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A magnitude-7.1 earthquake that shook California's Mojave Desert in 1999 also
unraveled some of the mysteries of Earth's mantle flow. Using satellite imagery,
a team of U.S. Geological Survey scientists analyzed deformations in Earth's
crust as a result of the Hector Mine quake. They came to the surprising conclusion
that the upper mantle is more plastic and flexible than the lower crust.
Fred Pollitz, Chuck Wicks and Wayne Thatcher of the USGS in Menlo Park, Calif.,
report their findings in the Sept. 7 Science. They comment that the mechanics
of stress in Earth's crust have remained an enigma. But their timely use of
technology to record the earthquake's aftereffects has produced valuable information
that could help predict crustal motions in response to future quakes.
Pollitz and colleagues analyzed the land's readjustment after the Hector Mine
earthquake using Interferometric Synthetic Aperture Radar (InSAR) and the Global
Positioning System (GPS). For nine months following the quake, InSAR recorded
distance changes between Earth's surface and an orbiting satellite. The authors
used three sets of InSAR images representing the crust's topography at three
intervals throughout the nine months to understand regional land changes. They
found that the velocity of uplift and subsidence after the Landers earthquake
was three to four times greater than before it.
Why? Because after the Hector Mine earthquake, the shallow upper mantle - about
19 miles down - flowed like a fluid.
The rupture of the Bullion and Lavic Lake faults in the Mojave Desert caused
the Hector Mine quake. These fractures run roughly parallel to and about 50
miles west of the San Andreas fault and are two of many northwest-trending,
right-lateral, strike-slip faults within the eastern California shear zone.
For thousands of years, pressure has built under these faults, finally breaking
in 1992 at the Lavic Lake fault, resulting in the Landers earthquake - magnitude-7.3.
Seismologists think that this quake triggered the rupture of the Bullion fault
and Hector Mine quake seven years later. The two large quakes caused significant
changes in regional subsidence and uplift but, luckily, nearby communities like
Barstow, Calif., suffered little damage because of their sparse population.
Jann Vendetti
See a related story, "After the
Quake, Into the Mantle," in the January 2001 Geotimes.
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