BAY AREA QUAKE : Scientists Aren’t So Sure San Andreas Was at Fault : Seismology: Survey teams fail to find the surface displacement they expected and are checking two smaller faults in the area as possible culprits.

A note of doubt Wednesday crept into earthquake scientists’ initially firm belief that the San Andreas Fault was responsible for the 6.9-magnitude quake that struck the San Francisco Bay Area.

Teams of seismologists who went into the region of the Santa Cruz Mountains where the quake had been centered failed to find the 3-foot surface displacement of earth, or fracture, that they had expected to see running for about 30 miles from the Lexington Reservoir on the north to the town of San Juan Bautista on the south.

U.S. Geological Survey experts said the quake may have been too deeply buried--10 to 12 miles underground--to cause a surface impression on the San Andreas. But they disclosed that they are also looking at two much smaller faults in the vicinity--the Zayante and the Sargent--in an effort to determine whether the quake may have occurred on one of them.

The Sargent Fault, in particular, has a junction with the San Andreas almost at the precise spot where the epicenter of the quake was located. The Zayante is just to the west.

Meanwhile, a minority view briefly surfaced within the Geological Survey as Pasadena-based seismologist Lucile Jones declared that she found that certain patterns of the earthquake were more consistent with it being a vertical-thrust quake along a possibly unknown fault buried under the earth close to the San Andreas.

Scientists at the Geological Survey’s Western regional headquarters in Menlo Park, and Jones’ own superior in Pasadena, were inclined to discount Jones’ reasoning, and later on they said they understood that she was recanting it. Jones could not be reached.

The scientists said that the pattern of shock waves from the earthquake was consistent with it being a strike-slip, or horizontal quake, along the San Andreas or the Zayante or Sargent, rather than a vertical one.

But after the inspection teams reported in without seeing any sign of surface displacement, they conceded that the matter bears further investigation.

“There is some uncertainty,” said Thomas Heaton, who is in charge of the Geological Survey’s Pasadena office. “We’re rather surprised. It is pretty unusual not to see displacement in a quake of this size. But we’re still guessing it’s a strike-slip quake and that it was on the San Andreas.”

Geological Survey scientists in Menlo Park said they are planning to fly over the fault line today, photographing the whole area, and hope to get a better idea of any ground fracturing that may exist.

Calling the mountain region where the quake was centered a “hellacious place to work,” Allan J. Lindh said he remains convinced that solid evidence will be found linking the San Andreas Fault to the quake.

“We’re snowed under with data,” Lindh said. “We could be missing something, but the odds are 9 to 1 she (Jones) is wrong.”

Thrust, or dip-slip earthquakes, which push up mountains in large folds, came to major public attention for the first time only two years ago when such a fault, the Elysian Park Fault, was found to be responsible for the 5.9-magnitude Whittier quake in Southern California.

Most study of earthquakes in California, by contrast, has been on the San Andreas and other great strike-slip faults that quite visibly intersect the Earth’s surface and often have caused obvious displacements in major earthquakes.

Other than the minor tempest over whether the San Andreas Fault could be proved responsible for the present earthquake, the mood in the Geological Survey’s headquarters here, just north of Palo Alto, was quite upbeat Wednesday--despite inconveniences caused by the quake.

Ceiling tiles were down, there were cracks in the walls, computer reels were strewn on the floor, bookshelves were down and the regular power supply was out, necessitating use of an emergency generator. But the scientists were clearly satisfied that their prediction of last year that the epicentral region of Tuesday’s quake was more prone than any other area of Northern California to produce a strong temblor had been borne out.

Geophysicist James Diederich typified the prevailing sentiment at the office when he declared, “Obviously, no one’s happy about the damage and the deaths, but there’s some satisfaction in having the prediction verified and in their being so accurate.”

Meanwhile, in another area of earthquake inquiry, the first teams of scientists to go into sections of San Francisco and Oakland where the heaviest damage in the earthquake occurred found just what they had expected--that the areas involved were ones where the soils were weakest, consisting of Bay Area mud or loosely compacted alluvial deposits or landfill.

A group of seismologists from New Zealand who walked around the devastated Marina District of San Francisco, where gas pipeline ruptures set off huge fires after the earthquake struck, said later that they had found clear evidence of liquefaction, a condition in which the soil, shaken up by a strong earthquake, turns to the consistency of quicksand.

It is in such circumstances that the most violent shaking occurs in a big earthquake, destroying buildings and rupturing pipelines. When gas lines break, major fires often occur, and then there is often insufficient water to fight these fires since the water lines have ruptured as well.

Peter North of the New Zealand group said his party had observed many sand boils--little volcanoes of sand associated with liquefaction--as well as large volumes of water coming out of the ground, even in areas where there had not been water pipes, as well as a great deal of settling of the ground.

Another area of vast destruction, the approach to the Bay Bridge on the Oakland side, as well as the parts of Oakland where the two-tier Nimitz Freeway collapsed, were known to have been over bay mud or alluvial deposits, scientists said Wednesday. There, shaking would have been intense, and in some places, liquefaction may have occurred.

Lindh said the fact that these areas of Oakland and San Francisco were 70 miles from the quake’s epicenter would give quake experts new pause about the devastating effects that a major quake can have on areas widely removed from the epicenter. The lesson has pertinence not only to the Bay Area but also to Los Angeles.

Jeanne Perkins, earthquake program manager for the Assn. of Bay Area Governments, said the collapse of the Nimitz Freeway demonstrates the importance of soil composition in construction.

“Basically, you can avoid building on bay mud or bay fill, or you can accept that you are going to have problems like we’ve had here,” she said.

Thomas H. Holzer, branch chief for engineering seismology for the Geological Survey at Menlo Park, said that regardless of whether one is talking about freeways, gas lines or apartment houses, “the bottom line is that people shouldn’t be building in these areas.”

Holzer said there is often no feasible way to counteract soil liquefaction through building techniques or design. The only sure thing, he said, is to treat the soil itself to prevent such a condition from occurring, either by putting columns of loosely packed gravel deep into the soil, thus allowing water to escape, or compacting the soil with huge machines so that it fits together more tightly and cannot become waterlogged.

Such techniques would not, in the present state of technology, be possible underneath the surface of the San Francisco Bay. But it is these areas of bay mud that underlie the Bay Bridge. A section of the bridge, subjected to some of the heaviest shaking, collapsed Tuesday night, and the mud may have been ultimately to blame for that shaking.

Maugh reported from Menlo Park and Reich from Los Angeles.

Liquefaction and long-distance destruction

Despite being 70 miles from the quake’s epicenter, some Bay Area neighborhoods are believed to have suffered widespread destruction because of the scientific phenomenon known as liquefaction. Much of the Bay Area is built atop mud or alluvial deposits, water-saturated soil that becomes like gelatin during intense seismic activity. Liquefaction also played a role in the Mexico City quake in 1985 and the Long Beach quake in 1933--which both involved destruction some distance from the epicenter.