Still Waiting for the Big One

By J. MADELEINE NASH

Even as the earth rocked and rolled, California's army of seismologists rallied into action. In Berkeley, University of California graduate student Anthony Lomax felt the sidewalk shiver and watched telephone poles sway, then rushed to his seismographic station. "The instruments were off-scale!" he marveled. Within minutes the scientists on duty had pinpointed the epicenter of the quake in the rugged Santa Cruz mountains some 50 miles away. The spot was no surprise: it lay on the San Andreas fault, a great gash in the earth that extends nearly the length of the California coast. Even before the quake, the Santa Cruz area had been identified as a prime candidate for a big tremor. "We still can't predict when an earthquake will occur," says geologist Clarence Allen of the California Institute of Technology, "but at least we can say where an earthquake is most likely."

As aftershocks jolted the area, geologists fanned out into the mountains to look for changes wrought by the quake. They examined winding roads for fractures and shot laser beams across the fault to measure expected shifts in terrain.

From the start, scientists had a firm answer to the question uppermost in every Californian's mind: the earthquake that hit San Francisco last week was not the long-feared Big One. While it packed a punch, measuring 6.9 on the Richter scale,* the 1906 earthquake was 25 times as strong, at 8.3. Warns Dallas Peck, director of the U.S. Geological Survey: "The question is not whether a bigger earthquake is coming. The question is when."

This quake did not begin to exhaust the pent-up energy in the 800-mile-long San Andreas system. In a list of seismic danger zones compiled by an expert panel last year, the section around Santa Cruz ranked only sixth. The area believed most likely to have a devastating quake in

the next three decades lies near Palm Springs.

Like a river with multiple tributaries, the San Andreas is associated with numerous lesser faults, among them the Hayward fault, which undercuts Berkeley and Oakland, and the San Jacinto fault, near San Bernardino. Some parts of the San Andreas are more dangerous than others. One segment that lies to the south of the Santa Cruz mountains does not appear prone to large jolts at all. "It just creeps along," says geophysicist Ross Stein of the USGS. "Probably & there's some remarkable material down there that, like talcum powder, acts as a lubricant."

The earth is constantly moving underfoot. Its surface, cracked like ancient pottery, is broken into 15 large pieces. These pieces of crust, called plates, restlessly roam about, driven by plumes of molten rock that roil up from the planet's superheated core. Many of the world's largest earthquakes occur at the boundaries of such plates. The San Andreas fault system divides the Pacific plate and the North American plate, which grind past each other at the pace of 2 in. a year. But this movement of the plates is not uniform. Along fault zones the plates tend to become "locked," resisting the overall motion. Explains Berkeley seismologist Robert Uhrhammer: "Stress builds up in these areas that are in effect welded shut. It's as if the rock were being stretched like a big rubber sheet." At a certain point the rock snaps, allowing the plates to slip and release stress. The result is an earthquake.

During the 1906 tremor, the plates on either side of the San Andreas lurched past each other by as much as 20 ft. Over time, such jumps add up. "In 30 million years," Berkeley seismologist Bruce Bolt says, "Los Angeles will become a new suburb of San Francisco."

Even though the mechanics of earthquakes are understood, accurate prediction of their occurrence has remained beyond reach. Earthquake forecasting is mostly based on past history. If a fault once generated a big earthquake, it can be assumed that it will do so again. But just where and when will the next big break occur? Here scientists are beginning to make headway. Geophysicist Wayne Thatcher of the USGS notes that the 1906 quake ruptured a 260-mile-long section of the San Andreas, extending from Cape Mendocino to San Juan Bautista. But the plate movement along the southern portion of the rupture was minor compared with the far greater movement in the north. To Christopher Scholz of Columbia University's Lamont-Doherty Geological Observatory, this meant one thing: the southern section of the quake zone had retained an enormous amount of stress. "It was," he says, "ready to go." And last week it did.

Still, there is a vast difference between suggesting that an earthquake is likely to happen and pinpointing when. For now, scientists cannot say whether a specific section of the San Andreas fault will snap in one year's time or in a hundred, but they are working on it. Seismic silence is one clue. Soundings taken along the San Andreas over the past 15 years showed that the small earthquakes that are a daily event along other parts of the system were not occurring in the Santa Cruz mountains. Scientists argued over the significance of this blank spot in the data. Then a year ago, activity ominously resumed, and last August brought a damaging earthquake. Such an increase in activity, notes Columbia's Scholz, seems to indicate that stress has built up to the point where a major release is imminent.

Halfway between San Francisco and Los Angeles, near the tiny town of Parkfield, scientists are conducting an experiment that they hope will open the door to a new era of earthquake prediction. Along a 20-mile section of the San Andreas, researchers have sunk strain gauges up to 1,000 ft. deep into the earth and laced the surface with "creep meters" that measure rock movement. "We're listening to the heartbeat of this section of the fault very, very closely," says the Geological Survey's Thatcher. The Parkfield section of the San Andreas is unusual in that it is the Old Faithful of earthquake zones, generating moderate tremors every 20 to 27 years. The last Parkfield earthquake occurred in 1966, which means that the next one should strike between now and 1993. By keeping detailed track of underground changes over time, scientists hope to identify precise signals that an earthquake is imminent.

The size of an earthquake is determined partly by the length of the fault segment that slips. In addition, large earthquakes tend to be spaced further apart than small ones, since it takes a much longer time to accumulate sufficient stress. While scientists cannot say exactly where or when the next Big One will hit, they are not without hunches. Southern California, which has not had a Big One since 1857, is every seismologist's first bet.

Still, as last week's quake in San Francisco demonstrated only too well, it does not take a Big One to deal a lethal blow. For this reason, some geologists think that the Big One has been overemphasized as a near-term threat. There are faults up and down the California coast capable of equaling the latest quake, and that is enough reason to worry. Likely candidates for significant quakes in Northern California include not only Berkeley and Oakland but also the Silicon Valley. The Los Angeles Basin, for its part, has experienced an increase in small earthquakes, which many seismologists find alarming. The message from Mother Nature seems unmistakable: Be prepared.

FOOTNOTE: *The Richter scale is logarithmic, so each additional point represents a tenfold increase in severity.