Northridge Quake Activity Has Apparently Subsided Says NASA

The Northridge fault surprised residents of greater Los Angeles with
a magnitude 6.7 earthquake on January 17, 1994, killing 60, injuring more
than 7,000 and causing more than $20 billion in damage. Now, it has
surprised scientists again.

“Recent measurements indicate the Northridge fault has slowed to a
crawl,” said Dr. Andrea Donnellan, a geophysicist at NASA’s Jet Propulsion
Laboratory, Pasadena, Calif. “Following a quake, Earth’s crust readjusts to
changing forces. We expected the Northridge region to re-adjust for at least
another 20 years, but it looks as though that readjustment is largely done.”

So what put the brakes on? Donnellan says evidence points to the
physical state of rocks deep in the region’s crust. The Northridge quake
began in the lower crust, which unlike Earth’s brittle upper crust is usually
“gooey.” “But with Northridge,” says Donnellan, “measurements of the
speed seismic waves travel through the region indicate the lower crust is
actually cold and hard — like frozen molasses. This cold crust apparently
brought Northridge to the conclusion of its earthquake cycle much sooner
than expected, or perhaps is causing the region to adjust so slowly that we
can’t detect it. In contrast, the lower crust in places such as Landers in the
Mojave Desert — site of a 1992 magnitude 7.6 quakeóis 260 to 538 degrees
Celsius (500 to 1,000 degrees Fahrenheit) warmer, so quake responses in
such regions can be measured for a longer time.”

Donnellan cautioned that this information doesn’t mean there still
couldn’t be aftershocks, or that neighboring faults couldn’t rupture. “It’s an
interacting system,” she said. “A quake on one fault may turn on or turn off
a quake on another. While an earthquake is not likely to happen directly on
the fault that broke in 1994, we don’t know the status of Northridge’s
neighboring faults — where they are in the cycle of building up stress,
releasing the stress through a quake, and then building up stress again.
There are indications neighboring faults were affected, and it’s possible one
of them may break, but we don’t know when. Understanding the way those
faults were affected will take time.”

In the study, presented recently to the Seismology Society of
America, Donnellan and colleague Dr. Gregory Lyzenga, a JPL geophysicist
and professor at Harvey Mudd College, Claremont, Calif., analyzed data
from global positioning system (GPS) receivers arrayed throughout Southern
California and radar images taken before and after the Northridge quake.

When the quake struck, Donnellan and other scientists had been
monitoring the area with GPS for eight years. Each receiver continuously
measures its location, detecting surface changes as small as 3 millimeters (a
tenth of an inch) horizontally and 7 millimeters (a third of an inch)
vertically.

Measurements show that during the 1994 quake, the fault slipped two
to three meters (6.5 to 9.8 feet), starting about 18 kilometers (11 miles) deep
in Earth’s crust and rupturing up to about 5 kilometers (3 miles) below the
surface. In the 18 months following the quake, it moved an additional 30
centimeters (about one foot), raising the nearby Granada Hills about 12
centimeters (about 5 inches).

“Ninety percent of the movement after the initial earthquake was a
quiet sliding motion, or aseismic,” said Donnellan. “Only 10 percent was
seismic — the shaking motion a seismometer measures. That’s why new
tools like GPS and advanced radar are so important. We’re just now seeing
how Earth behaves with geodetic measurement — we’re able to observe
these quiet processes and get insight into the whole system.”
In response to the Northridge disaster, NASA led a collaboration of
agencies to develop and install the Southern California Integrated GPS
Network, which now includes more than 250 stations in 14 California
counties and Mexico. The network allows scientists to monitor movements
of Earth’s plates. “We can now see how faults interact and look at whole
fault systems, not just individual ones,” said Donnellan.
But while GPS receivers are measuring continually, they’re not
measuring everywhere. For the most complete map of every change a quake
makes on the surface, scientists turn to satellite interferometric synthetic
aperture radar. By combining detailed radar images taken before and after a
quake, they can pinpoint surface changes caused by the initial quake and
track its aftermath. “You would need a GPS receiver every 20 meters (about
66 feet) to get the same information,” said JPL geologist Dr. Gilles Peltzer, a
specialist in interpreting radar data. Radar images of Northridge from the
European Space Agency’s European Remote Sensing Satellite-1 taken two
months before the quake, combined with others from December 1995, help
fill in the Northridge picture. Those results confirmed the GPS results,
indicating the Northridge fault continued to slip after the event. They also
shed light on the extent of the region subject to deformation as a result of the
quake.

Donnellan said increased use of space-based technologies will lead to
better computer models and should, in the next 10 to 20 years, give scientists
much clearer insights into how earthquakes behave and where and when
they may occur.

The two are now studying other California faults — Oak Ridge, Sierra
Madre, and San Cayetano — developing computer models to see how stress
is transferred between them.

JPL is a division of the California Institute of Technology in
Pasadena.

Note to Editors: Live satellite interviews are available with Drs. Donnellan
and Lyzenga on Tuesday, Oct. 1, from 3:15 p.m. to 7:15 p.m. EDT. “B” roll
and interviews will be carried on GE-2, Transponder 9C at 85 degrees West
longitude, with vertical polarization. Frequency is on 3880.0 MHz with
audio on 6.8 MHz. The full video package will run Sept. 30 and Oct. 1
during the NASA-TV Video File feed scheduled for noon, 3 p.m., 6 p.m., 9
p.m. and midnight EDT. To book time for this interview, call Jack Dawson
at (818) 354-0040 or email Jack at jack.b.dawson@jpl.nasa.gov.