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                                                                                SCIENCE, Page 54

News from the Underground


Recent quakes in California may have stirred the San Andreas
Fault, setting it up for the long-dreaded Big One

BY J. MADELEINE NASH LANDERS


    "Talk about bad luck!" says Caltech geologist Brian
Wernicke, squinting through a telescopic eyepiece at an aerial
photo of Landers, California, a small town in the middle of the
Mojave desert. "Wham! Right through this house. Wham! Right
through that house. The funny thing is, there aren't that many
houses out here."

    In more ways than one, the earthquake that rumbled through
this desolate region on June 28 was an ominous force. In a few
fearsome seconds, it rerouted roads, realigned parking lots and
reconfigured the landscape in countless capricious ways,
miraculously taking only one life. Rather than rupture a single
fault line, it swiped a 70-km (45-mile) diagonal slash through
several, at one point heaving up a raw ridge of rock roughly the
size and shape of a stegosaurus' spine.

    For weeks afterward -- even this past week -- the region
has been rocked by thousands of nerve-racking aftershocks, and
the quake ignited mysterious swarms of smaller earthquakes in
volcanic zones hundreds of kilometers away. But most alarming
of all, this quake, the largest to hit Southern California in
40 years, appears to have substantially altered subterranean
stress fields. In the process, it may have awakened a fitfully
sleeping dragon -- the mighty San Andreas, the nation's biggest
and most dangerous fault.

    With a mixture of excitement and dread, scientists with
the U.S. Geological Survey in Pasadena are rushing to augment
an already extensive seismic network with portable
instrumentation. "Before the San Andreas goes," reflects
geologist Ken Hudnut somberly, "maybe we'll catch a precursor."
A hot wind swoops across the desert as Hudnut retrieves a
plastic box from under an oleander bush and pops the lid to
reveal the small satellite receiver it shields from blowing
sand. Nearby, a tripod-mounted antenna straddles a survey pipe
like a spindly sentinel. Coded signals beamed down by orbiting
satellites, Hudnut explains, serve to pinpoint the location of
the pipe. The slightest shift in the pipe's position, and Hudnut
will know the earth around it is on the move.

    The southernmost section of the San Andreas has made
scientists jumpy for some time now. Between 1948 and 1986, the
region adjacent to the fault experienced only one earthquake of
magnitude 5.8 or higher.* Since then there have been seven,
including the Landers quake, which weighed in at an impressive
7.5. Moreover, this surge in seismicity appears to be occurring
on a worrisome schedule. Excavations of old lake-bed sediments
by Caltech paleoseismologist Kerry Sieh in the mid-1980s
indicate that large earthquakes have roared through this section
of the San Andreas at not quite 300-year intervals. The last
such quake took place circa 1680. "It's just a gut feeling,"
ventures Sieh, who is 41 years old, "but I think I'll witness
a great earthquake on the southern San Andreas in my lifetime."

    About 1,300 km (800 miles) long, the San Andreas Fault
system separates two sections of the earth's crust known as
plates. Like giant rafts, these plates glide across an expanse
of superheated rock, viscous as tar, that surrounds the planet's
molten outer core. At the rate of nearly 5 cm (2 in.) a year,
the Pacific plate to the west of the San Andreas is slowly
pushing north, past the North American plate on the east. One
possible result: 60 million or so years from now, a sliver of
the California coast that includes the megalopolis of Los
Angeles could become beachfront property in Alaska.

    Getting there, however, will not be fun. The slip of the
plates is not constant along the fault. The southern San Andreas
bends like a river and splits into multiple branches. Because
of this contortion, the Pacific and North American plates cannot
slip in a straightforward way but must strain against each
other like two sumo wrestlers. The battle of the plates has
created numerous smaller fault lines along the San Andreas,
giving the region the look of a smashed windshield. Over the
millenniums, the Mojave shear zone to the east may offer a path
of less resistance to the giant plates and replace the San
Andreas as a new plate boundary, suggests geophysicist Amos Nur
of Stanford.

    Only four years ago, scientists gave the stuck plates
along the southernmost section of the San Andreas a 40% chance
of snapping sometime in the next 30 years. At the same time,
they warned that a rupture of this part of the fault could
trigger earthquakes along neighboring segments, possibly as far
west as San Bernardino and nearly as far north as Bakersfield.
Result: the long-feared Big One -- an earthquake of magnitude
8, five times as powerful as Landers -- on the doorstep of the
populous Los Angeles Basin. Now, in the seismic spoor of the
Landers earthquake, scientists have found reason to suspect that
the timetable for this disaster may have been fast-forwarded.

