Puente Hills fault system could set off a devastating Quake
is a broad, rectangular area 25 miles long and 15 miles wide that
stretches from near Whittier to downtown L.A. Seismologists say it's
capable of producing a magnitude 7.5 temblor.
week's predawn earthquake in Pico Rivera woke up people around Southern
California, but the magnitude 4.4 temblor didn't cause major damage.
scientists said the quake should serve as a warning about a fault that
runs through a large swath of densely populated Los Angeles County and
is capable of producing a devastating, magnitude 7.5 quake.
The Puente Hills thrust fault is less well-known than other faults such as the San Andreas and Newport-Inglewood.
But there is growing scientific evidence that it is particularly dangerous.
part, that's because it runs under heavily populated areas. But another
factor involves the shape and direction of the fault, which experts
said would send the strongest shaking during a major temblor toward
downtown Los Angeles.
It is a broad, rectangular area 25 miles
long and 15 miles wide, stretching from the Puente Hills near Whittier
through downtown L.A., USC and Dodger Stadium, before veering west
toward Beverly Hills.
In the 1970s, seismologists had thought that
the worst quake to hit downtown would be a magnitude 5. But that all
changed in 1999 when they discovered the Puente Hills fault. Experts now
believe the 1987 Whittier Narrows quake, which killed eight people,
occurred along this fault.
The surface of the Puente Hills fault
plane cuts through the buried rocks like an angled sheet of paper, with
its highest edge slanting up to the west and its deepest to the east. It
lies about 2 miles under Dodger Stadium and 4 miles below downtown
L.A.'s high-rise district. Its lowest point is 9 miles underground,
north of Whittier.
"Any place that's over the rupture is likely to get strong shaking," said Caltech seismologist Kate Hutton.
problem is that the shape of the Puente Hills fault system funnels
energy toward Los Angeles' densest neighborhoods. Video simulations show
energy from a quake erupting, with the strongest waves rippling to the
west and south across the Los Angeles Basin.
By contrast, the
magnitude 6.7 Northridge earthquake in 1994 that killed 60 people
channeled its strongest shaking north to sparsely populated mountains.
Another problem in a Puente Hills fault quake is that the soft sediment underneath the L.A. Basin amplifies the quake's energy.
models developed in 2005 showed that a 7.5 quake under downtown L.A.
could kill as many as 18,000 people and cause as much as $250 billion in
damage if it occurred on a weekday afternoon. Such a quake would be 15
times more powerful than the Northridge earthquake. It is impossible to
say when such a large quake could occur.
When the Puente Hills
thrust fault shakes, one side of the fault moves up while the other side
slips below it. This fault is entirely underground, and there is no
visible scarring on the surface.
The Puente Hills fault system
was previously unknown to scientists when it ruptured in 1987, causing
the magnitude 5.9 Whittier Narrows earthquake that caused $358 million
in damage. Scientists discovered the fault in 1999.
years, experts have sounded the alarm about it. “This is the fault that
could eat L.A.,” seismologist Sue Hough told The Times in 2003.
geologist Edward Field of the Southern California Earthquake Center in
2005: “It’s one of the worst disaster scenarios you could imagine for
the United States.”