Hydrology, Cryosphere & Earth Surface News

The Permafrost Listeners

Geophysicists have discovered a way to monitor permafrost thaw by measuring seismic waves so gentle they don’t shake a thing.

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Stephanie James did not need to see the ice buried beneath her feet to know it was there. She just had to look at the plants that grew around her—and, according to new research, listen for seismic waves passing through the ground.

James was in Bonanza Creek Experimental Forest, about 35 kilometers southwest of Fairbanks, Alaska. She was there to study permafrost—permanently frozen soil that stands to become not so permanent as global warming continues. She hiked through the forest and saw tall white birches and white spruces. Then the scene morphed into a forest of short black spruces that she said looked scraggly.

“The trees get thinner and thinner, and shorter and shorter,” James said. “And then you start to see more and more dead trees.”

Tall birches and spruces grow where there is no permafrost, whereas shorter spruces grow where permafrost still permeates the ground. The dead trees James saw became waterlogged after the permafrost beneath them thawed.

Specialists call a place like the dead forest James hiked through a “collapse scar bog” because as the forest dies and the permafrost thaws, the area collapses into a wetland covered with moss (a bog). Boardwalks cross James’s bog site, which is part of the Bonanza Creek Long Term Ecological Research program.

James is a hydrogeophysicist with the U.S. Geological Survey, and she led a team that designed a first-of-its-kind method allowing scientists to measure seismic waves in such a way that they can chart the distribution of permafrost thaw without ever lifting a shovel.

They just have to put their ears to the ground.

Permafrost Listening

James’s “ears” are seismometers that she and her team put into the ground. The seismometers can detect ambient seismic waves made by the rustling of leaves in trees when the wind blows, by a car driving down a road a few kilometers away, and by ocean waves crashing at the edge of a continent. “You don’t even feel that it’s happening right now,” James said.

These ambient waves, the same kinds of waves triggered by an earthquake, travel at different speeds depending on what kind of material they pass through. They travel through solids like ice faster than they travel through liquids like water.

Map with different color shades—dark reds to light yellows and white. Darker shades are places underground where seismic waves move relatively slowly compared with lighter colored regions.
A map of seismic wave velocity changes that shows areas of greatest velocity reduction (darker shades) aligning with areas of deepest permafrost thaw. Credit: Stephanie James

The team’s seismometers, James explained, can detect this difference in wave speed, and researchers can use the data to map the distribution of icy permafrost.

“We can interrogate the ground repeatedly, day after day, and see the subtle changes as the ground thaws and refreezes,” James said.

And that is what James and her team did. They listened for the seasonal expansion and contraction of the permafrost from September 2013 to June 2015, proving that their method, described in the Journal of Geophysical Research: Earth Surface in June, can work. The research was funded through a grant awarded to Robert E. Abbott, one of the paper’s coauthors, by Sandia National Laboratories.

James explained that using ambient seismic waves is a step toward being able to calculate the levels of greenhouse gases scientists can expect thawing permafrost to emit.

“It’s been difficult to get a handle on actual numbers,” she said.

But now that researchers can more precisely measure the volume of permafrost thaw from season to season, getting a handle on actual numbers can happen—and it’s what James wants to do next at the Bonanza Creek bog site.

“Stephanie did a great job,” said Jonathan Ajo-Franklin, an applied geophysicist at Rice University in Texas who was not involved in the study.

What began at one site in Alaska should now, he explained, be expanded to a global scale. “That’s where we’re going to go next,” he said.

—Lucas Joel, Freelance Journalist

Citation: Joel, L. (2019), The permafrost listeners, Eos, 100, https://doi.org/10.1029/2019EO130483. Published on 07 August 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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