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Oct. 1 Quake Downgraded to 5.9 Magnitude : Experts Say Temblor Marks a New Hazard

November 05, 1987|THOMAS H. MAUGH II | Times Science Writer

Earthquake experts gathered at USC to review studies of the Oct. 1 Whittier quake downgraded it Wednesday to 5.9 on the Richter scale, but warned that the temblor appears indicative of a new source of earthquake hazard to the Los Angeles area.

Caltech researchers, drawing on new data from throughout Southern California, said the quake was actually 5.9, rather than the 6.1 previously announced. Seismologists also said that there is a growing consensus that the quake occurred on a previously unrecognized fault, buried nine to 12 miles underground, that stretches from Rosemead through Los Angeles and Santa Monica and out to sea off the coast of Malibu.

Potential for Earthquakes

This fault and others like it may represent a previously unknown potential for earthquakes, said independent geologist Thomas L. Davis, who is studying the area under a contract with the U.S. Geological Survey. Davis said that the Whittier temblor, which caused seven deaths and produced $213 million in damage, was the fourth earthquake of magnitude 5 or larger to have occurred along the fault since 1929. The other three were all offshore and were in 1929, 1930 and 1973.

This theory places an active fault directly beneath Los Angeles. Previously, seismologists have regarded the principal earthquake threat to Los Angeles as faults lying outside the city--for example, the San Andreas, which runs east of Los Angeles.

Davis' theory is still controversial, and many scientists do not fully accept it. Some argue that the full extent of the fault has not been documented, while others say that even if the fault exists, geologic strains on it could be relieved by mechanisms other than earthquakes.

Moreover, there is division within the scientific community about how large an earthquake could be generated by such faults, and how frequently they would occur.

"But (Davis' results) have raised the possibility of earthquakes under Los Angeles, not at its edge," said Geological Survey seismologist Lucille M. Jones. "The issue of size (of the possible earthquakes) can't be resolved yet, but we would have to say they would be the magnitude of the San Fernando earthquake (magnitude 6.4 in 1971) to be safe."

Davis' principal argument is that geologists have not correctly assessed the extent of the fault system that lies along the southern base of the Santa Monica and San Gabriel mountains. That series of faults is clearly visible on the surface, and the mountains themselves are clear evidence of strong geological activity.

Geologists have believed that the visible faults represented the southern edge of the fault system.

But Davis argues that the mountains and the exposed faults are only the visible tip of a geologic formation that was thrust up from the ocean floor millions of years ago. He said that the rest of the system has been buried by silt over the ages so that no surface signs of the fault are present.

Directly Under City

Because the fault system slopes downward in a southern direction, he said, the fault is nine miles underground and 10 to 12 miles south of the mountains. That area is directly under Los Angeles.

Davis conceded that more study needs to be carried out to document the faults. Jones noted, however, that the area beneath Los Angeles is difficult and expensive to study because of the city's dense population.

The re-evaluation of the magnitude of the Oct. 1 quake arose from data not initially available, Caltech seismologist Kate Hutton said. Data from many seismometers scattered across Southern California are recorded photographically. The film must be collected, mailed in, and developed before the data is available to researchers.

Like Richter's Data

Jones said it is important to use this data because it is collected in the same manner as data used by Richter in formulating his earthquake severity scale. Using the photographic data provides historical continuity so that valid comparisons can be made between earthquakes occurring at different times.

The researchers also downgraded the Oct. 4 aftershock from 5.5 to 5.3.

Geologist Ross Stein of the Geological Survey said at the Wednesday meeting that the Oct. 1 earthquake involved a horizontal slippage of about one yard as earth on the southern side of the fault forced its way over the northern side.

This movement raised Whittier Hills about two inches, while the surrounding area, including the cities of Whittier and El Monte, were raised about an inch.

'Strike-Slip' Motion

The Oct. 4 aftershock, according to Jones and seismologist Egill Hauksson of USC, occurred on a separate fault about 1.6 miles northwest of the original epicenter. It involved the more common "strike-slip" motion, in which earth on the two sides of the fault moved sideways past each other.

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