Scientists Giving New Importance to ‘Thrust’ Faults

The conclusion of seismologists that the Oct. 1 Whittier earthquake occurred on the eastern end of a previously unrecognized subterranean “thrust” or “dip-slip” fault has intensified interest in a geologic feature of Southern California that was previously thought to be of little importance.

The as-yet unnamed fault is markedly different from the more thoroughly studied San Andreas and Newport-Inglewood faults that have been the previous focus of concern about earthquake hazards in Los Angeles. Moreover, the magnitude 5.9 earthquake has opened new debate over the accuracy of previous assessments of earthquake hazard in the area.

“I am much more concerned about a quake of sizable magnitude under downtown Los Angeles this month than I was,” said Kerry Sieh, a Caltech seismologist.

The newly identified fault stretches from Whittier through downtown Los Angeles and Santa Monica and out to sea off the coast of Malibu. Geologists now believe that at least four earthquakes of magnitude 5 or larger have occurred on this fault since 1929.

Additionally, scientists have in recent years concluded that the 1971 Sylmar earthquake and the 1983 Coalinga temblor occurred on similar undetected dip-slip faults lying far below the surface.

All this has forced researchers to rethink their views on the importance of such faults.

“I’m convinced that in the next few years there will be a big push to study thrust faults (in the Los Angeles Basin) and a much more concentrated effort,” said USC seismologist Egill Hauksson.

Since the San Francisco quake disaster of 1906, the study of earthquakes in California has focused on the state’s great surface faults, most of all the famous San Andreas. Such faults are readily visible, appearing as cracks in the earth, dislocations of stream beds, and “saddles” or low points in mountain ranges.

Researchers concluded that such strike-slip faults, in which earth on opposite sides of the fault scrape sideways past each other, would produce the great quakes of the future. Hundreds of lesser surface faults in the state were carefully mapped as well.

In contrast, dip-slip faults thrust the earth on one side upward as the crust attempts to climb over the adjacent soil. This process creates great folds on the surface, miles from the epicenter.

Even before the Whittier earthquake occurred, geologists were discovering that a number of similar deeply buried faults lie under the Los Angeles area. None has been reliably mapped to date.

A consulting geologist retained in February by the U.S. Geological Survey to prepare a subterranean cross-section of the land between Glendale and the Palos Verdes Peninsula, including geological structures underlying downtown Los Angeles, says he has now reached the tentative conclusion that a fault under downtown could produce a magnitude 6 plus earthquake every 125 to 225 years.

The Los Angeles geologist, Thom L. Davis, says in a report prepared for a December meeting of the American Geophysical Union that “deep-level thrusts that do not reach the surface . . . suggest the earthquake potential of the Los Angeles Basin has probably been underestimated.”

In interviews, Davis said his studies have shown that the Los Angeles Basin is slowly being compacted as the huge Pacific tectonic plate, which forms the western side of the San Andreas Fault, presses against the equally massive North American plate that forms the eastern side.

As recently as 3 million years ago, he said, the distance from the Palos Verdes Peninsula to the crest of the San Gabriel Mountains was 84 miles. Today, it is only 62 miles, indicating that the basin is shrinking about half an inch per year.

This compaction produces folding just like pushing on the edge of a throw rug with your foot. Hills in Elysian Park and Montebello are the visible tips of these folds, but most of the folds have been buried miles beneath the surface as silt has filled in the basin.

Davis added, “There’s a direct relationship between folding and faulting. There’s a big fold under downtown which peaks out just to the north in the hill line in Elysian Park. The fault underlying it is certainly capable of quakes in the 6 to 7 range.”

But others are not sure about Davis’ conclusions, particularly his estimate of how often earthquakes could occur in such faults.

Many seismologists argue that the geologic tensions along such a fault may be relieved very slowly by a process called “creep” rather than abruptly by an earthquake. If that is so, the fault would represent little hazard.

Thomas Heaton, scientist in charge of the Pasadena office of the U.S. Geological Survey, said in an interview that while it is clear that “there are faults in these cross-sections” and that “undoubtedly at some point there will be large earthquakes on them,” he does not believe there is sufficient knowledge now to make even tentative estimates of recurrence rates.

