NASA Study: Retreating Glaciers Spur Alaskan Earthquakes

In a new study, NASA and United States Geological Survey
(USGS) scientists found that retreating glaciers in southern
Alaska may be opening the way for future earthquakes.

The study examined the likelihood of increased earthquake
activity in southern Alaska as a result of rapidly melting
glaciers. As glaciers melt they lighten the load on the
Earth’s crust. Tectonic plates, that are mobile pieces of the
Earth’s crust, can then move more freely. The study appears
in the July issue of the Journal of Global and Planetary
Change.

Jeanne Sauber of NASA’s Goddard Space Flight Center,
Greenbelt, Md., and Bruce Molnia, a research geologist at
USGS, Reston, Va., used NASA satellite and global positioning
system receivers, as well as computer models, to study
movements of Earth’s plates and shrinking glaciers in the
area.

“Historically, when big ice masses started to retreat, the
number of earthquakes increased,” Sauber said. “More than
10,000 years ago, at the end of the great ice age, big
earthquakes occurred in Scandinavia as the large glaciers
began to melt. In Canada, many more moderate earthquakes
occurred as ice sheets melted there,” she added.

Southern Alaskan glaciers are very sensitive to climate
change, Sauber added. Many glaciers have shrunk or
disappeared over the last 100 years. The trend, which appears
to be accelerating, seems to be caused by higher temperatures
and changes in precipitation.

In southern Alaska, a tectonic plate under the Pacific Ocean
is pushing into the coast, which creates very steep
mountains. The high mountains and heavy precipitation are
critical for glacier formation. The colliding plates create a
great deal of pressure that builds up, and eventually is
relieved by earthquakes.

The weight of a large glacier on top of these active
earthquake areas can help keep things stable. But, as the
glaciers melt and their load on the plate lessens, there is a
greater likelihood of an earthquake happening to relieve the
large strain underneath.

Even though shrinking glaciers make it easier for earthquakes
to occur, the forcing together of tectonic plates is the main
reason behind major earthquakes.

The researchers believe that a 1979 earthquake in southern
Alaska, called the St. Elias earthquake, was promoted by
wasting glaciers in the area. The earthquake had a magnitude
of 7.2 on the Richter scale.

Along the fault zone, in the region of the St. Elias
earthquake, pressure from the Pacific plate sliding under the
continental plate had built up since 1899 when previous
earthquakes occurred. Between 1899 and 1979, many glaciers
near the fault zone thinned by hundreds of meters and some
completely disappeared. Photographs of these glaciers, many
taken by Molnia during the last 30 years, were used to
identify details within areas of greatest ice loss.

Field measurements were also used to determine how much the
glacier’s ice thickness changed since the late 19th century.
The researchers estimated the volume of ice that melted and
then calculated how much instability the loss of ice may have
caused. They found the loss of ice would have been enough to
stimulate the 1979 earthquake.

Along with global positioning system measurements made by
Sauber and Molnia a number of NASA satellites were used to
document glacier variability. Data from Landsat-7 and the
Shuttle Radar Topography Mission (SRTM) were used to study
glacier extent and topography. Currently, NASA’s ICESat
satellite is being used to measure how the glacier
thicknesses are changing.

“In the future, in areas like Alaska where earthquakes occur
and glaciers are changing, their relationship must be
considered to better assess earthquake hazard, and our
satellite assets are allowing us to do this by tracking the
changes in extent and volume of the ice, and movement of the
Earth,” Sauber said.

For images and information about this research on the
Internet, visit:

http://www.gsfc.nasa.gov/topstory/2004/0715glacierquake.html