Scientists Issue Grimmer Forecast of Quakes for L.A. : Seismology: New study indicates a greater likelihood of jolts in the 7.2 to 7.6 range than earlier believed.

TIMES SCIENCE WRITER

There is a greater likelihood of major earthquakes in the Los Angeles area than scientists previously believed, and their destructive power may be stronger than engineers had anticipated, researchers at Caltech, USC and the U.S. Geological Survey said Thursday.

Researchers warned that the Los Angeles Basin may have been in an unusual earthquake lull for centuries and that a barrage of earthquakes the size of last January's 6.7 Northridge temblor or an even larger single major earthquake may be overdue.

Based on a new analysis of six major fault systems under the metropolitan area, earthquakes as large as magnitude 7.2 to 7.6 may be likely, the scientists said, far more powerful than any recorded since the region was settled by Spanish missionaries in the 1700s.

The findings, published today in Science, are part of a concentrated effort to re-evaluate the seismic and engineering risks posed by a lattice of faults underlying the Los Angeles Basin. With ground literally still shaking from the Northridge temblor--more than 11,000 aftershocks have been recorded so far--scientists have a heightened respect for the power of subterranean faults hidden in the folded carpet of earth on which the metropolitan area is built.

In related research based on data collected during the Northridge earthquake, new computer simulations show that the ground shaking in the kind of strong quake researchers now expect could be so severe that many high-rise buildings, previously thought to be virtually earthquake-proof, could be seriously damaged.

Even buildings using sophisticated base isolation systems to protect themselves from the ground shocks might sustain serious damage, scientists said.

"Designers of several types of buildings were not anticipating these large rapid motions when they designed buildings," said Thomas H. Heaton, the U.S. Geological Survey earthquake expert who led the group performing the simulations. "It will probably cause some consternation in the engineering community, and that is not an unhealthy thing."

The computer studies have major engineering importance for metropolitan areas such as Los Angeles, San Francisco and Tokyo, which are located on or near potential earthquake zones, experts said. In the United States, 37 states have a significant earthquake problem.

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The greater potential for damage to buildings "has worldwide implications," said seismologist Leonardo Seeber at the Lamont-Doherty Earth Observatory of Columbia University.

The findings, made public just before the first anniversary of the Northridge temblor, add to a growing store of sobering earthquake studies prepared by geophysicists, seismologists and other experts. They are using the experience of North America's most costly earthquake to reassess seismic hazards in Southern California.

Unable to actually predict earthquakes or even locate all the faults that may be responsible for them, geophysicists and seismologists must rely on satellite sensors, strain gauges, seismographs, mathematical models and old-fashioned shovel work for the raw material of their scientific guesswork.

To prepare the newest hazard analysis, this group of researchers calculated the strain building up in the area as tectonic plates squeeze the Los Angeles Basin together; then they attempted to determine the most likely scenarios in which the Earth will release its titanic pent-up energy. Working in more detail than previous studies of the basin, the researchers combined new calculations of the speed of the earth's crust with studies of past quakes to gauge how each of six major fault systems under the Los Angeles area could trigger major earthquakes.

The energy is generated by the global forces that move continents around like checkers. Southern California is at the juncture of the Pacific Plate and the North American Plate. As they lumber past each other, they compress the Los Angeles Basin. Satellite measurements show that the thousands of square miles of the Los Angeles area are being compressed at a rate of about half an inch every year.

"All this strain has to be released by earthquakes," said USC earthquake geologist James F. Dolan, who led one research group. "Whether that happens this year or 10 years from now or 50 years from now, we don't know. We do think it is inevitable that we will have more frequent, larger earthquakes."

In three research papers published today, scientists attempted to calculate the prospects for larger earthquakes along the relatively small faults in the metropolitan area and then assess the destructive power of the temblors their calculations show are most likely.

Among their findings:

* To relieve the seismic strain in the Los Angeles Basin, at least 17 earthquakes the size of the 6.7 Northridge temblor should have occurred during the past 200 years, but only two have actually taken place. A cluster of serious earthquakes like the Northridge, or a very large temblor--in the 7.2 to 7.6 range--may be likely, they concluded. "Given the level of damage from the Northridge earthquake, such a sequence would certainly strain the ability of the region, and the nation, to absorb the resultant losses," the researchers said.

* The average time between such very large earthquakes in the region should be about 140 years, yet at least 210 years have passed with no such temblor on record. An accompanying mathematical analysis concluded that, given the amount of pent-up seismic squeeze, at least one 7.4 to 7.5 earthquake should occur every two or three centuries.

Not everyone is convinced that the energy must be released by violent quakes. Many believe that the strain could be slowly and safely siphoned off by millions of imperceptible shifts of the ground called "aseismic creep."

In the aftermath of the Landers earthquake, for example, many researchers expected the earthquake risk along the San Andreas Fault to increase because the powerful temblor added to the tension along that fault near San Bernardino.

Geological Survey geophysicist Ross Stein, however, determined that the San Andreas fault may have relieved the extra stress miles below ground by quietly slipping about 5 inches without triggering another quake. Other scientists remain concerned that the stress may have simply been transferred to another section of the fault.

Whatever the actual probability of a large earthquake, scientists have gained a more precise--and more daunting--idea of how a 7.0 earthquake would affect the high-rise buildings and other structures in dense downtown areas of cities such as Los Angeles and San Francisco.

Using actual readings of ground motion gathered during the Northridge quake, Heaton and his co-authors used a computer to simulate the effects of a magnitude 7.0 earthquake on a 20-story steel-frame building built to the standards of the 1991 Uniform Building Code and on a three-story building using a high-tech base isolation system to insulate it from ground shaking.

When researchers tested out the ability of the high-rise building to withstand the more intense ground forces now expected in a large quake, the swaying was so pronounced that I-beams buckled and welds failed.

"The building collapsed," the researchers noted.

And when they subjected the building protected by a base isolation system to the same forces, they found "substantial damage."

Some engineering authorities were skeptical. "I am not sure all that really happens, because buildings survive; there were a lot of buildings in Northridge that had very little damage," said Loring Wyllie, president-elect of the Engineering Research Institute, which specializes in earthquake issues.

"In the profession as a whole, there is not a unanimous vote on these things yet," he said. "We have to approach some of these things with caution."

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Multiple Threats

Scientists who long worried about the threat posed to the Los Angeles area by the San Andreas Fault now have become more concerned about the potential of smaller faults, directly under the metropolitan area, to generate stronger quakes than previously predicted. Major fault systems are labeled with boldface type.

1971 Sylmar earthquake

1987 Whittier Narrows earthquake

1994 Northridge earthquake

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