PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA 91109.  TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov

Contact:  Mary A. Hardin at JPL                    AGU Newsroom
         (818) 354-0344                         (415) 905-1007

         Cheryl Dybas
         National Science Foundation
         Arlington, VA 
         (703) 306-1070

FOR IMMEDIATE RELEASE                     DECEMBER 17, 1996

1994 NORTHRIDGE EARTHQUAKE HASN'T STOPPED, HILLS HAVE RISEN

     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.

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#9697
12/13/96