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