the same time, ground temperatures at 2 to 4 meters depth have creeped slowly, but steadily upward. The researchers interpreted these linear trends as a sign that more of the permafrost layer is thawing, which has serious implications for global greenhouse gas emissions. As dead plant matter in the permafrost thaws, it is decomposed by newly unfrozen microbes, releasing carbon and methane into the atmosphere. There is more than twice the amount of carbon locked in permafrost soil than in the entire atmosphere, so these emissions would greatly speed up the feedback cycle by which even deeper layers of permafrost warm up in turn. Permafrost warming due to rising air temperatures has already been documented around the world.
“That ‘the Arctic is like the canary in the coal mine’ has long been understood,” says Gibbons. “Changes observed in the poles may be signals of greater changes coming to the regions we inhabit.”
Keeping an ear to the ground
Climate scientists use many tools to monitor permafrost, but passive seismic monitoring — the technique by which ambient seismic energy can be measured using seismic arrays — has distinct advantages. “Seismic methods are very sensitive and they allow us to target different depths, cover large areas and extend very localized observations provided by boreholes,” says Albaric. “Compared with other geophysical methods, the data are continuous, which is necessary for this type of long-term analysis.”
Thinking ahead, Albaric would like to see seismic monitoring methods expanded to visualize permafrost structure. She says comparing seismic measurements in the field with laboratory seismic measurements on core samples could also provide insight.
“If historical seismic data going back several decades can provide an indication of how the permafrost may have changed in this time perspective, it may help to fill some gaps where we lack data from direct field measurements,” says Gibbons.
With changes in the Arctic affecting the earth, sea and air, seismic monitoring is poised to become one more weapon in the fight against climate change. For instance, NASA’s Soil Moisture Active Passive is an orbiting satellite that measures the amount of frozen water in the uppermost layer of soil, letting scientists know where and how fast the permafrost is thawing from the perspective of space.
“The more multidisciplinary monitoring becomes, the more likely we are to see and understand the big picture,” says Gibbons.
References
Albaric, J., et al. (2021) Seismic Monitoring of Permafrost in Svalbard, Arctic Norway. Seismological Research Letters. doi.org/10.1785/0220200470.
Brouillette, M. (2021). How microbes in permafrost could trigger a massive carbon bomb. Nature News. doi.org/10.1038/d41586-021-00659-y.
Fountain, H. & Schwartz, J. (2021). ‘It Is All Connected’: Extreme Weather in the Age of Climate Change. Retrieved from https://www.nytimes.com/2021/07/16/climate/europe-floods-climate-change.html.
Rost, S., & Thomas, C. (2002). Array seismology: Methods and applications. Reviews of Geophysics, 40(3), 2-1. doi.org/10.1029/2000RG000100.
