BAY AREA QUAKE : Quake Fingerprints Provide Perplexing Clues for Scientists

TIMES SCIENCE WRITER

Geologist David Schwartz strides slowly along the four-foot-wide gash through Freda and John Tranbarger's lawn just off Summit Drive in the Santa Cruz Mountains.

It's chilly here, this Friday morning after the great earthquake of 1989, and gathering clouds portend an afternoon storm that may wash away some traces of the temblor. Already, Schwartz's search for clues to the quake's nature has been interrupted by three television crews and numerous reporters.

Schwartz has been exceptionally patient answering the questions. But now, as he follows the rapidly narrowing crack along a fire road, down a gully, back up the steep slope and through a heavily wooded area back to Summit Drive, a note of tension enters his voice--and a much larger amount of perplexity. This quake is just not behaving the way they expected it to.

In a magnitude 7 temblor along the San Andreas fault, like the Loma Prieta quake, geologists would expect to see a 30-mile-long gash here, near the quake's epicenter, with perhaps 3 to 6 feet of what they call right lateral movement. Standing on one side of the crack, it should look like the ground on the other side has moved three feet or more to the right as the giant Pacific tectonic plate slipped northwestward relative to the North American plate.

But what they have found instead is a 250-yard-long gash--as well as much narrower cracks that continue into the distance--and if there is any lateral movement, it unexplainably is mostly in the wrong direction. "To have this degree of cracking without a lateral event is just . . . well, it's just not what we expected."

The team will eventually conclude that the quake certainly did occur on the San Andreas fault, but they will probably be trying to explain the mystifying features for weeks.

Schwartz is one of a team of eight U.S. Geological Survey researchers who are clambering through the Santa Cruz Mountains, measuring the width of cracks, charting their lateral movement, surveying their lengths--trying, in effect, to take the earthquake's fingerprints. At least a dozen others from the survey's office in Menlo Park are out scouting other locations in the Bay Area, as they have been every day since the Tuesday quake, questioning witnesses and looking for evidence of the quake's appearance.

Evidence, witnesses, fingerprints. It sounds like a police investigation, and in many ways it is. Tracking down the evidence after a quake "is really like a detective story," says geologist Manuel G. Bonilla, who has been on the survey's beat since 1947. "You have just a few clues, and you have to make inferences about things. That's true of geology in general. You have a limited amount of information and you have to make the most reasonable interpretation. That's partly why it's so much fun."

Unfortunately, just like in a murder case, the clues are disappearing rapidly. Caltrans has already repaired breaks in Summit Drive that would have provided clues, TV crews and print reporters are trampling over cracks and fissures, farmers south of Watsonville are already plowing fields where good evidence had been spotted in an aerial survey, and the impending rain threatens to destroy much of the evidence that's still left.

This one is going to be difficult. At lunch outside the devastated Summit Market, where broken bottles and cans still litter the aisles and flies are beginning to collect, Schwartz observes that tracking a typical San Andreas quake with a pronounced surface fissure "is like solving a jigsaw puzzle with 10 pieces. This is like a puzzle with a thousand pieces." And in this case, someone else observes, "most of the pieces are blank."

But if they can put all the pieces together and learn exactly what happened in the Loma Prieta quake and why, it will give them crucial insights about how to prepare for future earthquakes and to minimize damage. Eventually, every geologist dreams, such information may lead them to a technique for predicting temblors before they occur.

Survey headquarters have been a beehive of activity since the quake occurred at 5:04 p.m. Tuesday, knocking power out and taking most of their instruments off-line. Photographers and TV news crews are disappointed Tuesday evening and Wednesday when they arrive at Menlo Park to take the now-de rigueur pictures of the seismograph's squiggly record of the temblor's shaking. The charts show only a quarter-inch blip before the seismographs died: No visuals.

Senior scientists are on the phone all day, trying to compare their information with that collected by other field stations like those in Pasadena and at USC. Allan Lindh, who had, in effect, predicted Tuesday's quake, is on the phone virtually all day. Over a year ago, the survey had predicted a 30% chance that a quake this size would appear at this location within the next 30 years.

With his stringy brown full beard and his long hair, Lindh looks the part of a campus guru from the 60s. But on the phone, he is the voice of calm reason, telling first a New York City reporter, then a Toronto reporter that "not many people would have predicted the deaths and destruction" in San Francisco, so far away from the quake's epicenter eight miles outside of Santa Cruz. "It clearly raises our level of concern about the Hayward and other faults in the area."

Lindh refuses to exult in the success of the prediction because of the death and damage, but there is some satisfaction in his voice when he later says, "For once, we seem to have been right."

On Wednesday, Lindh and others are perturbed by reports that Lucile Jones of the survey's Caltech office is contradicting them. The Menlo Park geologists are calling the Loma Prieta quake a classical slip-strike quake on the San Andreas fault. In a slip-strike quake, the two tectonic plates scrape sideways against each other. But Jones is saying in Pasadena that it's a thrust quake, in which one plate tries to climb over the top of its neighbor, and that it may not be on the San Andreas.

