EMBARGOED UNTIL NOON EST
NSF PR 96-84 - December 17, 1996
This material is available primarily for archival purposes. Telephone
numbers or other contact information may be out of date; please see current
contact information at media
contacts.
Northridge Earthquake Hasn't Stopped; Hills Have Risen
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 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. "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.
|