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Some Earth-Shaking Questions Prompted by 2 Recent Temblors

July 20, 1986|JOHN KISSELL | Times Staff Writer

The question last Sunday was the same as the Tuesday before:

Did you feel it?

For the second time in six days, Southern California awoke to the disquieting rock and roll of an earthquake.

Sunday's quake, centered off the coast near Oceanside, registered as moderate on the Richter scale at Pasadena's California Institute of Technology, the major center for earthquake research in Southern California, and caused no reported damage in the South Bay. But it was strong enough to bring to mind a second question:

Will the next one happen here?

Seismic activity has been recorded along all four faults that run through the South Bay, and they are considered active under both standards used to measure such activity: geologic time, measured over millions of years; and historical time, more or less within the last century or so, when frequency and magnitude could be measured with accuracy.

But the earth movement along these lines is very slow, according to Egill Hauksson, a research assistant professor in geophysics at USC in Los Angeles. "The San Andreas Fault moves an average of 25 to 35 millimeters a year," he said. "These faults move one-half to one or two millimeters a year."

Fewer Quakes

This slower rate means that quakes happen less often, and the faults in the South Bay do not cause the broken curbs and other physical manifestations seen along faster-moving faults like the San Andreas.

Of 42 damaging earthquakes in Southern California between 1800 and 1978, five had their epicenters in the South Bay and all of them occurred along the Newport-Inglewood Fault Zone, the major fault line through the South Bay, according to the U. S. Geological Survey.

The best-known was the Long Beach quake on March 11, 1933. The magnitude of the original quake was 6.2 on the Richter scale and it was followed by 12 aftershocks lasting into October. The quake took 115 lives and was blamed for $40 million in damage. (An increase of one number on the Richter scale indicates a 10-fold increase in ground motion.)

On June 21, 1920, a quake estimated at 4.9 on the Richter scale hit Inglewood, but it was highly localized. Buildings were damaged but there were no deaths.

Two in 1941

In 1941 there were two separate damage-causing quakes in the South Bay. The first, on Oct. 22, was centered in Gardena and was estimated at 4.9 on the scale. It caused nearly all the oil wells in the area to be shut down temporarily but did not do much damage.

The second quake, on Nov. 14, had centered in Torrance magnitude of 5.4 and caused about $1 million damage.

The oldest of the recorded damage-causing quakes in Southern California occurred on Dec. 8, 1812. That quake, believed to have had a magnitude of 6.9, was so severe it was reportedly felt in San Diego. Forty people attending services at Mission San Juan Capistrano were killed when the stone and mortar building collapsed.

Less dramatic are frequent small earthquakes along the faults in the South Bay, measuring 3 or 4 on the Richter scale, that cause little or no damage. Most are felt only by the sensitive equipment of the monitoring stations.

Buried in Ground

The monitoring stations--250 throughout Southern California, eight in the South Bay--are cylinders, 3 inches wide and 6 inches tall, with a coil inside. The cylinder is placed deep in hard rock or in a bored hole, like a well. Ground movement shakes the cylinder, which is connected to a telephone line, and the movement is recorded at USC and Cal Tech.

Such low-intensity quakes occurred in April in Torrance and in June off Hermosa Beach.

Most of the South Bay is covered with sedimentary soil that amplifies ground motion and has a tendency toward liquefaction, a phenomenon that happens when soil becomes saturated with ground water. During a quake, the soil temporarily loses strength and behaves like a liquid, causing structures on the surface to sink.

The Palos Verdes Peninsula, which is mostly hard rock, would not be susceptible to such hazards. But the slow-moving landslides on the peninsula, though not linked to fault activity, could be exacerbated by an intense quake, Hauksson said.

68-Mile Fault

The longest of the South Bay's fault lines is the approximately 68-mile Palos Verdes Zone. Most of the fracture is under the ocean, stretching from below Newport Beach in Orange County, cutting across the Palos Verdes Peninsula to the tidelands off Hermosa Beach and Redondo Beach.

The nearby Cabrillo Fault, to the west, is not a part of this zone. Half of the Cabrillo's 11-mile fracture lies in the ocean, the other half across the western end of the Palos Verdes Peninsula.

The shortest fault line is Charnock, about 6.2 miles in the northern part of Hawthorne.

The most active, and potentially most dangerous, is the Inglewood Fault Zone, five fault lines running from south of Long Beach through Inglewood. The 20 or so miles include the Inglewood, Potrero, Avalon-Compton, Cherry Hill and the south branch of the Newport-Inglewood faults.

The Malibu and Santa Monica faults on the Westside are close enough to the South Bay to cause damage if a major quake occurred there, Hauksson said. Of the 42 recorded major Southern California quakes, two occurred along the 16 1/2-mile Malibu Coast Fault, the most recent was in February, 1973, and caused some damage in Oxnard. The other was on Aug. 31, 1930.

No damaging quakes have been recorded on the Santa Monica Fault, which runs about 25 miles, but both faults have frequent low-intensity quakes.

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