Blind thrust fault, in 'replay' of 1971 quake, did the damage, scientists say

January 18, 1994|By New York Times News Service

LOS ANGELES -- Mountains and hills in the San Fernando Valley may have been heaved more than a foot higher early yesterday morning when an earthquake fault ruptured about nine miles beneath the Northridge section of the city, seismologists said.

The magnitude 6.6 earthquake was almost identical to one that struck the same area 23 years ago in the nearby Sylmar section, scientists said. Both earthquakes were caused by deeply buried faults whose existence was not known until the earth began shaking.

"The movement of the fault at that depth could have pushed the mountains up by a foot in 10 seconds," said Kate Hutton, a staff seismologist at the California Institute of Technology. Scientists believe that the mountains normally rise a few fractions of an inch each year.

Wilfred Iwan, a professor of applied mechanics at the California Institute of Technology, said the tremor yesterday "looks to me ,, like a replay," of 1971.

"We've seen a great deal of damage to transporation, pipeline systems and to structures," he said. "It's the same kind of damage we see over and over again. When are we going to learn from lessons in the past?"

This kind of earthquake is typical for the Los Angeles region, said James Dolan, a researcher in the Caltech seismology laboratory who spends much of his time digging trenches to look for evidence of such earthquakes.

Although scientists cannot predict the exact time and date of an earthquake, given the region's geology, Dr. Dolan said, yesterday morning's temblor was "not a surprise."

A huge network of faults, or fractures in the earth's crust, lies under the Los Angeles basin, extending over a 3,600-square-mile area, Dr. Iwan said.

Although smaller than the infamous San Andreas fault, which is about 60 miles to the east of downtown Los Angeles, he said, these smaller faults can cause more damage because they are directly beneath populated areas.

The earthquake yesterday occurred on a kind of fault called a blind thrust -- "blind" because they tend not to break the surface when they rupture, so geologists have a difficult time deducing their existence.

When a thrust fault ruptures, Dr. Hutton said, the movement resembles that of hands held in the prayer position that are made to slide past one another. One side of the fault is thrust upward over the other side, sending energy in all directions.

The temblor was felt over a wide swath of the West, from Baja, Mexico, to Northern California to Las Vegas, Nev.

The actual rupture lasted about 10 seconds, said Dr. Lucile Jones, a seismologist at the U.S. Geological Survey in Pasadena. But people farther away from the epicenter felt the shaking for as long as 45 seconds. This is because the shock waves emanating from the epicenter tend to bounce off rocks and sediments of different densities, she said.

The waves thus reverberate through the region with varying intensity, depending on local soil conditions and the type of structure affected.

Yesterday morning's earthquake was caused by the inexorable motion of two tectonic plates -- gigantic slabs of the Earth's crust that float on a hot, sticky mantle below.

In California, the Pacific plate is grinding past the North American plate at a speed of about one inch a year, creating at its border the San Andreas fault.

In Southern California this geologic flow is vastly complicated because the San Andreas turns abruptly to the west as it passes through the Los Angeles region.

The result is that the Pacific Plate no longer bumps and grinds past the North American plate. Rather, it smacks straight into the plate with gargantuan force.

As a result, the Los Angeles basin and the San Fernando Valley -- both bowl-like formations filled with sediment -- are being squeezed like an accordion by relative plate motions.

The region is being pushed together at the rate of a quarter of an inch a year, said James Mori, a seismologist at Caltech. At the same time, the Santa Monica Mountains and the San Gabriel Mountains are being shoved to new heights.

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