“Have these faults been active in the recent past?” Heaton asked. “And how much of their movement has been due to creep rather than earthquakes? We know many faults primarily creep. But we have very little to go on with some of these low-angle, thrust faults.”

Clarence Allen, a veteran Caltech earthquake expert, while saying that “in recent years, we’ve gained increased appreciation and understanding of these kinds of faults,” also questioned Davis’ conclusions, saying that while they are “intriguing, they will be subject to a lot of debate and discussion.”

“It’s a far cry from saying something is possible to saying how likely it is,” Allen said. “It’s possible a meteor will hit City Hall, but it’s not likely. A magnitude 7 quake under downtown Los Angeles is possible, but is it likely? I’d say no.”

It was the 6.7 Coalinga quake of 1983 that marked the real start of interest in thrust faults in California.

The Coalinga quake took place in an area of the San Joaquin Valley where no surface faulting had been observed before and came as a surprise to scientists. Later studies concluded that a deeply buried thrust fault had been responsible for the temblor.

Over many thousands of years, according to the studies, it has been pushing up an anticline, or fold, in the Coalinga area. There are other anticlines nearby that also are believed to be quake-related.

Ross Stein, a U.S. Geological Survey seismologist who studied the Coalinga quake, wrote later in Science magazine: “Apparently folds provide as good an indicator of earthquake sources as do faults.”

Since this and other similar analyses, a certain rush has been on to discover other places in California where the existence of such folds might indicate the potential for serious earthquakes.

And some major quakes have been re-evaluated. Both the 7.7 Tehachapi earthquake of 1952 and the 6.4 Sylmar-San Fernando earthquake of 1971 are now recognized to have been generated on subterranean thrust faults underlying large folds. Davis even suggests that the 6.3 Long Beach quake of 1933 may have been primarily of the thrust variety.

Stein remarks, “In this country, these hazards have been underestimated. Many earthquakes worldwide occur underneath folds. In Japan, in North Africa, in Greece and right here in California, there’s a history of earthquakes under these folds.”

Part of the new focus in thinking “represents the appalling depth of our ignorance,” the U.S. Geological Survey seismologist declared. “Most of the big earthquakes that have occurred in the United States have occurred in places where we never would have thought a major earthquake would appear.

Telltale Oil Fields

“But in California, we do have the ability to identify surface indications of such folds,” Stein went on. “You could practically go around California and wherever you find an old oil field, you’re probably in the vicinity of an active fold. If the folds were not there, the oil would not have been trapped.”

In the Coalinga case, scientists concluded that the fold has been uplifted at a rate of one to 1.5 millimeters per year over the last million years, and that during the 1983 earthquake it rose by nearly 2 1/2 feet. They tentatively estimated that a similar 6.7 magnitude quake might strike every 500 to 1,500 years.

Davis, studying the folds in the Los Angeles area and drawing his subterranean cross-section with the aid of oil-drilling records, says he thinks recurrence times in the Los Angeles area may be more frequent, although he cautioned that his 125- to 225-year recurrence estimate for downtown Los Angeles may later be revised.

The geologist, who works out of his home near the Glendale Freeway, may be described as a true believer in the new focus of earthquake studies. Doing his research on a $55,000 grant, he is calling on the U.S. Geological Survey to sponsor much more research into subterranean faults in populated areas of Southern California.

Davis has the support of some earthquake scientists at both the U.S. Geological Survey and Caltech. But others have serious questions whether the research, at the present state of expertise, is really worthwhile or cost-effective.

Seismologist Robert Uhrhammer of UC Berkeley commented, “The problem you have to consider is that you may not be able to easily map all the faults. . . . It takes a lot of time and money . . . and a lot of the faults identified will prove not to have been recently active.”

Seismologist Stephen Ward at the University of California, Santa Cruz, said, “Using oil company reflection data, you may be able to see structures that look like faults, but you have no idea whether they have been active in the last million years. . . . It is in that sense very difficult.”

