March 5, 1999
Scientists Identify a Major Fault Under Los Angeles That Could Produce Quake
By SANDRA BLAKESLEE
Using information once jealously guarded by oil and gas companies,
scientists have for the first time identified a major fault under
downtown Los Angeles of a kind whose existence there they previously
could only infer.
They say it is capable of producing a devastating earthquake.
Scientists have for years believed that such faults, called blind
thrusts, exist beneath the city. But this is the first solid proof
that one is actually present, said Dr. James Dolan, a professor of
geology at the University of Southern California.
The research adds support to the theory, emerging in the last decade,
that Los Angeles is threatened more by faults like this one, which
lie concealed underground right inside the city, than by the famous
and much larger San Andreas fault, a visible scar running across the
landscape 60 miles east of downtown.
Asked how many blind thrust faults might lie beneath Los Angeles,
experts said yesterday that they did not know. But Dr. Dolan said
there was unlikely to be another the size of the newly identified
one, which would measure 250 square miles on the surface and is
believed to be at least nine miles deep.
It is "the poster child of blind thrust faults underneath Los
Angeles," he said.
Dr. Dolan, who was not among the researchers, said they had "nailed
its location" and determined that it was under increasing strain.
Dr. Tom Henyey, co-director of the Southern California Earthquake
Center, a consortium of academic and government insitutions with
headquarters at U.S.C., said the finding would add to the data that
scientists and policy makers consider in evaluating building codes
and the design of crucial complexes like hospitals.
The fault was identified by Dr. John Shaw, a structural geologist at
Harvard University, and Dr. Peter Shearer, a seismologist at the
Scripps Institute of Oceanography in San Diego.
Their work is described in Friday's issue of the journal Science.
Thrust faults, which exist elsewhere and have long been thought to
lie beneath Los Angeles, in part because of surface features found
within the city, are so called because of the activity they induce in
In a quake caused by what is known as a slip-strike fault, like the
San Andreas, the opposing blocks of earth slide past each other
horizontally. But in a quake caused by a thrust fault, whose fracture
of the earth dips along at angles that can be either shallow or
steep, the blocks move diagonally, almost vertically.
Such movement may cause rocks to fold just below the surface. Or it
may create small hills on the surface. But that evidence of a quake
can easily erode away, or be bulldozed to make room for apartment
buildings, leaving the fault invisible, or "blind."
The blind thrust faults of Los Angeles, Dr. Shaw explained, are
caused by the relative motion of the North American and Pacific
plates, two huge masses of crust that are sliding past each other
along the San Andreas fault. Because of a dogleg in the fault, the
Los Angeles basin is being squeezed like an accordion between these
masses of earth.
The ground under the city is compressing by a fifth of an inch a
year, Dr. Shaw said, and some of that stress is building up on one
fault or another somewhere out of sight.
The first sign that the Los Angeles basin had potentially dangerous
blind thrust faults came in 1987 with the Whittier Narrows
earthquake, a magnitude-6 temblor that killed eight people and caused
property damage of $358 million. No fault had been mapped in the area.
The magnitude-6.7 Northridge earthquake of 1994, also on a blind
thrust fault that had not been mapped, killed 33 people and caused
damage worth $35 billion.
But when scientists began looking for blind thrust faults several
years ago, Dr. Dolan said, they could only infer the existence of
them from folds in sand and gravel layers near the surface. The
largest of these earlier, inferred faults is called the Elysian Park
fault, which, like the newly identified one, apparently runs under
parts of downtown. But its actual size is not known.
To find blind faults, Dr. Shaw said, scientists must identify
geologic structures underground, a task with which the energy
industry is quite familiar. Petroleum companies have been drilling
and exploring in the Los Angeles basin for decades, seeking pockets
of oil and gas. The methods they use can also be employed to gain
evidence of faults: setting off small explosions on the surface and
measuring the way sound waves propagate through the ground, or
measuring the velocity of sound in rocks during oil drilling.
The oil and gas companies long kept their findings secret, however,
regarding them as highly proprietary. But Dr. Shaw, who once worked
for Texaco as a petroleum geologist, said major petroleum companies
were now pulling out of Los Angeles because of strict environmental
regulations and the depletion of oil and gas there.
The companies are now willing to turn over their old data to
researchers, he said, in return for improved techniques that will
help them search for oil and gas elsewhere.
It was oil company information, Dr. Shaw said, that led to the
discovery of the newly identified fault, called the Puente Hills
blind thrust because it lies along the Puente Hills near the town of
Whittier, in Orange County. The fracture, he said, showed up clearly
on 10 seismic profiles -- so clearly, in fact, that distinctive
characteristics could be discerned on each side of the fault.
The fault runs east-west for about 25 miles from Century City to
Coyote Hills in northern Orange County. If it came to the surface, it
would be about 10 miles wide. Underground, the entire fault dips at
about 27 degrees to the north and appears to form three distinct
segments, termed Los Angeles, Santa Fe Springs and Coyote Hills.
Each fault segment is capable of generating a Northridge-size
earthquake, Dr. Shaw said. And if all three segments ruptured
simultaneously, a much more damaging, magnitude-7 quake could occur.
One of the primary goals of earthquake research is to be able to
reliably predict events. While the fullfillment of this goal is
currently out of reach, we can at least, make estimates of the
probability of earthquakes, given certain circumstances.
The following list details some of the computed probabilities for
events in Southern California.
PROBABILITY OF AN AFTERSHOCK ONE UNIT OF MAGNITUDE SMALLER,
(e.g. a M5 aftershock to a M6 mainshock):
within 1 day: 28%
within 7 days: 40%
after 7 days, but within 1 year: 26% PROBABILITY OF A LARGER
EARTHQUAKE AT THE SAME PLACE:
within one hour: 1.5%
within three days: 6% PROBABILITY OF AN EARTHQUAKE ONE UNIT OF
(e.g. a M6 mainshock to a M5 foreshock at the same place:
within three days: 1% PROBABILITIES OF A FUTURE LARGE EARTHQUAKE
ARE HIGHER NEAR A FAULT LARGE ENOUGH TO PRODUCE A BIG EARTHQUAKE
(e.g. the San Andreas).
Probability of a M7.5 earthquake in the three days after a M6 in the
southern Coachella Valley: 30%