On July 6, seismologists completed installation of the 250th and final station of the Southern California Integrated GPS Network (SCIGN). Unlike most seismological instruments, which record shaking, SCIGN tracks plate movement with the Global Positioning System (GPS) — a constellation of satellites originally designed for military navigation and used to determine precise and absolute locations on the ground.
The project was begun in 1996 to estimate earthquake potential in Southern California and Baja California, one of the world's most seismically active and highly populated areas, and to learn more about the mechanical movements along faults. Scientists have been using the system since its inception with some success. But with the denser SCIGN network, they hope to learn more about the complicated links between plate motion and the resulting earthquakes. Using SCIGN data to measure crustal deformation, which can occur as movement on faults or as slow intraplate distortion, scientists can determine how strain builds up slowly over time before being released suddenly during earthquakes.
The SCIGN project is significant because of the leaps in data processing that allowed it to happen. Ten years ago computers could only handle the analysis of occasional GPS measurements, and even then took a long time to process the information. Today the equipment is cheap and efficient enough to allow continuous assessment from all 250 sites and provide real-time data.
NASA began monitoring plate motions decades ago with large radio telescopes around the world. USGS scientists pioneered precise measurement of deformation to assess earthquake hazards, using lasers during the early 1970s through the late 1980s. Then GPS came along. Today GPS instruments can measure movement as small as a few millimeters a year.
The initial, permanent GPS sites went into the ground in the early 1990s. The Northridge earthquake boosted interest and funding in such projects, expanding the network to 50 sites. Since 1996, when SCIGN got going, an additional 200 sites were added to the area. SCIGN data are freely available to anyone over the Internet (www.scign.org). The majority of SCIGN data users are scientists working in universities and government agencies around the world.
The California Department of Transportation (Caltrans) is also working with SCIGN. Doug Failing, chief deputy of District 7, said, "not one major freeway structure collapsed during the Northridge earthquake of 1994 that had been seismically retrofitted prior to this earthquake. The damage we did sustain to those seven freeway structures that did collapse was the result of seismic retrofit work not being complete or not yet underway at the time." Failing says he hopes that SCIGN will allow Caltrans to focus on structures most likely to get hit with heavy shaking, and target them for seismic strengthening.
Another new earthquake monitoring device being installed for Caltrans is a laser strainmeter along the east side of the Glendale Freeway. Here a laser beam will travel back and forth inside a 2,000-foot long pipe to precisely measure the distance between the endpoints of the pipe. Any change indicates crustal deformation. The laser strainmeter makes observations of strain that are approximately 100 to 1,000 times more sensitive than the GPS system. "The strainmeter is so sensitive that if we were to take the LA basin and squeeze it over its entire breadth by no thicker than a human hair, the change would be easily detected," said Frank Wyatt of the Scripps Institution of Oceanography.
Scientists of the Southern California Earthquake Center designed and manage SCIGN. NASA's Jet Propulsion Laboratory, the Scripps Institution of Oceanography at the University of California at San Diego, and the U.S. Geological Survey (USGS) are the main participants. Funding for SCIGN is provided by NASA, the W.M. Keck Foundation, the National Science Foundation and USGS.
For more information about the Southern California Earthquake Center and SCIGN visit www.scec.org and www.scign.org.
Emily D. Johnson