GPS Measures the Earth's Movements

As a growing number of people use GPS systems to find their way across the ocean, through the wilderness or around the suburbs, a team of scientists has found an interesting new application for these devices: measuring movements of the earth's crust.

The motion they measured occurs in Scandinavia in a region that was covered by an ice sheet three kilometers thick during the Ice Age. As the ice melted and its heavy weight was lifted, the earth apparently started to bounce back. "The earth is not just springing back vertically, though," says Jerry Mitrovica of the University of Toronto. "It is also springing out in a horizontal motion, and that's something we couldn't measure until we started using GPS." Mitrovica is one of the authors of a new study on the phenomenon, published in today's Science.

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The researchers installed 34 advanced climate-controlled GPS sensors (image above right) throughout Sweden and Finland. "When used in cars, this system will tell a person where they are within a couple of meters on the earth's surface. We've taken that a few steps further," Mitrovica explains. "With the same idea and more sophisticated tools, we've been able to detect motions less than a millimeter per year." The 34 sensors (map below left)have been recording continuously for the past six years, using 24 different satellites. As the maps below show, there has been an uplift of several millimeters per year throughout Sweden and Finland (center), as well as horizontal expansion (right).

In prior research, Mitrovica and others created computer simulations of the earth's rebounding action and concluded that it was springing back horizontally and vertically. Until now, they had no way to prove the theory.