Southern California may soon be the best-surveyed area on the planet, thanks to powerful tools used by scientists seeking to understand the region's earthquake potential.
The National Science Foundation (NSF) Southern California Earthquake Center (SCEC) at the University of Southern California is installing 250 ground-based "monuments" to electronically track satellites of the Global Positioning System (GPS).
The resulting network -- the densest of its kind on the planet -- will enable scientists to follow, in unprecedented detail, movements of the Earth's crust in one of the world's most seismically active and highly populated areas.
"The Southern California scientific community can now pioneer the use of the most promising new tool in seismology since the invention of the seismometer," says SCEC director Thomas Henyey. "Eventual completion of the 250-station array will put the full potential of GPS technology to work in an earthquake-prone region particularly suited to the task."
GPS makes it possible to locate geophysical monuments with extraordinary precision. Such monuments, located at specific key points on the Earth's surface, are equipped with special electronic units to receive signals, around the clock, from the 24 GPS satellites orbiting the earth. Although the monuments may be separated by scores of miles, a change in their relative position -- even by no more than a single millimeter, or about one twenty-fifth of an inch -- can be detected by the GPS system.
In seismically active areas such as Southern California, where plate tectonic forces are at work, substantial earth movements of millimeters to centimeters occur continuously each year and are readily measurable by the state-of-the-art GPS technology.
These movements give scientists indications of how fast strain is building up, where it's concentrated, and where earthquakes might occur in the near future. Before and after the January, 1994, Northridge earthquake, the few GPS monuments then in service revealed important scientific clues about processes taking place far underground. GPS technology is particularly valuable for studying hidden faults, like the one that caused the Northridge earthquake, according to Henyey. Faults located far underground are more difficult to study by other methods.
According to Henyey, SCEC is rapidly expanding a current network of 40 monument receiver stations. In addition to providing coverage for the 25,000-square-mile area extending from the Tehachapi Mountains south to the Mexican border, and from the Pacific Ocean to the Colorado River, stations will be concentrated along a tectonically critical corridor extending through the Los Angeles basin. Each station will be monitored daily.
Funding for the project comes from NSF and the W.M. Keck Foundation.