In a landmark study for space-based observation of the motions of the earth's crust, geologists have used the same satellite navigation system used to guide motorists and boaters to monitor the movement of an entire continent and record the yearly growth of the Andes Mountains in South America to within a fraction of an inch.
The finding, the cover story in the Jan. 16 issue of the journal Science, holds promise for improved earthquake hazard estimation, as it shows that satellite data can help geologists to calculate the accumulating stresses along fault lines as the plates of the earth's crust slide over each other.
Researchers from Northwestern University, the University of Miami, the Carnegie Institution and the University of Memphis, along with Peruvian and Bolivian scientists, used the NAVSTAR Global Positioning System (GPS) satellites to measure movements at locations across the South American continent over a two-year period. Previously, geologists have had to rely on data that traced accumulated plate motions over millions of years' time.
"We're in a whole new world where we can actually look at geology in real time," said Seth Stein, professor of geological sciences at Northwestern and principal investigator on the study. "The GPS has changed the field from a historical approach to a present-time approach."
It was Darwin who first suspected that the giant mountains, huge volcanoes and great earthquakes that occur along the Pacific coast of South America were related. In the 1960s, geologists realized that these phenomena were all consequences of the fact that the oceanic Nazca plate was sliding under the South American plate.
But scientists studying plate tectonics have been able to observe changes only as they accumulated in the geologic record over millions of years, because the continent-sized plates move only a few inches each year -- about the speed fingernails grow.
Enter the GPS. The Global Positioning System, a network of two dozen orbiting satellites, was developed in the 1970s by the Defense Department to locate ships and aircraft to within a range of about 100 yards. The same degree of accuracy proved useful to trucking companies and shipping fleets and is now in widespread civilian use by drivers, boaters and even hikers.
But geologists have spent the past decade refining the technology to be able to pinpoint a location with far greater precision -- about an eighth of an inch.
"Obviously the military has no need to pinpoint targets to within three millimeters -- and neither do drivers," Stein said. "If you just turn on a GPS receiver and wait two minutes, it will tell you where you are within a hundred 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 drove large pins into firm ground at 43 separate locations on the South American continent. They traveled to each site and used a tripod-mounted precise optical plumb system to position an antenna directly over the landmark, then tracked the satellite for a couple of days as the GPS receiver recorded the position of the satellite.
"That gave us a huge data set so we could get very precise ground positions," explained Northwestern graduate student Lisa Leffler-Griffin, whose Ph.D. dissertation is based on the work. "We processed the satellite data to get rid of errors from uncertainties in the orbits and from atmospheric influences on the radio signals."
The results show that about three inches of motion per year occurs between the Nazca and South American plates, and is divided three ways. About 1.4 inches per year of the Nazca plate slides smoothly under South America, giving rise to volcanoes. Another 1.3 inches per year is locked up at the plate boundary, squeezing South America, and is released every hundred years or so in great earthquakes. About 0.3 inches of motion per year crumples South America, building the Andes.
"The Andes were a prime target for us, because along with the Himalayas, they are the highest and broadest mountain range that is actively building now," Stein said. "This is an enormously seismically-prone area. Some of the largest earthquakes in the world happen here."
The researchers compared their findings to the predicted values of NUVEL-1, the standard plate motion model developed at Northwestern University that describes the relative velocities and directions in which all of the earth's plates move.
"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," said Stein, who was one of the developers of NUVEL-1. "We've had no idea of how geologic processes over millions of years correspond to those on a time scale of years. We're now beginning to understand that they're similar. So plate motion is a very steady phenomenon."
Stein said the new data demonstrate that space-based measurements can show in detail how plate motion occurs at the boundary zones between pairs of plates, such as the Andes, Himalayas or San Andreas. The data also show that the interiors of plates are mostly rigid, a key assumption of most plate tectonic models. But in addition, he said, it is now possible to study the mysterious zones in the interior of plates that shouldn't move but do, such as the New Madrid zone in the central U.S., where giant earthquakes occurred from 1811 to 1812. Stein is part of another team of researchers using the GPS to quantify the rate and distribution of strain accumulation in the New Madrid seismic zone.
In addition to Stein and Leffler-Griffin of Northwestern, other authors on the Science paper are Edmundo Norabuena and Leonidas Ocola of the Instituto Geofisico del Peru, Ailin Mao and Timothy Dixon of the University of Miami, Selwyn Sacks of the Carnegie Institute and Michael Ellis of the University of Memphis.
The research was supported by NASA and the National Science Foundation.
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Science