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       Earthquake researchers measuring the movement of the Earth's surface with the Global Positioning System (GPS) have concluded that the Northridge earthquake has continued in a "quiet" way and the Granada Hills have risen about 16 centimeters (about 6 inches) since that first jolt in January 1994.

       Scientists from NASA's Jet Propulsion Laboratory will present their findings this week at the annual Fall meeting of the American Geophysical Union in San Francisco.

       "The Northridge quake occurred on a thrust fault that did not break all the way to the surface. However, the sedimentary layers of rock, in the top five kilometers located just below the surface near the epicenter, have continued to move in a fluid- like manner -- sort of like honey flowing off a spoon -- since the earthquake," said Dr. Gregory Lyzenga, a JPL geophysicist and professor at Harvey Mudd College, Claremont, CA. "The amount of motion that happened because of this 'stealth' earthquake is equivalent to the displacement that would accompany a magnitude 6.0 earthquake."

       Lyzenga and his JPL colleague Dr. Andrea Donnellan studied data from about a dozen GPS receivers that continuously measure the constant, yet nearly physically imperceptible, movements of earthquake faults throughout Southern California. These temporary GPS sites were part of a preliminary earthquake study that helped lead to a large effort called the Southern California Integrated GPS Network (SCIGN). SCIGN uses an array of permanent GPS receivers placed throughout the region.

       GPS uses data transmitted from a constellation of 24 Earth- orbiting satellites that are jointly operated by the departments of Defense and Transportation. The satellites are arranged so that several of them are "visible" from any point on the surface of the Earth at any time. Scientists at JPL can determine the position of a user with a GPS receiver to better than 1 centimeter (0.4 inches) per day by correlating signals from the satellites and knowing the satellite orbital locations very accurately.

       "It is not clear yet if this continued post-Northridge 'after-slip' represents a loss of stress along a fault or if it is a transfer of stress to other areas," Lyzenga said. "Our GPS processing techniques are now better refined, making it easier to resolve vertical as well as horizontal movements of the Earth's surface."

       What is clear is that the force of the after-slip has added about 16 centimeters (about 6 inches) to the Granada Hills since the earthquake. Granada Hills is a foothill community just to the north of the city of Northridge.

       "While similar post-seismic movements have been seen after earthquakes in other regions, this observation is significant because it highlights the difficulty of fully accounting for all of the strain that can potentially lead to earthquakes. If we hope to make realistic assessments of earthquake potential in different parts of the Los Angeles basin, we need to understand the processes and amounts of quiet movement, as well as the more obvious shifts that occur immediately during seismic events."

       In a related observation, researchers studying GPS measurements from a single site located in the foothills behind the Jet Propulsion Laboratory in Pasadena have seen the rate of motion at that site change significantly since the Northridge quake.

       "This extra motion cannot be easily explained by means of additional slip on the fault which ruptured during the Northridge earthquake, suggesting the possibility of slip on a second fault closer to JPL," said Dr. Michael Heflin, a JPL geophysicist. "The extra motion may represent a significant release of strain energy which is occurring without earthquakes. If such events turn out to be common, the overall earthquake hazard may need to be re-evaluated."

       The on-going measurements of the new and growing SCIGN array will help clarify the "earthquake budget", or the amount of strain accumulation that has built up in Southern California. If the observations show that significant strain energy is released quietly, then less total energy is left to be released and we may experience fewer damaging earthquakes, Heflin added.

       The GPS earthquake research is funded by NASA's Office of Mission to Planet Earth, the United States Geological Survey and the Southern California Earthquake Center (SCEC), a National Science Foundation Science and Technology Center headquartered at the University of Southern California.

For more information, visit the Southern California Global Positioning System home page.