Standing in the wash of the Santa Ana River, you can see mountains all around you. The San Gabriel, the San Bernardino, and the San Jacinto mountains form a long, semicircular are to the north and east pierced by the passes that lead variously to such outposts on the California desert as Barstow, Indio, and Palm Springs. To the south and west are the lower hills that separate the San Bernardino Valley from the booming conurbations of Orange County and from Los Angeles itself, which lies seventy miles due west.
On those days, infrequent during summer, when opaque brown smog has not blown inland to fill the valley and lap over the rim of the mountains, the contours of the ranges are clearly visible from the valley floor. There is no natural obstruction to the view more substantial than the waist-high scrub. The sharp edges and escarpments of the mountains suggest their dramatic origin, as products of the pushes and pulls on the earth's surface that are even now creating mountain ranges in the West. The white gravel in the dry riverbed has been washed own from the mountains during the flash floods that from time to time strike this bone-dry region. The "river" runs with water for only a few days out of every year.
It can seem the bleakest of settings, especially when the Santa Ana winds blow down from the high deserts at temperatures well above 100 degrees. Yet, like many other harsh regions of the Southwest, this has been a scene of phenomenal growth. As people continue to move to southern California, as Los Angeles has filled up, as real estate prices in Orange County have shot to the sky, many people have taken the next step in from the coast and have brought a population explosion to the "Inland Empire." The orange trees that line Interstate Highway 10 are the remnants of groves that were until the past two decades an economic mainstay of the region. They succumbed first to the smog and then to the subdividers. Some 400,000 people lived in the San Bernardino Valley in 1960. More than 700,000 live there now. There are plans for an "instant city" of 200,000 more people, to be built in the hills outside Chino, where the corners of San Bernardino, Orange, and Los Angeles counties touch.
People have moved to this valley seeking cheaper real estate, the good life in the sun, and the opportunities that economic growth creates. They are an hour's drive from the mountains, an hour from the ocean, and far, far away from such northern, urban cares as frost and economic decline. And their search has led them to the area that, according to many scientists, will be the site of the next catastrophic earthquake to strike the continental United States.
Those who live in the vicinity, as I did when I was growing up, are accustomed to earthquakes. Many, pride themselves on surviving them, as those on the Gulf Coast might on surviving a hurricane. But the prospect that scientists now suggest is different from anything within living memory in southern California.
"We have known for years that major earthquakes in California are inevitable," says Clarence Allen, a geophysicist at the California Institute of Technology. "The scientific attitudes have not changed as much as the public perception that this is a real problem." But both scientific and public attitudes have been heavily shaped by two scientific developments of the past decade.
The first, which has been widely publicized since the early 1970s, is the evolution of the theory of plate tectonics. This theory postulates that the earth's crust consists of a number of large "plates," floating on a molten mass within the earth. There are seven of the larger plates—one bearing most of Africa and half of the Atlantic and Indian oceans, another holding virtually all of the European and Asian land mass—and a number of small ones. As the plates have drifted apart from, and into, each other, their motions have changed the world's geography. They opened the basin that became the North Atlantic Ocean, when the North American plate slid away from the Eurasian. They created the Himalayas, which mark the region where India was driven northward and collided with southern Asia. They stimulate volcanic action, including the recent eruptions at Mt. St. Helens. (As the small Juan de Fuca plate is pushed beneath the North American plate off the Washington and Oregon coasts, it melts at the tremendous temperatures and pressures of the earth's depths and then flows upward, in molten form, as lava.) The motion of the plates can also cleave regions from one another, as is now taking place in California.
The most famous of the "faults" that are responsible for California's earthquakes is the San Andreas—which, from the perspective of plate tectonics, is not just any fault but the boundary between two great plates. To the east of the fault lies the North American plate, which contains more than 99 percent of the land area of the nation, along with Canada, Mexico, and roughly half of the Atlantic Ocean. To the west of the fault is the Pacific plate, which runs beneath the ocean to the Mariana trench—where it disappears beneath the Philippine plate. The city of San Francisco is on the North American plate. Santa Cruz and Monterey, immediately to the south, are on the Pacific plate. So are Los Angeles, San Diego, and the other population centers of southern California.
Through the millennia, the two plates have been moving past each other along the San Andreas Fault, the Pacific regions slipping north, the North American south. The Baja California peninsula resembles in length and width an indentation on the western coast of Mexico. That is exactly where it came from, geologists say, before it moved north with the rest of the Pacific plate. The motion of these two plates goes on at rates as high as 5 or 6 centimeters a year—very fast, by geologic standards. Rocks on the Pacific side of the fault near Point Arena match those on the North American side at Taft, 350 miles to the south. The two regions abutted one another until the fault motion started about 25 million years ago. Ten million years from now, Los Angeles and San Francisco should lie in the same latitude.
For California, the significance of the tectonic theory was to explain the basic mechanism of earthquakes, not only on the San Andreas Fault but also on the many others that parallel it and the few that run at right angles to it. Year by year, the plates creep past each other; from time to time, rocks must yield. The largest recent earthquake in California, the 1906 "fire" (as civic boosters chose to call it at the time) in San Francisco, could be seen from the perspective of plate motion as the event through which the northern section of the San Andreas Fault released the strain that had been accumulating over the years.
It took a second and more localized scientific development to add the note of certainty to forecasts that a catastrophic earthquake will strike southern California. The man most often identified with this development is Kerry Sieh, a geologist at Caltech. His contribution has been to estimate the schedule on which the rocks along the fault give way.