THE ORIGINS
OF KILLER
QUAKES
A FEVERISH SEARCH FOR DANGER SIGNS
Adherents of the theory of plate tectonics, which
revolutionized earth science during the 1960s, say
that the world's surface is a mosaic of rigid plates
floating slowly around a semimolten interior. Some
plates are giant slabs of ocean floor, others are a
combination of continent and ocean floor. The
constantly moving plates sometimes glide past each
other with relative ease. But they occasionally grind
together like ice floes, sometimes sticking together,
with pressure building up, then breaking free with
explosive force, causing an earthquake. The San
Andreas Fault, where the Pacific Ocean plate rubs
against the North American plate, is particularly
susceptible to this phenomenon. Scientists say that
the way the plates in that region sometimes grind
against each other is responsible for the earthquakes
that regularly erupt in California.
The main fault line, which stretches 800 miles
almost along the entire length of California and
reaches within one mile of San Francisco and 30
miles of Los Angeles, is the best understood of all
fractures in the earth's crust. The San Andreas Fault
moves at a rate of 1.4
inches each year, occasionally producing substantial
quakes. But that is where certainty stops. California
actually has a spider's web of smaller cracks-not all of
them visible branching off from the San Andreas
Fault, all with different rates of movement. And two
years ago, seismologists discovered a major hidden
group of subterranean faults in the Los Angeles basin
that constitute a whole new class of earthquake
hazards. They have added a host of imponderables to
the inexact science of earthquake prediction.
Foreshocks: Dieter Weichert, acting director
of the Pacific Geoscience Centre near Victoria, said
that the number of major earthquakes that scientists
have been able to predict accurately is "negligible."
There are various signs that could signal a quake, he
added, but none of them is definite. One sign is a
series of smaller foreshocks in an area where that
kind of activity is unusual. Foreshocks alerted
scientists to the China quake of 1975, but those
warning signals are easily overlooked in California,
which experiences thousands of tremors every year.
As for last week's earthquake, seismologists said that
there were one or two
shocks that in hindsight could have been interpreted
as foreshocks, but they were not particularly unusual
for the area and did not appear to be cause for alarm.
Severity: Weichert added that when the ground
begins to rise-usually by a few inches-in the vicinity
of a fault, it can be a sign that a quake is imminent.
But that phenomenon, too, offers no certainty that
an earthquake will occur, nor of the likely severity of
an upheaval. As a result, most scientists rely on the
seismic gap theory, which was developed at New
York City's Columbia University during the 1970s.
According to that theory, the likeliest place for an
earthquake to occur is the spot along a fault that has
been quietest for the longest time, building up the
most tension.
Using that reasoning, the U.S. Geological Survey
predicted in 1988 that there was a 60per-cent chance
that another so-called great quake-which would
dwarf last week's upheaval-would strike southern
California within 30 years. Clarence Allen, a
seismologist at the California Institute of
Technology in Pasadena, said that the highest
probability of such a quake appears to be in the
Coachella Valley, south of Palm Springs, Calif. That
segment of the San Andreas Fault has been dormant
for at least 196 years, far longer than most others.
Rupture: A "great quake" measures more than
8 on the open-ended Richter scale (a system devised
in 1935 by American seismologist Charles Richter),
with each succeeding number representing a tenfold
increase in the strength of the tremor. California has
had only two quakes of that magnitude in recent
history: one in 1857 that produced a 180-mile-long
rupture in the Mojave Desert, east of Los Angeles,
and the San Francisco earthquake of 1906, which
scientists estimate would have measured 8.3 on the
Richter scale and which killed 700 people, many of
them from a disastrous fire that followed the quake.