It’s not quite like yodeling in the Swiss Alps, but researchers at Scripps Institution of Oceanography at UC San Diego recently discovered that glaciers in southern Switzerland can emit a harmonic “hum.”
In an effort to probe the inner dynamics of glaciers, researchers at Scripps and their colleagues analyzed data from a network of seismometers deployed at Gornersee, a lake that forms each spring at the confluence of two glaciers in the southern Swiss Alps. Scientists say Gornersee and similar glacier regions are tantalizing exploration targets because of their role as large storehouses for fresh water—several million gallons in Gornersee’s case—as well as the hazards they pose to nearby village populations due to unpredictable summer flooding episodes.
Examining water flow dynamics and other structural details inside and below glaciers is challenging. Drilling into ice is expensive and time consuming, so measurements made at the surface are the most practical.
“This can be tricky,” said Debi Kilb, a Scripps seismologist and coauthor of a new paper published in the journal Geology. “One analogy is trying to figure out if a watermelon is going to be juicy by looking at only a small portion of the watermelon’s outside skin.”
Thus surface seismometers and the signals they captured of Gornersee’s “icequakes” gave scientists an idea about ice cracking and water flow—key to understanding what’s happening within the glaciers—not unlike how a doctor employs a stethoscope to detect a heart murmur.
To their surprise, the researchers found that during a 21-hour period the icequakes at the glacier base gave off a signal with a frequency on par with humming, or what the scientists call a “gliding harmonic tremor.” (Hear the humming glaciers)
Such glacier humming had been found previously in ice systems at the polar regions, but the Geology paper describes a first for mountain glaciers in the Swiss Alps.
So what causes the curious hum? According to the researchers, the intriguing sound is tied to a complex network of fluid and fracture processes inside the glacier.
“It can be clearly linked to glacier hydraulics, such as the lake drainage and hydro-fracturing at the glacier base,” said David Heeszel, formerly at Scripps and now based at the U.S. Nuclear Regulatory Commission, and lead author of the report. “To our knowledge, our study is the first documentation of such tremor signals in an Alpine, non-polar environment.”
The fresh results, the researchers say, are leading to new insights into processes within the elusive glacier interior as well as the potential to improve glacial outburst flood hazard predictions for citizens populating the nearby valley.
The study is a demonstration of the power of continuous seismic data to capture signals the scientists did not anticipate finding, they said.
“These unexpected signals often contain information that moves our understanding of the physical world forward in surprising ways,” said co-author Fabian Walter of the University of Grenoble in France. “We discovered a new way to look into the glacier without having to cut it open.”
The study was funded by U.S. National Science Foundation’s Early-concept Grants for Exploratory Research (EAGER), the Swiss National Science Foundation, the Cecil H. and Ida M. Green Foundation at the Institute of Geophysics and Planetary Physics at Scripps, and the European Union Seventh Framework Programme.
-- Mario C. Aguilera