TIME: Almanac 1993/TIME Almanac 1993.iso/time/103089/10308900.053

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Still Waiting for the Big One


The science of quake prediction is improving, but don't bet on
it

By J. Madeleine Nash


    Even as the earth rocked and rolled, California's army of
seismologists rallied into action. In Berkeley, University of
California graduate student Anthony Lomax felt the sidewalk
shiver and watched telephone poles sway, then rushed to his
seismographic station. "The instruments were off-scale!" he
marveled. Within minutes the scientists on duty had pinpointed
the epicenter of the quake in the rugged Santa Cruz mountains
some 50 miles away. The spot was no surprise: it lay on the San
Andreas fault, a great gash in the earth that extends nearly the
length of the California coast. Even before the quake, the Santa
Cruz area had been identified as a prime candidate for a big
tremor. "We still can't predict when an earthquake will occur,"
says geologist Clarence Allen of the California Institute of
Technology, "but at least we can say where an earthquake is most
likely."

    As aftershocks jolted the area, geologists fanned out into
the mountains to look for changes wrought by the quake. They
examined winding roads for fractures and shot laser beams across
the fault to measure expected shifts in terrain.

    From the start, scientists had a firm answer to the
question uppermost in every Californian's mind: the earthquake
that hit San Francisco last week was not the long-feared Big
One. While it packed a punch, measuring 6.9 on the Richter
scale,* the 1906 earthquake was 25 times as strong, at 8.3.
Warns Dallas Peck, director of the U.S. Geological Survey: "The
question is not whether a bigger earthquake is coming. The
question is when."

    This quake did not begin to exhaust the pent-up energy in
the 800-mile-long San Andreas system. In a list of seismic
danger zones compiled by an expert panel last year, the section
around Santa Cruz ranked only sixth. The area believed most
likely to have a devastating quake in the next three decades
lies near Palm Springs.

    Like a river with multiple tributaries, the San Andreas is
associated with numerous lesser faults, among them the Hayward
fault, which undercuts Berkeley and Oakland, and the San
Jacinto fault, near San Bernardino. Some parts of the San
Andreas are more dangerous than others. One segment that lies
to the south of the Santa Cruz mountains does not appear prone
to large jolts at all. "It just creeps along," says geophysicist
Ross Stein of the USGS. "Probably there's some remarkable
material down there that, like talcum powder, acts as a
lubricant."

    The earth is constantly moving underfoot. Its surface,
cracked like ancient pottery, is broken into 15 large pieces.
These pieces of crust, called plates, restlessly roam about,
driven by plumes of molten rock that roil up from the planet's
superheated core. Many of the world's largest earthquakes occur
at the boundaries of such plates. The San Andreas fault system
divides the Pacific plate and the North American plate, which
grind past each other at the pace of 2 in. a year. But this
movement of the plates is not uniform. Along fault zones the
plates tend to become "locked," resisting the overall motion.
Explains Berkeley seismologist Robert Uhrhammer: "Stress builds
up in these areas that are in effect welded shut. It's as if the
rock were being stretched like a big rubber sheet." At a certain
point the rock snaps, allowing the plates to slip and release
stress. The result is an earthquake.

    During the 1906 tremor, the plates on either side of the
San Andreas lurched past each other by as much as 20 ft. Over
time, such jumps add up. "In 30 million years," Berkeley
seismologist Bruce Bolt says, "Los Angeles will become a new
suburb of San Francisco."

    Even though the mechanics of earthquakes are understood,
accurate prediction of their occurrence has remained beyond
reach. Earthquake forecasting is mostly based on past history.
If a fault once generated a big earthquake, it can be assumed
that it will do so again. But just where and when will the next
big break occur? Here scientists are beginning to make headway.
Geophysicist Wayne Thatcher of the USGS notes that the 1906
quake ruptured a 260-mile-long section of the San Andreas,
extending from Cape Mendocino to San Juan Bautista. But the
plate movement along the southern portion of the rupture was
minor compared with the far greater movement in the north. To
Christopher Scholz of Columbia University's Lamont-Doherty
Geological Observatory, this meant one thing: the southern
section of the quake zone had retained an enormous amount of
stress. "It was," he says, "ready to go." And last week it did.

    Still, there is a vast difference between suggesting that
an earthquake is likely to happen and pinpointing when. For
now, scientists cannot say whether a specific section of the San
Andreas fault will snap in one year's time or in a hundred, but
they are working on it. Seismic silence is one clue. Soundings
taken along the San Andreas over the past 15 years showed that
the small earthquakes that are a daily event along other parts
of the system were not occurring in the Santa Cruz mountains.
Scientists argued over the significance of this blank spot in
the data. Then a year ago, activity ominously resumed, and last
August brought a damaging earthquake. Such an increase in
activity, notes Columbia's Scholz, seems to indicate that stress
has built up to the point where a major release is imminent.

    Halfway between San Francisco and Los Angeles, near the
tiny town of Parkfield, scientists are conducting an experiment
that they hope will open the door to a new era of earthquake
prediction. Along a 20-mile section of the San Andreas,
researchers have sunk strain gauges up to 1,000 ft. deep into
the earth and laced the surface with "creep meters" that measure
rock movement. "We're listening to the heartbeat of this section
of the fault very, very closely," says the Geological Survey's
Thatcher. The Parkfield section of the San Andreas is unusual
in that it is the Old Faithful of earthquake zones, generating
moderate tremors every 20 to 27 years. The last Parkfield
earthquake occurred in 1966, which means that the next one
should strike between now and 1993. By keeping detailed track
of underground changes over time, scientists hope to identify
precise signals that an earthquake is imminent.

    The size of an earthquake is determined partly by the
length of the fault segment that slips. In addition, large
earthquakes tend to be spaced further apart than small ones,
since it takes a much longer time to accumulate sufficient
stress. While scientists cannot say exactly where or when the
next Big One will hit, they are not without hunches. Southern
California, which has not had a Big One since 1857, is every
seismologist's first bet.

    Still, as last week's quake in San Francisco demonstrated
only too well, it does not take a Big One to deal a lethal blow.
For this reason, some geologists think that the Big One has
been overemphasized as a near-term threat. There are faults up
and down the California coast capable of equaling the latest
quake, and that is enough reason to worry. Likely candidates for
significant quakes in Northern California include not only
Berkeley and Oakland but also the Silicon Valley. The Los
Angeles Basin, for its part, has experienced an increase in
small earthquakes, which many seismologists find alarming. The
message from Mother Nature seems unmistakable: Be prepared.