Retreating Glaciers Spur Alaskan Earthquakes
NASA/Goddard Space Flight Center <http://www.gsfc.nasa.gov/>
Date: 2004-08-03
http://www.sciencedaily.com/releases/2004/08/040803095217.htm
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.
Editor\'s Note: The original news release can be found here
<http://www.gsfc.nasa.gov/topstory/2004/0715glacierquakes.html>.
This story has been adapted from a news release issued by NASA/Goddard
Space Flight Center.
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