A serendipitous discovery by a University of Colorado at Boulder-led team has shown for the first time that satellite signals from the Global Positioning System are a valuable new tool for studying earthquakes.
CU-Boulder Associate Professor Kristine Larson of aerospace engineering sciences said seismic waves from a 7.9 magnitude earthquake in Alaska's Denali National Park in November 2002 were detected using Global Positioning Satellite, or GPS, receivers as far away as 2,350 miles from the event. The quake also was picked up by scores of GPS receivers in Canada and the United States.
GPS is a constellation of satellites originally designed by the U.S. military to provide precise positions of ships, tanks, airplanes, other military equipment and even people. Currently there are 27 GPS satellites orbiting Earth at roughly 12,500 miles above the planet.
"This is the first time GPS has been used to track seismic waves," Larson said. "The signals were large enough to be recorded by GPS receivers as far away as Colorado Springs, Colorado."
A paper on the subject will be published electronically by Science magazine on Science Express May 15. In addition to Larson, co-authors include Paul Bodin from the University of Memphis and Joan Gomberg from the U.S. Geological Survey's Memphis office.
"The nice thing about GPS is it's great versatility," said Larson. "In this study we were able to track seismic waves that traveled from Alaska through Canada to Washington, Montana and Colorado."
GPS also has a number of other scientific uses, like measuring ice sheet movements, the inflation of magma under volcanoes and plate tectonics, Larson said. More practical uses of GPS include navigating aircraft, boats and cars, as well as helping lost hikers find their way to safety.
GPS users -- like hikers, boaters and car drivers -- decide how frequently their position determination is needed. For measuring seismic waves from Denali, Larson's team used GPS receivers that were set to measure positions once each second, or 1 Hertz.
Ordinarily, she said, scientists study earthquakes with seismometers, but these often are set for a particular sensitivity range. Because the earthquake in Alaska was so big, however, many seismometers in the United States and Canada were not able to measure it. "But GPS researchers love very big signals," Larson said. "The bigger the better for us."
The Denali quake ruptured almost 200 miles, causing surface displacements of more than 25 feet in some places, she said. "This is permanent deformation. The deformations we observed with GPS in the lower 48 states also were large, but were caused by the seismic waves and did not cause permanent displacement."
For a sense of how big the seismic waves were, a GPS receiver in eastern Washington moved nine inches horizontally in just 10 seconds, even though it was 1,500 miles from the Denali earthquake.
There are many continuously operating GPS receivers in the United States, she said, and scientists use them primarily to monitor small motions on faults. Roughly 250 GPS receivers are operating in Los Angeles County, for example, installed in response to the 6.7 magnitude Northridge earthquake in 1994.
The National Science Foundation recently funded a research project known as Earthscope to study the structure and evolution of the North American continent, Larson said. Earthscope also is designed to decipher what causes earthquakes and volcanic eruptions and as part of that effort, Earthscope engineers will soon be installing 800 additional GPS receivers in the western United States.
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