March 11, 1998
Earthquakes have always been hard to study. Although the ground motion during an earthquake is dramatic -- and destructive -- the tremendous strain it releases builds up very slowly. Before GPS, scientists were able to observe the motion of the plates of the earth's crust only as changes 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. Now, using radio signals from GPS satellites, geologists can see the incremental motion in a single year.
"We're in a whole new world where we can look at geology in real time," says Seth Stein, professor of geological sciences at Northwestern University. "The Global Positioning System has changed the field from a historical approach to a present-time approach." Stein spoke today at the Seismological Society of America meeting in Boulder, Colo.
According to Stein, GPS adds three new dimensions to earthquake studies:
First, GPS shows that the plate velocities that were previously known only as averages over millions of years in fact apply equally to time scales of only a few years. Thus, the models of plate motion used to infer how often large earthquakes on average occur in Japan or California, whose application used to be questionable, can now be used with confidence in earthquake hazard studies, he said.
Second, GPS lets geologists study the slow squeezing within the interior of plates that gives rise to earthquakes like the great New Madrid earthquakes that shook the interior of the U.S. in 1811 and 1812. Although the motions that cause these earthquakes are especially slow, Stein said, GPS can measure them and thus provide valuable information about how often these earthquakes can occur. Geologists from Northwestern, University of Miami, University of Missouri and Grand Valley State University are now monitoring a set of markers spanning the New Madrid seismic zone.
"Although most of the motions are probably the after-effects of the earlier quakes, some could be building up for the next quake," Stein said.
Third, GPS makes it possible to study what happens in the zones along plate boundaries, where most earthquakes, volcanoes and other geological actions occur. For example, new GPS data show what is happening in the spectacular zone where the oceanic Nazca plate subducts under the South American continent, Stein said. Measurements of a network of benchmarks established by U.S. scientists from Northwestern, University of Miami and the Carnegie Institution, together with Peruvian and Bolivian scientists, show that about three inches of motion occurs between the Nazca and South American plates each year, and is divided three ways. About 1.4 inches of the Nazca plate slides smoothly under South America, creating deep pressure that gives rise to volcanoes. Another 1.3 inches is locked up at the plate boundary, squeezing South America, and is released every hundred years or so in great earthquakes. About a third of an inch crumples South America permanently, building the Andes.
"This is an enormously seismically-prone area," Stein said. "Some of the largest earthquakes in the world happen here."
Graphics available at http://www.earth.nwu.edu/research/geodesy.html