For years, analysts at the Alaska Earthquake Center, while tracking about 100 temblors a day in the most seismically active U.S. state, have dutifully filtered out some of the Earth-shaking events that trip the sensors.
The analysts would mark these distinctive readings with the letter, "G," for "glacier."
The readings, considered a curious by-product of the effort to track earthquakes, were from calving glaciers. Some registered as high as magnitude 3.
Now, Alaska's state seismologist Mike West, the center's director, argues that this data is a valuable record that could yield new understanding on climate change. In a presentation Thursday at the Seismology Society of America's annual meeting in Anchorage, West showed that long-ignored data within the state's earthquake records faithfully capture dynamic change occurring above ground: ice breaking off of glaciers and falling into water, the phenomenon known as calving.
Some calving occurs quietly; ice can slip off a glacier beneath the water without generating seismic waves. But when blocks of ice topple off the front of glaciers into the water, much like swimmers doing cannonballs into a pool, the impact resonates and is picked up by sensors.
"It turns out to be a spectacular data set in its own right," he said. "We have many years of records of these activities at these glaciers – winter versus summer, glacier 'X' versus glacier 'Y' – all kind of sitting there."
Sensors have captured several thousand so-called "ice-quakes." Hundreds happen annually along the south-central coast of Alaska, where glaciers reach the sea. Records at the center, located at the University of Alaska, Fairbanks, date back to the 1970s, when seismologists first recognized that calving glaciers created unmistakable seismic signals hundreds of kilometers away, West said. But after the Alaska earthquake network's hardware was upgraded in 2007, the number of ice-quakes picked up by the sensors increased.
"We have a half-dozen years of really high-quality glacier data," West said. "We're in the early stages of being able to look [at what has happened] over a period of years."
Glacier seismology is a relatively new area of science, but interest has been growing in the possibilities for detecting the extent of global warming's impact in the vibrations it causes beneath the Earth's surface.
Atmospheric measurements, from below ground
It may seem incredible that atmospheric change can be measured below ground, but seismic sensors record all types of surface movement – even delineating day from night, or weekends from weekdays when placed near cities. "It doesn't matter if it's an earthquake, or a train going by, or an animal, or a nuclear explosion, or glaciers," West said. "Our sensors don't know any better – they just measure how the ground shakes."
Seismologists can distinguish ice-quakes from tectonic earthquakes by location and the unique shape of the waveforms they generate. This has led to significant findings. Seismologists at Harvard and Columbia Universities first reported on their tracking of ice-quakes from glaciers in Greenland in 2003, and later showed this glacial earth-shaking was on the rise. Scientists from University of Colorado, Boulder detailed the seismic signals produced at the rapidly retreating Columbia Glacier in Alaska in 2007.
West singled out Columbia as the best example to date of an Alaska glacier with a clear change in ice-quake production in recent years. After it retreated into shallow waters and became grounded in 2009 and 2010, the end of the glacier rose high enough above sea level that it led to "tall icebergs that fall over, smacking the water" – and a corresponding increase in seismic signals.
Opening up possibilities
West's new findings are different from previous studies where scientists instrument and study a single glacier in a targeted way. His research opens up the possibility of tracking what is happening over all of Alaska, one of the most dynamic glaciated regions of the world.
Alaska is the most glaciated U.S. state, with glaciers covering about 29,000 square miles, about 5 percent of its surface. More than 99 percent of the state's low-lying glaciers are retreating.
"The ability to routinely monitor calving events is a new approach," said Bruce Molnia, research geologist with the U.S. Geological Survey, who has written extensively on Alaska's glaciers and has documented their change over time. He noted that the equipment to track the seismic signals from calving glaciers is expensive, and it's necessary to have a number of sensors in place – as the Alaska network does – because to pinpoint the location of a calving, glacier scientists have to triangulate data from several sensors. "And most people who have the equipment are more focused on bigger seismic events," said Molnia. "But the data is there."
Only a subset of glaciologists have focused on the potential of seismology to inform their research, while the vast majority of seismologists are focused on tectonic and volcanic events. The two branches of geology have not fully recognized the potential ways their research could intersect.