Scientists can now watch earthquakes and volcanic eruptions develop with far greater precision than ever thought possible, researchers at Northwestern University report.
The subtle movements of the Earth’s crust that over centuries and millennia will raise up mountains and cause quakes and eruptions can be traced year to year by using the same satellite system that gives boaters their bearings and helps military strategists keep track of troops in the field.
Reporting in Friday’s issue of the journal Science, geologists at Northwestern and colleagues in North and South America say they were able to measure the growth of the Andes Mountains in South America to within a fraction of an inch.
The landmark study is the first to document that Global Positioning Satellite (GPS) technology can track movements in the planet’s crust down to an eighth of an inch. The system was devised 20 years ago by the U.S. military to track troop movements and recently has been adopted by boaters, motorists and even hikers using locators made for the consumer market.
“Obviously the military has no need to pinpoint targets to within 3 millimeters, and neither do drivers,” said Seth Stein, the Northwestern geologist who led the study. “If you just turn on a GPS receiver and wait two minutes, it will tell you where you are within 100 meters. But if you keep recording for a couple of days and use a $20,000 receiver instead of a $300 receiver, you can get sub-centimeter accuracy.”
Researchers staked out 43 sites in South America to do the Andes study and they have done similar work in the central U.S. to study the New Madrid fault.
Movement of the Earth’s crust around New Madrid in Missouri and southern Illinois is much less than is seen in South America, where the Andes still are growing. The observations confirm expectations that large earthquakes along the New Madrid fault occur on average every 1,000 to 2,000 years rather than on a scale of tens or hundreds of years, as is true around the Andes.
Besides using sophisticated receiving equipment, the researchers use complex mathematical analysis to filter out errors introduced by uncertainties in satellite orbits and by atmospheric interference in radio signals.
The pinpoint accuracy of the project doesn’t bother military planners, who have complained previously that some civilian uses of GPS technology rival the accuracy of armed forces applications.
“Our work takes months of analysis,” said Stein, “so it has no tactical field application.”
Stein’s work is funded by NASA. When his collaborators started making measurements five years ago, no one was certain the technology could produce the needed precision.
“So far, our greatest surprise is that we can measure these things on this time scale,” said Stein. “To detect an inch of movement along more than a thousand miles of coastline is amazing.”
The information gathered by the technology will greatly improve measurements of the forces that produce earthquakes and volcanic eruptions, and it will help scientists predict eruptions with greater accuracy, Stein said.
But it won’t improve earthquake prediction because scientists don’t understand the physics of earthquakes well enough to apply the information.
“What’s really nice is that it looks like something we’ve suspected for a long time, that these movements occur fairly smoothly over a 25 million-year period,” Stein said.
