Quakes Shift Attention to San Andreas Risk : Seismology: Experts advise increased readiness but are uncertain whether the Big One is imminent.

Scientists trying to analyze the ongoing Southern California earthquake sequence focused their interest Monday on a section of the San Andreas Fault after several aftershocks were detected near the state’s longest and most infamous quake zone.

Cautioning that they cannot be certain what it all means, Caltech and U.S. Geological Survey quake experts suggested that Southern California increase its quake preparedness in case the signs are advance warning of a great temblor along the San Andreas, possibly in the area between Yucaipa and Desert Hot Springs.

“Whether there are direct, first order consequences on the San Andreas, we can’t say,” said Allan Lindh, a branch chief for the Geological Survey, at a briefing in Pasadena. “But there is evidence we’re near the jumping off place for a big one on (the San Andreas). . . . Anyone who thinks they can ignore this should be dissuaded now.”

Seismologists also said the earthquake sequence that they officially proclaimed Sunday--following a magnitude 7.4 quake near Yucca Valley, a 6.5 quake near Big Bear and hundreds of aftershocks--started with a 6.1 temblor on April 22 near Desert Hot Springs.

That April 22 quake had the potential to also exceed magnitude 7--a level of intensity that typically causes major damage--but its release of energy was interrupted by the crossing of another underground fault. Nonetheless, the quake was strong enough to be felt widely in Southern California.

Also on Monday, researchers at Stanford University said they have evidence that the flurry of seismic activity in the desert area may be the sign of a major new fault forming across the Mojave with the potential to rival the San Andreas in importance.

The speculation about the San Andreas on Monday began after Sen. John Seymour (R-Calif.), emerged from a private briefing by Caltech and Geological Survey scientists and said he had been told that there was concern about the San Andreas and that another major quake could strike.

“Safety would suggest that we be prepared for another shaker, possibly along the San Andreas,” Seymour told reporters.

Later, seismologist Lucile M. Jones of the U.S. Geological Survey confirmed that scientists suspect that the San Andreas may become involved in the current series of quakes and aftershocks, but she said it would be rash to conclude that a catastrophic quake is imminent.

Jones said scientists studying the recent earthquakes believe that a triangle of faults have possibly been affected. Sunday’s magnitude 7.4 Landers quake and the earlier April 22 Joshua Tree quake--on the Camp Rock Emerson, Johnson Valley and perhaps two other faults--formed the right leg of the triangle.

The left leg of the triangle would be the unnamed, deeply buried fault that was responsible for Sunday’s 6.5 Big Bear temblor. The base of the triangle, Jones said, would be a 40-mile-long segment of the San Andreas between Yucaipa and Desert Hot Springs.

Jones noted that a magnitude4.4 aftershock hit near Yucaipa at 7:41 a.m. Monday. Other aftershocks were reported later on or near the San Andreas in the Coachella Valley. A series of magnitude-4 quakes along this base of the triangle would raise concern, but Jones said conclusions now are premature.

Occasionally--as in 1981 in the Imperial Valley--all three faults in such a triangle have become active within a short time, Jones said, but she added that the chance of a big earthquake happening on the San Andreas “this week is probably not that much higher than normal.”

But, she added, the longer run prospects for a big quake on the fault may have been “fundamentally changed” for the worse by this week’s events.

Jones and other scientists, however, said they hesitated to draw any conclusions that the San Andreas was about to spring back to life along the 40-mile segment where it has had no sizable earthquakes since 1680.

She was supported in this assessment by Randall Updike, associate chief of the Geological Survey’s Office of Earthquakes, Volcanoes and Engineering, interviewed by telephone from Reston, Va.

Asked if he felt that the Big One was imminent, Updike responded, “We always kind of keep that in the back of our mind as a potential scenario, any event that has proximity to the San Andreas.”

But, he added, “I haven’t heard anything today that suggests that our seismologists are fearful.”

Updike reported, meanwhile, that various faults in the Imperial Valley, more than 100 miles south of the quake area, are showing “a little bit of slip, not enough to generate a quake.”

It seems “to be some kind of sympathetic response,” he said. Geologists have been sent to conduct inspections there.

Caltech survey teams in the field said they have uncovered some areas of 21-foot horizontal displacement along the 44-mile rupture caused by the Landers earthquake. A number of magnitude 5 aftershocks continued to be felt, and earthquakes well away from Sunday’s epicenters were also felt but were said to be unrelated.

Seismologists said that neither a magnitude 3.9 tremor centered near the Rose Bowl in Pasadena Monday afternoon nor a 5.6 reported along the California-Nevada border 75 miles northwest of Las Vegas were connected to Sunday’s events.

Jones, meanwhile, said that the current sequence of related quakes started with the Joshua Tree quake more than two months ago. That earthquake may have been a magnitude 7 too, had its growth not been blocked by the intersecting Pinto Mountain Fault northeast of Desert Hot Springs.

Noting that an unusual number of aftershocks followed the Joshua Tree quake, Jones said that between April 22 and June 28, energy may have “migrated across the Pinto Mountain barrier” and triggered the Landers earthquake, which in turn triggered the Big Bear quake.

“Pinto Mountain probably caused a delay of two months in the spreading of a compound, multiple-fault earthquake,” Jones said.

But whether such migrations across barriers would bring activity on the San Andreas, she and other scientists could not say.

Meanwhile, Stanford geophysicist Greg Beroza said that seismic activity may be intensifying over a long term in the Mojave Desert region where the Landers quake occurred.

Beroza said he and a colleague, Amos Nur, have been mapping what they believe to be a new fault being formed across the Mojave, which could rival the San Andreas in big-quake potential.

“When we heard about the Landers quake, we realized we had a much larger-scale fault than we had originally thought,” he said. It could, he suggested, form a new boundary between the North American and Pacific tectonic plates, which have been moving apart along the San Andreas.

The Quake Aftermath

Here are some key facts about the aftershocks:

The aftershocks: Four sharp aftershocks Monday measured between magnitudes 4.9 and 5.4.

The toll: Three dozen more people were reported injured Monday, in addition to the 1 child killed and hundreds hurt Sunday.

The damage: Estimate for San Bernardino County still set at $16.3 million. Riverside County reported $81,000 in property damage.

The call for help: About 27,000 people either had no water or had contaminated water. More than 600 people sought help at shelters in Yucca Valley, Morongo Basin and Big Bear.

The warning: State emergency officials stood by their advisory that there is a 50-50 chance of a 6.0 aftershock by week’s end.

A Triangle of Faults

The U.S. Geological Survey and Caltech on Monday said they believe a rough triangle of faults near the high desert has played a key role in the current earthquake sequence. Scientists also found a 44-mile-long surface rupture left by Sunday’s first quake that roughly follows the Johnson Valley and Camp Rock Emerson faults (see inset).

Here are the faults that make up the triangle:

On the triangle’s right leg: The Camp Rock Emerson and Johnson Valley faults, where Sunday’s magnitude 7.4 Landers earthquake occurred. (The 6.1 magnitude quake in Joshua Tree on April 22 also occurred in this area.)

On the triangle’s left leg: An as-yet-unnamed, deeply buried fault where Sunday’s 6.5 Big Bear quake took place.

At the base: A segment of the San Andreas Fault where no major quakes have occurred since 1680. But several small aftershocks reported on or near the San Andreas on Monday have caused some concern among scientists about that section of the fault coming to life.