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Temblor Did Little to Ease Fault Pressures

July 09, 1986|THOMAS H. MAUGH II | Times Science Writer

The magnitude 6.0 earthquake that struck northwest of Palm Springs did little to reduce the risk of larger quakes in the future, because the Tuesday morning temblor released only an insignificant amount of stress on the San Andreas Fault, according to earthquake experts.

"Every earthquake relieves some stress, said Caltech seismologist Kate Hutton, "but a 6.0 is just a drop in the bucket."

Although the relative remoteness of the quake's epicenter from densely populated areas helped restrain damage, earthquake experts said that even had the temblor occurred in a more populous area, damage probably would not have been particularly severe. Damage would have been limited primarily to buildings constructed before 1933, when a comprehensive earthquake-safety building code was enacted, they said.

1971 Earthquake

In comparison, the magnitude 6.5 earthquake centered in the San Fernando Valley in 1971, which killed 65 people, was about 2.5 times more severe in terms of ground movement and involved the release of at least 10 times more energy by the Earth.

The Richter scale that scientists use to measure earthquakes is logarithmic, which means that an increase of one unit on the scale corresponds to a tenfold increase in the severity of the earthquake. The amount of energy released during an earthquake, however, goes up by a factor of 30 for every increase of one unit on the Richter scale.

A magnitude 8.0 earthquake, which scientists predict will occur along the southern section of the San Andreas Fault sometime within the next 50 years, would thus be 100 times more severe than Tuesday's quake and would release nearly 1,000 times as much energy.

Even after several hours of seismological study Tuesday, scientists were not certain exactly which fault the quake was associated with, primarily because of the complexity of fault systems in the area. Compared to the rest of the San Andreas Fault, the area near the epicenter is anomalous, according to Edna King, a spokeswoman for the U.S. Geological Survey in Menlo Park.

For most of its inland path southeast from San Francisco, the San Andreas Fault is a relatively simple fracture, in which the large Pacific plate on the Earth's surface is being submerged under the North American plate. This type of geological zone is called a "thrust" fault, because the North American plate is being thrust upward.

Thrust activity is normally associated with mountain building.

Just east of San Bernardino, however, the San Andreas splits into two large strands, called the Banning and Mission Creek faults. The faults intersect several smaller faults, before rejoining south of Thousand Palms.

This region is not only more complex, but the faults are of the type known as "strike-slip" faults, because the plates move sideways with respect to each other. Preliminary evidence, according to seismologist Thomas Heaton, scientist in charge at the U.S. Geological Survey's office at Caltech, is that Tuesday's quake was a strike-slip temblor that involved a lateral motion of about two inches.

Strike-slip earthquakes generally cause less damage than thrust earthquakes, Hutton said at a press conference Tuesday.

The epicenter of the quake fell between the Mission Creek and Banning faults, so scientists are not yet sure which fault moved.

A similar earthquake occurred in the same area on the afternoon of Dec. 4, 1948. That temblor had a magnitude of 6.5 and was associated with the Mission Creek Fault. Tremors from the quake were felt as far east as Colorado and as far south as Mexico.

Buildings swayed in Los Angeles and San Diego in the 1948 quake and plaster fell from some ceilings, but no significant damage was incurred either in Los Angeles or in the immediate vicinity of the epicenter.

Scientists hope to learn a lot from Tuesday's quake, Hutton said.

"This is the first quake in that area since we put in sensors in 1974," she noted.

"The most hazardous fault in California now is the southern San Andreas Fault, which includes the Banning and Mission Creek faults," Heaton said.

The section of the fault between Cajon Pass and Lake Hughes has experienced a magnitude 8.0 or larger earthquake an average of every 145 years. The most recent was in 1857, 129 years ago.

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