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From the "JPL Universe"
December 18, 1992

Earthquake fault motion observed in videotape produced by JPL geologist
By Mary Hardin

     Using satellite images taken before and after the June 28
Landers earthquake, JPL geologist Dr. Robert Crippen has produced
a video which shows the motion of the Emerson and Homestead
Valley faults in the Mojave Desert.
     "This is the first time fault motion has been observed
through the use of images acquired from space," said Crippen,
from the Tectonics and Geophysics Group, Geology and Planetology
Section 326. "The observation from space of newly fractured
ground along fault zones is also believed to be a first," he
added.
     Crippen used images from the French Satellite Pour
l'Observation de la Terre (SPOT) acquired under a data exchange
agreement between NASA and the French space agency, Centre
National d'Etudes Spatiales (CNES).
     The technique used to create the fault motion video is
similar to how weather satellite images are used to show cloud
motion on television weather reports.
     Crippen took a "before" SPOT image of the Landers quake site
that was taken on July 27, 1991, nearly a year before the
magnitude-7.5 earthquake, and he then matched that up with an
"after" image that was taken on July 25, 1992, only 27 days after
the quake.
     In order to minimize distortions, he said, it was important
to use matching images that were taken from the same orbital
position and on nearly the same day of year, to achieve the same
seasonal effects and sun angle.
     After precisely lining up enlarged portions of the images on
a computer display, Crippen flickered between the two and
observed the differing ground motions across each of the faults.
He repeated this process with other parts of the images taken of
several different sites along the faults, and in some cases, he
observed newly formed cracks in the fault zones.
     Working with JPL colleague Dr. Ronald Blom, Crippen is also
using the images as digital maps that show where the ground was
before the earthquake and where it ended up after the quake. By
using a supercomputer, the scientists hope to measure ground
motions at a level of geographic detail that has never been
achieved before, Crippen said.
     "By observing the details of fault motions we hope to better
understand the mechanisms of fault breakage and ultimately,
contribute to the understanding of the threats that earthquakes
pose to society," Crippen explained.
     Crippen presented his finding last week at the fall meeting
of the American Geophysical Union in San Francisco.
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