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 nearby Granada Hills have risen about six inches since that first jolt in January, 1994.
Scientists affiliated with the National Science Foundation's (NSF) Southern California Earthquake Center (SCEC) in Los Angeles present their findings on December 17 at the meeting of the American Geophysical Union in San Francisco.
"SCEC scientists have identified a possible shortage in the `earthquake budget,' calculated from the amount of strain accumulation in southern California. Measurements from a new and growing array of instruments will help clarify what is happening," explains Jim Whitcomb, director of NSF's geophysics program, which funds the earthquake center.
"The Northridge quake occurred on a thrust fault that did not break all the way to the surface. However, the sedimentary layers of rock 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," explains Gregory Lyzenga, a geophysicist at Harvey Mudd College in Claremont, California. "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 NASA Jet Propulsion Laboratory scientist 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 thoughout 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. "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," says Lyzenga. "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, he adds, is that the force of the after-slip has added about six 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," adds Lyzenga. "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."
The research is also funded by the U.S. Geological Survey and NASA's Office of Mission to Planet Earth.