Experts Hike Chances of the Big One : Seismology: Articles say new stresses from Landers quake increase probability of a major temblor by 2018 on segments of the San Andreas Fault.

Stress changes along the San Andreas Fault triggered by the June 28 Landers earthquake have brought a small increase in the probability of a great quake in Southern California in the next 25 years, but precisely when the Big One will occur remains uncertain, according to three scientific reports released today.

The 1988 prediction of a 60% chance of a major quake along that section of the fault by the year 2018 should be increased, according to reports in the current issues of Nature and Science magazines. But the articles gave no precise percentages.

Stress has increased the most along the San Bernardino Mountains segment of the San Andreas Fault, and particularly in the portion from North Palm Springs west through the San Gorgonio Pass to just beyond Yucaipa.

Increases also occurred in the Coachella Valley segment of the fault from east of Desert Hot Springs to the Salton Sea, the earthquake scientists said. But the Landers temblor also caused reductions in stress in the Mojave segment of the San Andreas, extending from Cajon Pass to Ft. Tejon, reducing the likelihood of a Big One there soon, the researchers said.

This means that the probability of a magnitude 7.5 or 8.0 quake occurring north of Los Angeles has gone down, the scientists said, whereas the probability of it occurring east of Los Angeles has increased.

In addition, parts of the San Jacinto Fault near San Bernardino also have experienced increased stress from the Landers quake, the scientists said. A magnitude 6.8 quake occurred on this fault in 1918.

The Landers earthquake had a magnitude of 7.5 but was centered in a remote desert area where damage was limited, and there was one death. A quake of similar strength centered close to the urban areas of the Inland Empire, San Bernardino and Riverside would be expected to cause much more damage and many casualties.

“The fact is that aftershocks from the Landers quake are occurring on or near the San Andreas in areas where we indicate a stress increase,” said Ross S. Stein of the U.S. Geological Survey, an author of one of the articles appearing today.

“This is a sign that some parts of that region are close to failure,” he said. “The question is whether the right spot on the San Andreas, the place where the fault is really stuck, has been critically affected. Since we don’t know exactly where that spot is, we are unsure what may happen or when the quake will occur.”

The scientists believe, however, that the San Gorgonio Pass, where the San Andreas bends and where the fault has been locked for as long as 500 years, may be a particularly sensitive area.

The last great earthquake east of the San Gorgonio Pass is believed to have occurred about 1680. Some distance west, near Cajon Pass, the last great quake took place in 1812. Neither quake is thought to have ruptured the San Andreas in the San Gorgonio Pass, and any quake that causes such a rupture would probably be at least magnitude 8.

Although there are variations on some details in the conclusions, all three articles found generally that the Landers earthquake increased stress in the San Bernardino Mountains and Coachella Valley segments of the San Andreas.

A seismic review panel appointed by the U.S. Geological Survey and the California Office of Emergency Services, meanwhile, has been reviewing the 60% probability for the Big One assigned in 1988, and its tentative conclusions are expected to be released Nov. 30. But two of the authors of Wednesday’s articles said privately that they have read the reports and that it does not vary from their own conclusion that the figure is higher than 60%.

The stress calculations made by the scientists came from computer models based on observations, measurements of displacements and other studies of the Landers quake sequence, which also included the magnitude 6.6 Big Bear earthquake of June 28 and the 6.1 Joshua Tree earthquake of April 22 as well as thousands of aftershocks.

The researchers--from the Woods Hole, Mass., Oceanographic Institution, Columbia University and USC, as well as the USGS--said the stress increases have moved up the date of the Big One from two to 45 years in certain areas.

But because scientists cannot measure total stress on a fault and do not know how much stress is required to trigger a quake, they cannot specify precisely when or where such a quake will occur.

Nonetheless, they see some ominous significance in the fairly short term in the number of moderately strong magnitude 5 and 6 earthquakes that have occurred in Southern California since 1986, noting that such clusters of quakes have preceded other great quakes.

“These studies are certainly significant,” said study co-author Jian Linn of Woods Hole. “However, we are not anywhere near being able to predict any earthquakes yet. There’s a lot of work that has to be done. That’s the message I try to get across to people.”

Co-author Robert W. Simpson of the USGS cautioned: “There’s a very large uncertainty involved in these probabilities. But the people of Southern California obviously are going to experience a large earthquake in the future, and they should prepare for it.”

By coincidence, a magnitude 3.6 aftershock of the Landers quake occurred early Wednesday four miles southwest of Yucaipa in an area viewed as being sensitive, and where there have been occasional aftershocks. No damage was reported.

Quake Update

Several earthquake scientists now say that as a result of the Landers earthquake, stress changes on the southern San Andreas Fault have increased their probablity of a great quake in that region--roughly between the Cajon Pass and Salton Sea--over the next 25 years.

Stress changes have occurred mainly in three segments--increases in the San Bernardino Mountains and the Coachella Valley portions of the San Andreas, and a decrease in the Mojave portion.