The 4.5-magnitude earthquake that struck Monday near Montebello was but the latest in an unusual rash of temblors in the region over the last three years, and bolstered a growing sense among scientists that a newly discovered fault running from Whittier to Malibu could be an active supplier of moderate or even strong quakes.
Earthquakes have occurred in Los Angeles and surrounding cities more frequently in the last three years than during any other period since roughly 1930, when detailed recording of earthquake activity began, seismologists and scientists said Monday.
Several of the most significant of these temblors--including the quake Monday, which was followed by an almost equally sharp aftershock--have occurred on a deeply buried thrust fault that was unknown to the experts until after the Whittier earthquake of Oct. 1, 1987.
Named the Elysian Park Fault, it follows hill lines extending from Whittier through Montebello, Elysian Park and the Cahuenga and Sepulveda passes to Malibu and Point Dume, but it never breaks the surface.
Experts from Caltech, USC and the U.S. Geological Survey agreed at a briefing Monday afternoon at Caltech that at some time in the future, the Elysian Park Fault could generate a quake in the magnitude 7 range. Since the fault passes underneath downtown Los Angeles and other highly populated parts of the Los Angeles Basin, this constitutes a sobering prognosis.
The experts said the accelerated rate of quakes does not say anything--at least, that they now can understand--about the probability of a really big quake hitting soon. The major quakes that struck Long Beach in 1933 and the San Fernando Valley in 1971 were isolated events.
One seismologist, Lucile Jones of the Geological Survey’s Pasadena office, said that the succession of moderate quakes in recent months increases chances that those in the near future will be of similarly moderate power.
Before the spring of 1986, Jones said, there was usually about one 4.5-magnitude earthquake in the Los Angeles area every three years.
But since then, there have been six quakes of equal or stronger strength, ranging up to the 5.9 Whittier temblor. This does not count two of the significant aftershocks to the Whittier quake.
Meanwhile, the number of small earthquakes, in the 2-magnitude range, has just about doubled, Jones said.
Jones said that during the period 1938 through 1942, Los Angeles experienced a similar increase in the frequency of moderate quakes, and USC geologist Egill Hauksson recalled that in the late 1940s, there were several 5.5- to 6.5-magnitude earthquakes along the San Andreas Fault near Banning.
Neither episode of high-quake frequency was followed by a giant quake, they said.
“Nothing that happened today makes us fear a larger earthquake more than we did yesterday,” Jones said.
But, she added, the quake on a segment of the Elysian Park Fault several miles west of the segment that caused the Whittier quake “confirms that we have an active fault structure west of where we’ve had it in the past.”
Other scientists said that quakes off Malibu in January and near Point Dume in 1973 now appear to have been on the Elysian Park Fault system, indicating that it is active over a considerable distance.
Kerry Sieh, a Caltech geologist, said: “Not any of us would be surprised to have a flurry of 5s and 6s on this fault in the next 30 years. I would not be surprised by a 7.5.”
“There’s no way we could rule out a 6.5- or 7-magnitude quake,” Jones agreed.
A 7.5 quake would be 1,000 times stronger than the Monday quake, which incidentally was only about one-twentieth as strong as the 5.9-magnitude Whittier earthquake.
Sieh said that certain studies indicate that over the last 2 million years there has been an average movement of several millimeters a year along the Elysian Park Fault system. Over the last 100 years, there has apparently been very little movement--"so we may see more,” he said, a prediction based strictly on the law of averages.
Study of Earthquakes
Unlike the San Andreas and the state’s other well-known fault systems, in which earth on opposite sides scrapes sideways, the Elysian Park Fault thrusts the earth on one side upward, while the other side slips lower. This process creates great folds on the surface that can be seen in the form of the Whittier, Montebello and Hollywood hills.
After the San Francisco quake disaster of 1906, the study of earthquakes in California focused for decades on the state’s great surface faults, its so-called strike-slip faults. For the most part, experts concentrated on the San Andreas.
After the Whittier earthquake, however, considerable attention began to be paid to the thrust, or dip-slip, faults. After recognition of the Elysian Park Fault, scientists said the destructive Coalinga quake of 1983 and the Sylmar-San Fernando quake of 1971 could have been caused by other dip-slip faults.
Hills Pushed Higher
The earthquake Monday, as did the Whittier one, had its epicenter deep beneath the surface, in this case eight or nine miles, the scientists said.
The Whittier quake, USC’s Hauksson said, thrust the Whittier Hills up about two inches higher than they had been before. Monday’s smaller quake probably made the Montebello Hills about 0.2 inches of an inch higher than before, he added.
The shaking of the latest quake was very similar to the kind felt in the Whittier quake, only less, Hauksson said.
Since scientists became aware of the Elysian Park and possibly other thrust faults in the greater Los Angeles region, there has been some attempt to map the subsurface cleavages, but this undertaking has not yet been anywhere near as extensive as the charting of surface faults.
Hauksson said Monday that while such mapping could delineate potential hazard zones, it would “not say anything about probabilities.”
THE ELYSIAN PARK FAULT--The Elysian Park fault was unknown to scientists until after the 5.9 Whittier Narrows earthquake of Oct. 1, 1987. But since then, this deeply buried, fold-and-thrust fault has come to be appreciated as perhaps one of the most dangerous in the Los Angeles area. Extending along hill lines northwestward from the Whittier Hills through the Montebello Hills, East Los Angeles, Elysian Park, the Hollywood Hills and the Santa Monica Mountains to the coast off Malibu and Point Dume, the fault, which never comes to the surface, may be capable of generating a quake as strong as magnitude 7, scientists believe.