BAY AREA QUAKE : Scientists Find More Clues to Earthquake’s Origin : Geology: While no significant rupture has been located, evidence appears to support the belief that the San Andreas Fault is to blame for the temblor.

Scientists on Thursday found geological evidence that, although inconclusive, supports their belief that Tuesday’s Bay Area quake was a “strike-slip” earthquake along the San Andreas Fault.

State Geologist James F. Davis said Thursday that teams of state personnel had observed “what they believe is displacement” along sections of the San Andreas Fault near the Santa Cruz Mountains epicenter of Tuesday’s earthquake, “but it’s quite small . . . one or two inches.”

“At this point, it’s pretty clear that no one has found, at the surface, the really significant rupture that we were expecting to see,” said Thomas Heaton, who is in charge of the U.S. Geological Survey’s Pasadena office.

Heaton said that no displacement had been found on either of two small faults near the earthquake epicenter, the Sargent and the Zayante. He said that while scientists were going on the “working hypothesis” that the San Andreas was responsible for the earthquake, “we will really have to investigate the data more thoroughly to come to that definite conclusion.”

He said the investigation could last for weeks.

Davis, however, seemed more satisfied that the San Andreas was definitely involved.

“The displacement is not as large as one would normally expect,” he said. “But the discussions I’ve had with the Geological Survey and other people to this point suggest that the San Andreas Fault in the area where this quake occurred is a complex and fairly wide zone, so the displacement may have spread over a number of strands.

“And this was a relatively deep earthquake,” he added. “So perhaps there would be less surface rupture.”

One geophysicist with the U.S. Geological Survey in Menlo Park, Manuel G. Bonilla, said that in deep earthquakes, it sometimes takes weeks for the ground rupture to work itself up to the surface. This was the case, he pointed out, with an earthquake in Parkfield in 1966 and the 6.6-magnitude Imperial Valley quake in 1979.

Meanwhile, scientists at the Geological Survey offices in Menlo Park and Pasadena downplayed the significance of the discovery by geologists from San Jose and the campus of UC Santa Cruz of a large gap--four feet wide, 15 feet deep and 750 feet long--in the hamlet of Loma Prieta, east of Santa Cruz.

They said this fissure did not extend far enough to represent the kind of surface displacement, or fracture, that would definitely establish the San Andreas as the source of Tuesday’s 6.9-magnitude earthquake.

As the search for the origins of the earthquake continued, a spokesman for the Geological Survey said that while the intensity and frequency of aftershocks have been normal for this size temblor, there were signs Thursday that the aftershocks may be migrating into more northerly portions of the San Andreas Fault zone, closer to San Francisco.

This migration, a matter of some concern, prompted the Geological Survey’s lead seismologist in Menlo Park, Paul Reasenberg, to issue this cautionary statement Thursday morning:

“Most probably, additional earthquakes in the next few days will be smaller than Tuesday’s magnitude 7 earthquake. As for the possibility of an earthquake comparable or larger than Tuesday’s quake, scientists characterize the chances for that as ‘very small but not zero.’

“In a small fraction of the cases observed in California, a large earthquake has triggered a comparable or larger earthquake on an adjacent segment of the same fault or on a neighboring fault. Such triggering, however, is considered unlikely.”

A Geological Survey spokesman, David P. Schwartz, said the “adjacent segment” being referred to in this case is the portion of the San Andreas Fault from Redwood City on the north to the San Jose-Santa Cruz highway on the south. South of that point is the previously locked segment believed likely to have been the focal area of Tuesday’s earthquake.

The Geological Survey on Thursday conducted aerial reconnaissance of the nearby Calaveras and Hayward faults, east and north of San Jose, looking for any signs of “sympathetic” movement in the wake of Tuesday’s earthquake centered between San Jose and Santa Cruz. There were no reports that they had detected anything that could be taken as a sign of an impending quake on one of those faults. The Hayward Fault, in particular, which has not ruptured since 1836, is a source of concern about eventual catastrophic earthquakes in the Bay Area.

Another Geological Survey aerial reconnaissance, meanwhile, found less evidence of liquefaction around San Francisco Bay than had been expected. Liquefaction is the process by which quake shaking changes water-logged soil into something resembling quicksand, thus endangering structures built on it.

The reconnaissance, however, did discover liquefaction at the Alameda Naval Air Station and Oakland Airport. At both installations, runways have had to be closed because of quake damage.

While the chief state and federal earthquake scientific agencies were busy, UC Santa Cruz Chancellor Robert Stevens authorized establishment of a temporary information coordinating center on the campus, a few miles from Tuesday’s epicenter, for quake investigators who have arrived from as far away as New York to inspect the mountainous area where the earthquake originated.

On Thursday, in addition to the Geological Survey and the state Division of Mines and Geology, earthquake scientists from UC Santa Cruz, Caltech, Columbia University and the Livermore Laboratories, as well as a few private consulting geologists, were all tramping through the mountainous terrain from Page Mill Road in Palo Alto to the town of San Juan Bautista, looking for signs of earth displacement or fracturing that would definitely establish the origin of the quake.

In this connection, a UC Santa Cruz geologist, Jeff Marshall, told Times reporter Eric Bailey Thursday that the fissure seen at the hamlet of Loma Prieta was “the most extensive ground rupture we’ve found so far.”

But the Geological Survey’s Schwartz said it was not the surface displacement seismologists have been looking for on the San Andreas.

“There are a lot of lateral spreads, slumps, various types of soil fractures, a lot of cracking through these mountains,” Schwartz said. “This feature has been looked at . . . but it was not tectonic surface faulting of the type we have been looking for. Such faulting would have consistence in its extent. The rupture would extend 30 to 40 kilometers (more than 20 miles).”

Most such fissures that the inspection teams have discovered are ground cracks along roads, Schwartz said. “When you try to trace those out over a distance, you get lost.”

Finding Fault With the San Andreas

Scientists searching the Santa Cruz mountains have found surface signs--fissures and small ruptures--that suggest, but do not confirm, that the San Andreas Fault was responsible for Tuesday’s 6.9 temblor. “Strike-slip” movement may have occurred along the San Andreas when the Pacific Plate, on one side, slipped southward and the North American plate slipped north.

The pattern of aftershocks near the quake’s epicenter at Loma Prieta has begun to migrate to the north, giving rise to concern by scientists that an aftershock comparable to the 6.9 jolt could happen in the near future, although is still regarded as unlikely.