Those "Goose Bumps" on Glaciers

Seated in her office overlooking the Pacific Ocean, Debi Kilb, an expert on earthquakes at Scripps Institution of Oceanography, UC San Diego, points out the objects of her current research--multicolored blocks that pop up rapidly on her computer screen like an ongoing video game.

The popping blocks on her computer are not earthquakes occurring, as one may have guessed. In fact they depict a chronological evolution of surface icequakes--tremors that occur almost constantly on glaciers all over the globe.

Icequakes, like earthquakes, are caused by stress changes. However, unlike earthquakes that respond to large stress changes within the earth, icequakes respond to relatively smaller stress changes driven by processes in glaciers such as surface crevasse openings, iceberg breakaway events known as calvings and the flow of water below a glacier’s surface. As a result, icequakes get recorded in negative values on a scale that measures the magnitude of earthquakes.

Kilb joined glaciologist and Scripps postdoctoral fellow Fabian Walter to study icequakes in Gornersee, a glacial lake on the Gornergletscher glacier in Switzerland in the summer of 2004. Their plan was to monitor icequake activity for an extended period during a lake drainage event — a study that Kilb and Walter expected would give insights into the behavior of water under the surface of glaciers.

Glacial lakes form within, below, above, or even next to glaciers as meltwater accumulates in spaces dammed either by glacial ice or moraines. Moraines are debris carried and piled up by moving glaciers. Such ice dams are extremely unstable. Hence, glacier-dammed water bodies can drain unexpectedly, sometimes within days or even hours in a potentially catastrophic manner. In 2004, Gornersee released about 2.7 million liters (713,000 gallons) of water in a flooding event that damaged parts of Zermatt and Tasch, two Swiss towns downstream from the glacier.

“Everybody just knew that icequake activities increased during glacial lake drainages,” said Kilb. “The idea was when glacial lakes drain, the excess water is injected into surrounding ice causing stress to build up that will dramatically increase icequake activity.”

Her smile gives away that her research on icequakes suggests otherwise. In an unprecedented survey, Kilb’s team recorded some one million tremors over three summers from 2004 to 2007 using a high-density network of seismometers that cover the western flank of lake Gornersee. Their analysis did not show strong evidence of the expected dramatic increase in icequakes during lake drainage events. But their discoveries were even more interesting.

Kilb and Walter found that during lake drainage, icequakes seem to halt in some regions and become heavily active in other regions but causing no net change in the rate of icequake activity. Kilb suggests that it is possible that during some of these drainage events, the excess water drains into locations that are already so heavily inundated that new meltwater cannot be accommodated. This forces the “new” water to find other outlets or wait until water pressures within the inundated locations abate. This could explain the temporary halt to icequake activity in certain regions and increases in others.

Although preliminary, Kilb and Walter’s research suggests that an increase in the number of icequakes may not be a good predictor of lake drainage. It may be more realistic to monitor changes in icequake activity in multiple areas of the glacier to better forecast lake drainages. The team also recorded an increase in the number of icequakes during the day compared to at night that suggest day/night melt cycle has a large impact on surface seismicity within critical locations of the glaciers. These new findings challenge well-established hypotheses on movement of glaciers.

Kilb said she hopes that the new data her team has collected will not only help scientists like herself to learn the unusual ways in which icequakes operate but also contribute meaningfully toward a better understanding about behavior of glaciers that are experiencing rapid melting due to warming air temperatures in the 21^st century.

“The possibilities are endless and it is so cool,” Kilb said.

Atreyee Bhattacharya