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Scientists Search For Surface Rupture From San Andreas Fault With AM-SF Quake-Bjt

October 20, 1989

LOMA PRIETA, Calif. (AP) _ Scientists converged on the Santa Cruz Mountains on Friday to look for the origins of Northern California’s killer earthquake as mountain residents resumed life in the fault zone.

Geologists were investigating whether a fissure hundreds of yards long and as wide as 20 inches in some spots was in fact the result of the San Andreas Fault rupturing to the surface.

″The question we’re trying to answer is whether that break is the primary fault or something else,″ said Tom Fumal, of the U.S. Geological Survey in Menlo Park.

Seismologists believe the infamous fault’s southern Santa Cruz Mountains segment was responsible for Tuesday’s disastrous 6.9-magnitude quake. But doubts have been raised by their difficulty finding where the fault broke through to the surface of the ground.

Finding the surface trace of the fault is important because it would confirm the San Andreas indeed caused the quake. A 1988 USGS report said the fault’s Santa Cruz Mountains segment had a 30 percent chance of causing a strong quake within 30 years.

If it turns out that Tuesday’s quake was caused by some fault other than the San Andreas, that risk likely would remain unchanged. If the San Andreas did cause the quake, severe quakes on that segment of the fault would be less likely in coming decades.

By Friday afternoon, state geologists found fissures caused by seismic ground settling. But they failed to turn up evidence that the underground rupture of the San Andreas had broken to the surface of the ground, said Max Flanery, of the California Division of Mines and Geology in Sacramento.

Tuesday’s major quake should have caused ground on one side of the fault to slip past the ground on the other side by at least several inches to a few feet. But such ″offset″ was found only on two cracked roads near the epicenter, and it totaled only a couple of inches, said Bob Streitz, a geologist with the state.

The aftershock zone from Tuesday’s quake stretches from the Lexington reservoir south of Los Gatos southward some 25 miles to San Juan Bautista, so the fault could have broken through to the surface along that line, Streitz said.

USGS scientist David Schwartz said the actual fault rupture causing the quake probably occurred as deep as 12 miles underground. He said it is possible the breakage might not migrate upward, or might take weeks to reach the surface. Some say rupture of the San Andreas deep underground might cause several smaller surface ruptures instead of a single crack in the soft soils at surface level.

The search for fissures has been complicated by the remote region’s thick brush and trees, mountainous terrain and difficult access.

The crack on which scientists focused Friday is about five miles from the quake’s epicenter. The fissure is nearly 17 feet deep at one point, runs across a two-lane mountain road, and up Linda and Roger Wolleson’s gravel driveway, which they jokingly dubbed ″Allstate (insurance) Alley.″

The Wollesons were not in their secluded home at the time of the 5:04 p.m earthquake. The temblor knocked their home off its foundations and flipped their water bed mattress out of its frame.

″Most of the people up here are pretty sturdy and we’ll try to rebuild, but to know so many people who have to glue their lives together is pretty impressive,″ Mrs. Wolleson said.

The Wollesons, who have been living in their home shrouded by redwoods for 16 years, were unaware of any fault beneath their property.

″You know it’s very beautiful here,″ Wolleson said, explaining their decision to remain. ″We’re just hoping the worst part is over.″

But Lucia Sutton, a longtime resident, said, ″My husband is absolutely ready to leave this area. I’ve never seen my husband shaking like he was and it wasn’t because of the problems yesterday. He was just so mad and he just wanted out.″