Tuesday, 03 January 2012 12:54 Written by Doug O'Harra, Alaska Dispatch

Photo: Kathryn Hansen/NASA. Alaska Dispatch.

A Bering Sea swell rises 20 feet and compresses the seabed with tons of extra mass, sending a tiny seismic wave rippling through Earth.

Heavy surf pounds a bedrock beach on Alaska's west coast, and the Earth beyond rumbles with miniscule but detectable motion.

The home planet is perpetually ahum with such ocean-generated microseismic waves. Well-placed sensors can record this low-power energy, allowing scientists to eavesdrop, in a sense, on the restless sea.

But what happens when the ocean starts to freeze and the growing pack ice dampens the size and power of swell and surf? Do ice floes change the signal — or simply shut it down?

A new study in Geological Research Letters reports a dramatic correlation between the formation of Bering Sea ice and the power of this "microseismic signal," suggesting that devices similar to earthquake sensors may someday be able to monitor sea ice strength.

In some cases, these sensors may provide a more accurate picture than satellites, the authors concluded.

"Because strong sea ice prevents large ocean waves from forming, sea ice can therefore significantly affect microseism amplitudes," wrote Victor Tsai of the California Institute of Technology and Daniel McNamara, with the U.S. Geologic Hazards Science Center in Colorado, in their report, Quantifying the influence of sea ice on ocean microseism using observations from the Bering Sea, Alaska. "This link between sea ice and microseism is not only a robust one but can be quantified."

Seismic energy variations because of sea ice

The scientists discovered that 75 to 90 percent of the variability in the seismic energy generated by the Bering Sea could be explained by the spread, density and thickness of sea ice. Visualizing the immense mechanical power of the roiling sea is critical to understanding how this might work.

"When an ocean wave swells, the sudden change in water column mass sends a pressure wave down to the ocean bottom," the authors explained in a release about the study. "If the wave lasts long enough to strike the shore, its kinetic energy is transferred to the rock."

Both of these natural processes — the rise and fall of ocean swells, and the slamming of surf against the beach — create low-powered seismic waves that last one to 20 seconds. The waves are recorded by earthquake-monitoring instruments as part of the planet's regular background motion, the scientists said.

The amplitude of this rumbling varies, depending on the distance from source to the seismic station and the time of year. "Big winter storms kick up larger waves," the scientists pointed out.

Since this microseismicity depends on the existence and size of ocean swells, Tsai and McNamara wondered whether sea ice thick enough to interfere with wave action might change the seismic signal — and what those changes might reveal about the ice itself.

"To find out, the authors built a simple computer model and pulled together twice-weekly observations of sea ice extent, along with the hourly readings of three seismic stations," the story explains.

One station on Unalaska Island in the Aleutian Chain is far south of the reach of seasonal ice, and served as a control in the experiment. The other two — one on St. Lawrence Island and the other near the tip of the Seward Peninsula at Tin City — were often surrounded by ice.

The scientists then gathered and processed data collected by the stations between 2002 and 2010. "It may be noted that these three stations represent the only stations bordering the Bering Sea with at least seven years of nearly continuous, mostly glitch-free data," the scientists wrote in the paper.

Over the seasons, as sea ice thickened and concentrations morphed, so did the signal recorded at the two northern stations. In the end, up to 90 percent of the variability could be explained by the changes in the concentration of nearby sea ice.

"Using observed microseism variability to predict ... strength of sea ice can be more accurate than using satellite observations," the scientists concluded. "Being able to predict sea ice mechanical strength would be useful on a number of different levels, including knowing how close this ice is to disintegrating. Such knowledge could potentially be useful for the shipping industry as well as climate scientists."

Although more research is needed, and several technical issues must be worked out, the results suggest that the ocean's faint seismic voice may be whispering accurate information about the strength and extent of surrounding sea ice to those listening with the right tools.

Contact Doug O'Harra at doug(at)alaskadispatch.com