Geophysicists find
puzzling new faults in Mojave
BY DAWN LEVY
Move over, San Andreas.
There's a new fault on the block. At the meeting of the
American Geophysical Union in December, Stanford
scientists presented evidence that geophysical forces are
creating new faults in the Mojave Desert. What's more,
the faults ruptured in patterns that puzzled scientists,
who expected earthquakes along the established north-west
and east-west lines of mapped, mature faults. Instead,
they saw ruptures both along old faults and along new,
previously unrecognized faults that ran north-south and
north-east and cut across many old faults. Some of the
ruptures broke the Earth's surface; many did not.
"What is really
striking to us about these earthquakes is that they
rupture faults with multiple orientations," says
Greg Beroza, associate professor of geophysics.
In the past 67 years,
seven moderate-to-large quakes have traced a
100-kilometer (62-mile) line through the Mojave. Their
magnitudes ranged from 5.4 to 7.4. The largest was the
1992 Landers earthquake; the latest was the Hector Mine
quake in October 1999.
"It has been a
misconception that faults in the Mojave could not produce
such large earthquakes," says Hagai Ron, visiting
professor of geophysics from the Geophysical Institute of
Israel and Hebrew University of Jerusalem.
Faulting at Hector Mine,
which is near Twentynine Palms, Calif., occurred entirely
within the U.S. Marine Corps Air Ground Combat Center.
While most Mojave faults traverse similarly sparsely
populated areas, at least six major faults cross
California's well-traveled Interstate 40. The new faults
cut across old ones that have rotated into orientations
that make slippage difficult.
Quakes happen when faults
slip. In a single earthquake, more than one fault may
rupture, but the ruptures may not manifest themselves
along a straight line. "The different orientations
of faults are clear from the surface rupture patterns and
from the aftershocks, and that's what we're trying to
understand," Beroza says.
Ron, Beroza and geophysics
Professor Amos Nur have developed a simple model that
invokes fault-block rotation to explain nonlinear rupture
patterns after a single earthquake. The model may explain
the behavior of Mojave earthquakes in the past few
million years.
The Stanford geophysicists
employ Coulomb friction theory to predict the orientation
of faults that form in the Earth's crust in response to
stress. The orientation of a fault depends on the
material properties and the relative magnitude of stress.
The theory predicts, and experiments have shown, that
faults usually develop at an angle of 30 degrees relative
to the direction of the stress. Therefore, Ron says, only
stress acting at certain angles can activate faults.
In the simplest analogy,
think of the Earth's surface as a net: Tug on one corner,
and all points in the net move. A fault slips at point A,
and points B through Z move in response.
Now imagine a slightly
more complicated analogy to demonstrate how geophysical
forces change as land shifts. Just like a row of books
will start to lean when the bookends that support it are
removed, fault blocks of the Earth's crust can rotate and
deform. And just like a row of books that slides too far
to one side, blocks of crust can eventually stop slipping
and lock up.
For further crustal
deformation, a new set of faults must develop.
"Formation of new faults is not a well-accepted
concept," admits Ron. "But things have to be in
equilibrium before and after an earthquake."
"We seem to be the
only group that is seeking to explain the pattern of
faulting seen in the Mojave with a mechanical
model," Beroza says.
"Adversaries say the
Earth is heterogeneous and things are complicated,"
Ron adds. "But we believe stress is pretty much
uniform over an area like the Mojave."
Old faults in the Mojave
may be 5 million to 6 million years old, whereas new
faults may be only 10,000 years old. Because of the
mechanics of faulting, new faults require greater force
to slip than old ones. (There's more friction on a new
fault, so greater stress is needed to shear the rock and
drive slippage.) But both fault sets can slip
simultaneously when activity shifts from old to new
faults, Ron says.
As faults rotate and age,
they become unfavorably oriented relative to stress and
therefore less able to accommodate slippage. Older faults
do not need much slip to rotate them to the angle --
about 60 degrees -- where they "lock up," Ron
says.
Can new faults eventually
stop old faults dead in their tracks? "Ask us in
100,000 years," Beroza says.
Scientists use two key
methods to prove a fault has rotated. First, they can
locate sedimentary rock, which is assumed to have formed
when sediment deposited horizontally, and observe tilts
in enormous sections.
Second, they can take
paleomagnetic measurements -- that is, measure the
direction of the magnetic field in a rock. "The
Earth is a sphere, so we cannot tell if the crust is
rotated relative to a point without an absolute frame of
reference," explains Ron. "Our absolute frame
of reference is the Earth's magnetic field because it
always runs north to south. When rock is being formed
from lava or even sediments, it contains magnetic grains.
When lava cools or sediment deposits, the grains acquire
the magnetic properties of the field and align parallel
to the Earth's magnetic field, which is always
north-south." In essence, Mother Nature has
implanted tiny compass magnets in the rock.
"We can collect
samples from any rock containing enough ferromagnetic
material and measure the direction of these frozen
compasses," Ron says. "This direction tells us
how much a block of the Earth's crust has been rotated
and in what sense -- say, clockwise or
counterclockwise."
This technique revealed
that older east-west faults in the Mojave have rotated
clockwise 40 to 50 degrees, Ron says. This rotation
effectively locks them up and sets the stage for the
formation of new faults.
"Fault ruptures tell
us about how faults work," Beroza says. But the
lessons he, Ron and Nur unearth may extend well beyond
the Mojave. New faults are forming around the world, and
maps of surface faults alone may not provide enough
information for scientists and engineers to adequately
assess earthquake hazards. They will have to examine
evolving geophysical forces to obtain a truer,
three-dimensional picture of seismic risk.
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