California’s Quakes​—When Will The Big One Come?

THE ground swayed. Gas lines snapped. Buildings collapsed. Fires raged. Was this the recent Los Angeles earthquake? No. It was the quake that struck San Francisco on April 18, 1906. That quake, and the three-​day fire that followed it, destroyed 512 blocks in the center of town and took some 700 lives.

What causes such catastrophes?

Scientists try to explain by using the theory of plate tectonics. They say that earth’s crust lies on some 20 rigid plates, or slabs, of rock that move slowly, sliding past and at certain points under one another. The Pacific plate slides slowly northward, past the North American plate. The zone of slippage between these two plates is called the San Andreas Fault. It stretches northward some 650 miles, from the head of the Gulf of California, disappearing into the Pacific Ocean near San Francisco.

These plates move very slowly, at something like the rate at which your fingernails grow​—an inch or so a year. Over many years stress builds when the plates stick as they try to slide past each other. Then they may break free with explosive force.

The San Andreas Fault passes 33 miles [53 km] northeast of Los Angeles and passes into the Pacific Ocean near San Francisco. Is it any wonder that Californians are concerned about the so-​called Big One?

San Francisco

After the 1906 quake, the northern end of the San Andreas Fault remained relatively quiet. Then, at 5:04 p.m. on October 17, 1989, an estimated 50 million Americans were glued to their TVs to watch baseball’s World Series from San Francisco. Suddenly, the cameras started to bounce. Some 60 miles [100 km] south of San Francisco the two sides of the San Andreas Fault had shot past each other, causing a quake that killed 63 people, smashed freeways, crushed automobiles, and left thousands homeless. But that temblor was far less powerful than the predicted magnitude 8 for the anticipated Big One.a

Back in the spring of 1985, the U.S. Geological Survey predicted that a quake of magnitude 6 would occur within five years of 1988 near the small town of Parkfield, about halfway between Los Angeles and San Francisco. By studying ground movement in advance of this anticipated quake, they hoped to learn how to predict earthquakes and to be able to give a warning perhaps hours or even days before a quake strikes. This study cost $15 million, but the quake never occurred. As William Ellsworth of the U.S. Geological Survey once said, “the interpretation of seismic patterns is an inexact science.”

Landers Quake

Thus, on June 28, 1992, no one expected a magnitude 7.5 quake to strike in a sparsely settled region near Landers, in southern California’s Mojave Desert. Of this quake Time magazine said: “In a few fearsome seconds, it rerouted roads, realigned parking lots and reconfigured the landscape in countless capricious ways, miraculously taking only one life.” For a quake of this magnitude, the damage was slight.

So this too was not the Big One. In fact, it was not even on the San Andreas Fault but on one of the smaller faults that surround it.

It is possible, however, that the Landers quake, along with a smaller one at nearby Big Bear, may have awakened nearby sections of the San Andreas. Scientists have said the stuck plates along the southernmost section of the San Andreas have a 40 percent chance of snapping loose sometime within the next 30 years. That might trigger the long-​feared Big One, at magnitude 8 some five times as powerful as that at Landers.

Los Angeles

Then, on January 17 of this year, Los Angeles was jolted awake at 4:31 a.m. Some 11 miles [18 km] beneath the surface of heavily populated San Fernando Valley in Los Angeles, a patch of rock is thought to have slipped some 18 feet [5.5 m] along a deeply buried fault. This ten-​second jolt of magnitude 6.6 took at least 57 lives. Tragically, 16 people died in one collapsing apartment building. A man who survived was trapped for eight hours under 20 tons of concrete in a fallen parking structure. A freeway collapse severed the city’s main route to the north. Churches, schools, stores, and a major hospital closed. As is frequently the case, lower-​income families suffered the most because of living in older buildings that had been constructed before modern earthquake codes were established.

This quake demonstrated the problems that may develop with even smaller local faults directly under a major city. As far as people are concerned, any earthquake is the Big One to them if they are sleeping on top of it!

The destruction would have been far greater had it not been for strict local building codes. Each earthquake teaches lessons that may make it easier next time. Some freeway overpasses that had been strengthened after previous quakes survived this one; others did not. But the real test will come if a greater earthquake​—a really big one—​strikes near a major city. Los Angeles again, maybe?

A Second Big One to Come?

‘Oh, No! Not another one! One’s too much!’ Nevertheless, another big one is what some geologists see in the offing. New Scientist magazine, January 22, 1994, said: “Dangerous fault lines that run beneath Los Angeles could cause a ‘Big One’ every bit as devastating as the one expected on the San Andreas fault, experts warn. . . . The Los Angeles basin is particularly rich in thrust faults, because the San Andreas fault​—which at most places runs north to south through the state—​kinks to the west at Los Angeles, causing extra stresses at that point. Somehow, the migrating land on the Pacific plate must get past that bend and continue its ride northwards.”

As the Pacific plate moved, geologists think, the network of thrust faults was created in the Los Angeles basin, one of which was responsible for the quake felt there earlier this year. Concerning that quake New Scientist magazine followed up its first report with this one a week later: “Scientists still believe the fault responsible is a thrust fault​—one where a slab of rock slides up and over another. During last week’s quake, the Santa Susana Mountains north of the epicentre were lifted at least 40 centimetres [16 inches] and simultaneously moved 15 centimetres [6 inches] northward.”

Kerry Sieh, a geologist from Caltech, feels that the smaller thrust faults crisscrossing the Los Angeles basin could be just as dangerous as the still-​expected magnitude 8 on the San Andreas. Sieh then asks with Los Angeles in mind: “Is it possible we could get a really big one, a magnitude 8, under downtown?” A frightening question, considering the millions sitting on top of it!

Californians seem to live with earthquakes, as other people live with hurricanes, floods, or tornadoes.

[Footnotes]

[Map on page 16]

(For fully formatted text, see publication)

Thrust fault lines in the Los Angeles basin

San Andreas Fault

Los Angeles

PACIFIC OCEAN

[Picture on page 15]

Freeway damage left by the 1994 Los Angeles quake

[Credit Line]

Hans Gutknecht/​Los Angeles Daily News

[Picture on page 17]

Flames erupting from a gas line snapped by the 1994 quake

[Credit Line]

Tina Gerson/​Los Angeles Daily News

[Picture on page 18]

This collapsed portion of a Los Angeles freeway was left in the wake of a ten-​second jolt of magnitude 6.6

[Credit Line]

Gene Blevins/​Los Angeles Daily News