Ward also pointed out that oil drilling holes generally go down only a few miles and that many thrust earthquakes have been centered at greater depths. So he questioned whether such really deeply buried faults as the one believed to have caused the catastrophic Mexican quake of 1985 would be discovered, even if they existed, under populated areas of California.

The U.S. Geological Survey’s Heaton adopted an extremely cautionary attitude.

“We have very poor knowledge about these things,” Heaton said. “The whole question of the mechanics, how these fairly flat, thrust faults work, is still to be understood. It would be inappropriate to give warnings at this point when we still have questions of where we are and there are pieces of the puzzle that are not in place. . . .

“Things are evolving,” he went on. “Coalinga is disturbing in that a substantial quake occurred in an area where no one could discern a fault. There was a fold there. So do we assume a quake can occur anywhere there’s a fold? The recent quake, even though it was smaller, since it was close to Los Angeles, has given sharper focus to that issue.”

He said he is urging extensive further study.

Everyone agrees that more study of the new fault is necessary, but the resources are not available. Currently, there are only 16 seismological stations scattered through the Los Angeles Basin, USC’s Hauksson said. “To get a good picture of such deep faults, we would need 50 to 100,” he said.

Because the seismometers must be buried deeply underground, he added, each monitor costs about $10,000 to install. “We approached USGS (U.S. Geological Survey) just recently about this, and they don’t have any money,” he said.

“If we’re going to study this fault, the city or the state will have to pay for it,” he concluded.

Times researcher Tracy Thomas also contributed to this article.

UNDERSTANDING EARTHQUAKE THEORY

For many years, the study of earthquakes in California has focused on the state’s surface faults, such as the San Andreas, that are so-called “strike-slip” faults. But the Oct. 1 Whittier earthquake and other recent temblors have spurred new scrutiny of subterranean “thrust” or “dip-slip” faults. The existence of dip-slip faults beneath Los Angeles is the increasing preocupation of seismologists, and the source of some scientific controversy.

STRIKE-SLIP FAULT

The San Andreas is a well-known “strike-slip” fault, long a principal preoccupation of earthquake scientists in California. This kind of fault intersects the surface on a more-or-less vertical plane, and when a quake occurs, arrows show how the earth on one side slips horizontally in one direction, while the earth on the other side slips the opposite direction.

DIP-SLIP FAULT

The Sylmar-San Fernando quake of 1971, the Coalinga quake of 1983 and the Whittier Narrows quake of 1987 all occurred on what is called a “thrust” or “dip-slip” fault. This fault may lie along a mainly horizontal or shallow-dipping plane. There are few or no surface indications. When a quake occurs, often rather deep, arrows show how the thrust upward of one side of the fault produces sizable elevation gains in “folds” at the surface, while ground on the other side of the fault sinks.

BELOW LOS ANGELES

Cross-sections of the kind now being drawn for the U.S. Geological Survey by consulting geologist Thom Davis use oil drilling records to show subterranean geological structures, including “anticlines” or “folds.” Thrust or dip-slip faults presumably have periodically been responsible for 6-to-7 magnitude quakes in the Los Angeles Basin. Debate centers on how costly and difficult it will be to map such subterranean faults, which may occur at depths beyond present drilling capabilities, and how to gauge the recurrence rates of quakes on these faults.

This cross-section of the earth under the Los Angeles area shows that “dip-slip” faults abound (labeled at right as A-F), sometimes quite deep underground, researchers say. The Eagle Rock and Raymond Hill faults, about 20,000 feet deep, are such dip-slip fault. In a quake, the earth would thrust toward the surface.

“Strike-slip” faults in the region are labeled at right as 1-3. One, the Newport-Inglewood fault (3) is vertical, underground and not visable on the surface at this point. In a quake, one side of the earth would slip by the other.

This model of the geology beneath downtown Los Angeles was built based on information from the Department of the Interior for a 1987 National Earthquake Reduction Program. A cross-section from the San Rafael Hills to Gardena was combined with surface maps of known faults.