During the last Ice Age, much of the north, including Scandinavia and Canada, was covered in sheets of ice that measured some 2 miles (3 kilometers) thick, Mitrovica explained. As these ice sheets melted, Earth responded by rebounding, springing back to toward its original position after the loads of ice that once pressed on it retreated.

"The Earth is not just springing back vertically, though," said Mitrovica. "It is also springing out in a horizontal motion, and that's something we couldn't measure until we started using GPS.

"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. With the same idea and more sophisticated tools, we've been able to detect motions less than a millimeter per year. And that data is able to tell us conclusively just how stable a region of land really is," he said.

For the past six years, 34 GPS sensors situated throughout Sweden and Finland have each been continuously recording simultaneous observations from up to 10 satellites. In total, 24 satellites, each orbiting Earth at a distance of about 16,156 miles (26,000 kilometers), were used.

Mitrovica said some scientists believe Sweden and Finland are subject to potentially hazardous tectonic motions. While the countries are not near tectonic plate boundaries, there was a belief that large faults ran through the area that caused motions in the range of several millimeters per year.

"That's important because if you're building a nuclear power plant, for example, you want to know the stability of the region you're building in," Mitrovica said. "This research shows that there is no evidence for tectonic faulting in Sweden. This is an incredibly stable region of the world that will not be faced with large earthquakes."

Mitrovica said the same results would be seen in Canada. The difference, however, is that the rebound would cover a larger area and be centered over Hudson Bay.

The group's other significant finding deals with global sea-level rise. Sweden is known for its data set of tide-gauge measurements, some dating as far back as 200 years. However, Sweden's data set has never been usable, precisely because of contamination from the rebounding effect, he said.

"There was no way of knowing if sea-level change was as a result of land popping up because of crustal rebound or global sea-level rise, which made the tide gauge measurements unusable. But now, for the first time, scientists can use these measurements by using the GPS data to correct for rebound."

The group determined that sea level in that area was, in fact, increasing by 0.08 inches (2 millimeters) per year, confirming earlier research reported by Mitrovica, Milne and Davis in a paper published in the Feb. 22 issue of Nature.

Mitrovica said this research has far-ranging applications and can be used in any other region of Earth where tectonic stability is in question.

Researchers have in recent years used GPS to measure and study earthquake faults, and while the results have yielded information about long-term movements, there is still no ability to predict specific earthquakes.

The study was funded by NASA and the National Science Foundation, along with several international organizations. Others who participated in the research include: Glenn Milne of the University of Durham; James Davis of the Harvard-Smithsonian Center for Astrophysics; Hans-Georg Scherneck and Jan Johansson of Onsala Space Observatory; and Martin Vermeer and Hans Koivula of the Helsinki University of Technology.