The Next Big One. . .

By J. Madeleine Nash/Chicago

The Big One. For decades Californians have lived in fear of the tectonic monster that inhabits the San Andreas Fault, a spectacular, 800-mile-long slash through the earth's surface. But last week's earthquake was a sobering reminder that the mighty San Andreas is not the state's only seismic menace. A web of smaller cracks crisscrosses the fragile California crust. Many of these faults are well known. But others lie hidden deep underground, like the one that gave Los Angeles its latest disaster. Until the earth moved, the residents of the northwestern suburb of Northridge had no idea that a deadly fault lay right below them, nine miles down.

Registering 6.6 on the moment-magnitude scale, a measure of earthquake energy that among scientists has largely replaced the Richter scale, the Northridge temblor didn't qualify as a Big One. (The San Andreas Fault, 30 miles east of Los Angeles, could produce a magnitude-8 quake, which would be more than 85 times as powerful.) But don't tell that to the people of Northridge and surrounding communities.

Soon after the first tremor, seismologists began trying to map out the newly revealed fault and determine how it is connected to other fissures in the region. To do this, the scientists will have to track the locations of hundreds of aftershocks, a lengthy and tedious process. At first it was thought that the quake might have resulted from a previously unmapped extension of the Oak Ridge Fault, which angles past the city of Ventura and into the Pacific Ocean. But as researchers fanned out through the San Fernando Valley, other theories emerged, including the possibility that the fault was not connected to any known system. Observes geophysicist Mark Zoback of Stanford University: "Individually these faults are smaller than the San Andreas and give rise to earthquakes that are less frequent and less severe. But collectively they represent a huge hazard because they are everywhere."

California is an earthquake zone because it lies on the boundary, marked by the San Andreas Fault, between two huge sections of the earth's crust, known as plates. Gliding atop a sea of superheated rock that surrounds the planet's molten outer core, the Pacific plate -- a thick slab to which Los Angeles is attached -- is very slowly pushing its way north and west, past the North American plate to the east, which is moving in the opposite direction. Most of the time, in most places, the two plates are snagged; they block each other's progress, and tremendous pressure builds up. Every so often, the snag breaks loose in one spot, the plates slide past each other and the ground suddenly shifts: earthquake.

A similar process goes on along smaller cracks in the crust outside the main fault line. But while the earth slides horizontally along the San Andreas, many of the other fissures, including the one under Northridge, are called thrust faults because they cause the ground to move vertically. Given enough time, they help form mountains and valleys.

Surprisingly, the hazards of thrust faults were largely overlooked until 1983, when a fierce temblor hit the small central California town of Coalinga. The culprit turned out to be a deeply buried fault (four to 10 miles down) that no one had known about. Its only sign on the surface had been a fold, or buckling, in the earth's crust. Many scientists had thought such folds were harmless, formed by an imperceptibly gradual lifting of the ground. But when Ross Stein, a geophysicist with the U.S. Geological Survey, and geologist Robert Yeats of Oregon State University examined the seismic record of fold belts all around the world, they uncovered a different story. Folds, they warned, also grow through repeated earthquakes.

Around the same time, other researchers began examining maps drawn up by petroleum geologists. Thrust faults, and the folds they form, are excellent traps for gas and oil, and many such subterranean spots have been found in the Los Angeles region. But were these structures still active? In recent years, nature has provided an unequivocal answer. Since 1987, when the Whittier Narrows earthquake caused eight deaths and $350 million in property damage, about half a dozen quakes of significant size have rattled along thrust faults beneath greater Los Angeles. All this activity, many scientists speculate, may . be a symptom that overall tectonic pressure in the region is increasing. For while temblors on secondary faults relieve stress locally, they often put greater strain on larger faults nearby. Of particular concern is the southern part of the San Andreas, which in 1992 was greatly perturbed by a major, 7.3 quake centered in the Mojave Desert town of Landers.

The good news, say scientists, is that last week's tremors had only a mild impact on the San Andreas itself. The bad news is that they increased subterranean stresses closer by. Caltech geologist Kerry Sieh, for one, is worried that the violent release of energy may have adversely affected the Elysian Park system, a deeply buried network of thrust faults directly under Los Angeles. Parts of this system have lain dormant, Sieh says, "since before Abraham." But he cannot predict when the faults might awaken.

What scientists can do, with fair reliability, is estimate the magnitude of tremors likely to occur on a particular fault. In general, the longer the rupture, the more powerful the expected quake. Unlike the San Andreas, the Elysian Park system is not large enough to unleash an earthquake of magnitude 8. But some scientists believe it might be capable of a 7 or even a 7.5, especially if more than one fault segment should give way at the same time. This is what happened in 1992, when the Landers earthquake hopscotched from one fault to another, in the process gathering enough power to push up a 6- ft.-high ridge of rock. Should the Elysian Park system, which snakes beneath downtown Los Angeles and the Hollywood hills, let loose with similar force, it would make last week's monster seem tame.