Metropolitan Los Angeles Under A Slow Squeeze

The measurements suggest that new mountains may be forming south of the high San Gabriel Mountains.

The results come from the Southern California Integrated Global Positioning System (GPS) Network, an array of 60 GPS receivers that continuously measures the constant, tiny movements of earthquake faults throughout Southern California.

"We've known for some time that the area between the coastline and the Mojave Desert is being squeezed together by the constant movement of Earth's crust," said Dr. Donald Argus, a geophysicist at JPL. "This new research helps pinpoint the area thatÕs being squeezed. Specifically, downtown and West L.A. appear to be moving toward the San Gabriel Mountains at about one-fifth of an inch (half a centimeter) per year."

Argus is presenting his findings Oct. 29 at the annual meeting of the Geological Society of America in Toronto, Canada.

"While this research does not mean that an earthquake in Los Angeles is imminent, one possible conclusion is that the earthquakes that occur in Los Angeles might be concentrated in the northern part of the basin," Argus said.

The GPS surveying system uses radio signals transmitted from 24 Earth-orbiting satellites operated by the U.S. departments of Defense and Transportation. Equipment on the ground receives signals from several satellites at a time, allowing scientists to pinpoint the position of a receiver to better than 0.4 inch (1 centimeter).

"The regional project is designed for exactly this kind of study. Our goal is to observe and monitor the slow, small motion, called strain, of the ground in greater Los Angeles," said JPL's Dr. Frank Webb, chair of the Southern California network. "This research helps us learn where earthquakes are more likely to happen, and helps with estimating the regional earthquake hazard in Southern California. It enables other agencies to make priorities about earthquake mitigation activities, including emergency preparedness and retrofit strategies."

There are about 60 GPS receivers on the ground around Southern California. Two new sites are being added every week with plans for a total of 250 more. The earthquake network began in 1990 with only four GPS receivers as a prototype funded by NASA. It detected very small motions of EarthÕs crust in Southern California associated with other California earthquakes in June 1992 in the town of Landers and in January 1994 in Northridge.

The Southern California network includes a number of institutions using GPS for earthquake research. The consortium is coordinated by the Southern California Earthquake Center, a National Science Foundation science and technology center at the University of Southern California (USC). The array is operated by JPL, USC, the U.S. Geological Survey and the Institute of Geophysics and Planetary Physics at the University of California at San Diego's Scripps Institution of Oceanography.

The JPL research is part of NASAÕs Earth Sciences enterprise, which seeks to understand the total Earth system and the effects of natural and human-induced changes on the global environment. JPL is a division of the California Institute of Technology.