PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA 91109. TELEPHONE (818) 354-5011
Contact: Mary A. Hardin at JPL (818) 354-0344
AGU Newsroom (415) 905-1007
Cheryl Dybas National Science Foundation (703) 306-1070
FOR IMMEDIATE RELEASEDECEMBER 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
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
"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
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
"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
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.