Analysis uncovers unusual earthquake-related deformation,
paves the way for methods to identify new active faults

On Oct. 16, 1999, approximately 37 miles from Palm Springs,
Calif., a magnitude 7.1 earthquake ripped through 28 miles
of faults in the Mojave Desert. Because of the area’s sparse
population and development, the massive quake caused
virtually no major measurable injuries or destruction.

Yet the “Hector Mine” event, named after a long-abandoned
mine in the area, has produced a treasure of information
about earthquakes, faults, and ruptures for scientists at
Scripps Institution of Oceanography at the University of
California, San Diego. In results published in the Sept.
13 issue of Science, the scientists, along with a colleague
at the California Institute of Technology (Caltech), reveal
that they used satellite and radar technologies to uncover
never-before documented characteristics of faults. These
include the first evidence that faults move backwards,
contrary to conventional observations, and indications that
the material within faults is significantly different than
its surroundings.

Scripps’s Yuri Fialko, the lead author of the study, says
the implications of the study include providing a new way
to identify potentially active faults, helping to track
when the last earthquake occurred in a fault zone, and
perhaps better understanding the earthquake process.

Fialko calls the Hector Mine event the “perfect” earthquake
for the satellite and radar technologies that he and his
colleagues used. It is the first event comprehensively
imaged using interferometric synthetic aperture radar
(InSAR), as Fialko and coauthors demonstrated in an earlier
study published in Geophysical Research Letters. InSAR uses
a series of satellite recordings to detect changes in
Earth’s surface.

According to Science study coauthor David Sandwell, the
fresh data gave researchers an uncommon and immediate
window into earthquake processes in fault areas that are
only typically imaged after being altered by natural forces
such as rainstorms and unnatural forces such as off-road
vehicle disruption.

Fialko, Sandwell, and coauthors Duncan Agnew, Peter Shearer,
and Bernard Minster of Scripps, and Mark Simons of Caltech,
studied the information to find unusual signatures of fault
displacements caused by Hector Mine in the Eastern California
Shear Zone (ECSZ) in an area thought to be relatively

The most surprising finding was the first evidence that
faults can move backwards. Prior to an earthquake, faults
are locked in position by the “glue” of friction. Changes
due to energy released during earthquakes cause faults to

“Even small stress perturbations from distant earthquakes can
cause faults to move a little bit, but it’s only been known
to cause this motion in a forward sense,” said Fialko. “Here
we observed the faults coming backwards due to relatively
small stress changes, which is really quite unusual.”

The study argues that the backward motion on the faults is
caused by the dissimilar material within the faults, rather
than the frictional failure.

“We used an analysis model that effectively says that material
within the faults is mechanically distinct from the material
surrounding the faults,” said Fialko, of the Cecil H. and Ida
M. Green Institute of Geophysics and Planetary Physics at
Scripps. “The rocks within the faults appear to be softer.”

He says the fault zones become strained during periods of
stress, acting like a soft, sponge-like material. The soft
area thus becomes squeezed during periods of energy release.

According to Fialko, the results will guide new seismic
studies to areas with contrasting fault material, such as
that seen in the Eastern California Shear Zone. They can
then be used as a way of identifying potentially active

Another possibility emerges through studying the properties
of fault zones over time.

“Measurements of changes in the mechanical properties of
faults may yield valuable information about the earthquake
cycle. For example, we might be able to say how long it was
before the fault experienced an earthquake and how long it
takes to heal,” said Fialko.

Coauthor Shearer attributes these detailed results to the
“breakthrough” offered by InSAR technology.

“Prior to InSAR, all we had were spot measurements of the
deformation field,” said Shearer. “At best we had maybe a
few hundred points across southern California. You had a
point here and there so you didn’t really know what was
happening. With InSAR we have millions of points and thus a
continuous picture of deformation across southern California.”

The scientists say the findings became possible due to highly
successful satellite missions of the European Space Agency.

“We hope that NASA will launch the U.S. InSAR satellites to
monitor surface changes in California and elsewhere,” Fialko
said. “This will dramatically improve our ability to study
earthquakes as well as other potentially hazardous phenomena,
such as volcanic activity and man-made deformation.”

The research was supported by the Southern California
Earthquake Center and the National Science Foundation (NSF).
Synthetic aperture radar data were purchased with funding
from NASA, the U.S. Geological Survey, and NSF.