    A fateful chain reaction, seismologists believe, started
in April, when an earthquake of 6.3 magnitude rattled the
vicinity of Palm Springs and Joshua Tree National Monument. On
a map, the fault that was then broken looks like a shotgun
taking dead aim at Landers, and in fact it was. Two months
later, a minor earthquake started on a fault with no name. For
a few seconds, this temblor rattled at a magnitude of 3.
Suddenly, seismometer readings soared as the fracture unzipped
a sequence of larger faults nearby. Then three hours after the
Landers earthquake shivered to a stop, a 6.6 aftershock
terrified the environs of Big Bear Lake, collapsing chimneys and
toppling buildings.

    Why Big Bear? In recent weeks research teams at the U.S.
Geological Survey in Menlo Park have put this question to two
different computer models. The results, while differing in
detail, are strikingly similar. Before the effects of the
Landers earthquake are taken into account, neither model flags
the region around the Big Bear fault as particularly menacing.
But as soon as scientists factor in the degree of ground
movement and its direction, it pops up on their computer
screens, color-coded red for danger.

    Of late, the two teams have begun to use their computer
models to peer into the future. What they see in these high-tech
crystal balls is unsettling. "To relieve the stress Landers
placed on it," says geophysicist Ross Stein, "the southern San
Andreas would have to produce a 6.5-magnitude earthquake of its
own."

    The type of stress that has increased on the southern San
Andreas is known as shear stress. It runs parallel to the fault,
enhancing its tendency to slip. There is, however, another kind
of stress, clamping stress, which retards slippage. It runs
perpendicular to the fault, pinning the sides together like an
invisible row of staples. "The situation we worry about most,"
says UCLA geophysicist David Jackson, "is when the shear stress
increases and the clamping stress decreases. This is precisely
what we think has happened."

    The Landers and Big Bear earthquakes cut through faults
that form two sides of a triangle. When these faults fractured,
the huge block of earth contained within the triangle shifted
about a meter to the north, unclamping the San Andreas at the
triangle's base. In deference to the menace posed by this
singular geometry, Jackson calls the area the "Bermuda
Triangle."

    What everyone who lives in Southern California wants to
know, of course, is not whether the southern San Andreas is
going to slip, but when. To their frustration, scientists cannot
answer that. The most careful calculations of stress transfer
are based on the assumption that faults separate large blocks
of earth, which stretch and compact in predictable ways. But the
Southern California crust is so crisscrossed with faults that
the material between them may behave more like sand. "Squeeze
a block of wood," muses UCLA'S Jackson, "and it will become
longer. But sand will behave in unforeseen ways."

    In addition, scientists can only guess how much total
stress accumulated along the southern San Andreas prior to the
Landers earthquake. Geophysicist Geoffrey King of the Institut
de Physique du Globe in Strasbourg, France, compares the
predicament to trying to push a car uphill while blindfolded.
Will the car move or not? "One person won't accomplish much,"
he observes, "but 10 people might. Our problem is that we don't
know how many other people are already pushing on the car."

    Indeed, scientists still do not know how much stress is
required to start an earthquake in the first place. In
laboratory experiments, explains University of Nevada, Reno,
seismologist James Brune, two blocks of granite forced past each
other generate a tremendous frictional heat. But earthquakes
apparently do not. "Nature," says Brune, "has figured out an
easier way of moving things around." After all, when carpet
installers try to move a rug, they do not attempt to drag it all
at once. "Instead," says Tom Heaton of the U.S. Geological
Survey in Pasadena, "they put a little ripple in it. As the
ripple moves from one end of the rug to the other, the rug moves
with it."

    What might cause such a ripple to spread across a fault
remains a mystery. Numerous ideas have been suggested. Brune
believes sliding rock physically deforms like tires squealing
on pavement. In this case, what greases the skid is an invisible
air pad that prevents the two surfaces from establishing
frictional contact. Just last week, in a paper published by the
science journal Nature, a team of researchers from the U.S.
Geological Survey in Menlo Park offered an alternative
possibility. Groundwater, they theorized, trapped under high
pressure, might also serve to pry faults apart, allowing them
to slip with a minimum expenditure of energy.

    Yet another mechanism capable of inducing fracture has
been suggested by the Landers earthquake. Because the quake
triggered scores of sympathetic vibrations in volcanic and
geothermal regions, some scientists have speculated that the
Landers event shook underground magma chambers as though they
were big cans of soda. The gas that fizzed forth could, in turn,
have forced open a gap that eased the slip of surrounding rock.
Whatever the mechanism, experts agree, it has only hastened the
fracture of a fault zone that was already stressed up and ready
to go.

    What scientists fear is that the southern San Andreas has
reached a similarly critical threshold. "If the Landers
earthquake put a little stress on the San Andreas," exclaims
Allan Lindh, chief seismologist of the U.S. Geological Survey,
"then what about the accumulated stress of 300 years of plate
motion?" For Lindh and other experts, the Landers quake and its
resulting tremors are all too reminiscent of the increased
seismic activity that preceded the great San Francisco blowout
of 1906. "I mean," says Lindh, with a dramatic pause, "how much
more on the edge of our chairs can we be?"