"Lucy is a wonderful lady, but she's a long way from here and talk is cheap," Lindh says. "If we had done the same thing and contradicted her account of the Palm Springs quake, she would have blown her top." By the end of the week, the quake is definitely pinpointed on the San Andreas, although it may have some thrust components, but Jones' comments still rankle.

While senior officials are collating data and fending off the press, everyone else is pressed into service, scattering throughout the region searching for evidence of the quake activity. One team walks through the devastated Marina District, searching for evidence of liquefaction--ground failure that turned the filled-in soil to a form of "instant quicksand" incapable of supporting buildings.

They find plenty of evidence, mostly in the form of "sand boils," volcano-like mounds of sand that are forced to the surface when the soil goes through liquefaction. They chart the locations of each boil and collect samples of the sand.

Another team goes to check the base of the Bay Bridge, where a collapsed segment promises to foul Bay Area traffic for weeks. They search for evidence of ground failure around the bridge supports like that in the Marina, but find none. They conclude that simple shaking did in the bridge. Another team checks the ground at the Cypress structure, the now-infamous section of Interstate 880 where the upper level of roadway collapsed, crushing 50 or more cars underneath. Again, no ground failure.

Ken Lajoie and Dan Ponti charter a single-engined plane and cruise at low altitude around the entire fringe of the bay. "Even flying at 500 feet and 100 m.p.h.," signs of liquefaction and surface damage are clearly visible, he reports later. They are surprised that liquefaction has not occurred in many areas where they thought it might.

Aside from the Marina, most of the liquefaction seems to have occurred in the northwest corner of the bay under the Naval Air Station in Alameda and Oakland Airport. Both are built on reclaimed areas over a Pleistocene-era formation called the Merritt Sands. The formation itself has settled firmly into place by now, but the sand was used to fill in the airports' land and has liquefied badly.

Lajoie notes that he once worked for PG&E;, excavating trenches in that area, and they excavated hundreds of Indian skeletons. "People always bury their dead where it's easiest to dig," he notes.

Every lab has a "Doc," a senior scientist respected or even revered by other researchers for his or her experience and knowledge. At Menlo Park, it's Bonilla, who has been there since 1947. Thursday afternoon, Doc Bonilla takes a reporter along as he goes to check damage in Foster City.

Developer T. Jack Foster built Foster City on bottom land that had been reclaimed by building a levee around it. The construction of Foster City was controversial because it is built on fill like that in the Marina District and is thus susceptible to liquefaction.

And the whole extent of Foster City is lower than the Bay at high tide. If the levees were to break, Doc said, the flooding would be "devastating."

Local reporters have persistently been asking why there wasn't more damage in Foster City, and survey officials are clearly concerned about its fate in future temblors. But after visiting there, Doc said: "It doesn't appear that much happened (in Foster City) this time. Still, that doesn't mean it won't happen in the big one." Nonetheless, Doc is now "more confident" about Foster City's fate in the future.

Now it's Friday, and Schwartz and his colleagues are tramping along Summit Drive--in part because the news media and one or two geologists have identified the Tranbarger's yard as the site of the quake.

But work is difficult because of the distractions. Airplanes and helicopters are flying overhead, news crews are filming, tourists and neighbors are gawking.

As Schwartz examines one crack on the side of Summit Drive, a taxi pulls up and three Japanese scientists emerge. With gestures and a little English, Prof. Moriya of Nihon University explains that he has just flown in and has come to see the quake. He shows Schwartz a book he prepared about the Spitak quake in Armenia and indicates he will do the same here. Clutching a newspaper map, he asks for directions to the epicenter. Another team of Japanese are subsequently spotted in a dark blue limousine.

Other geologists abound, from the University of Nevada, UC Berkeley, Cornell, the state geologist's office. Schwartz sees two men carefully mapping the cracks near the Tranbargers' house, a process that he is just starting. One is young Burt Hardin, who works for a geological consulting company. They don't have a contract to study the quake, he says, but "it's a once-in-a-lifetime chance."

Gently, perhaps resignedly, Schwartz assures him: "There'll be other chances."

As the day wears on, the geologists carefully measure and plot the cracks, drawing each site in their notebooks and recording their observations. Schwartz says he plotted 3,400 sites along a fault in Guatemala for his doctoral thesis: "Looking back at my notebooks, I can visualize every one of them."

On the roadways, they spray-paint sharp-edged stripes across the cracks so subsequent movements can be detected. At other sites, they force a row of 18-penny galvanized nails into the ground across the cracks.

The team is putting in benchmarks so they'll know if further movement occurs. Sometimes, slipping along the fault continues for weeks. Sometimes, from a deep-centered quake like this one, the surface doesn't rupture until days or even weeks later.

By the end of the following week, the media will be gone, the repair crews will have finished along Summit Drive, life will go on pretty much as usual. Despite the extensive ground cracking, damage has not been severe.

But Schwartz and his brothers-at-arms will be back, daily at first, then less frequently. They will continue returning until they know the quake has finished. Then they'll write up their report and wait for the next earthquake.

Geologists say there's a 50% chance of another magnitude 7 quake in the Bay Area in the next 30 years, and there are always lots of smaller temblors to chase. They probably won't have to wait long.

Copyright © 2019, Los Angeles Times
EDITION: California | U.S. & World
67°