National Geographic : 1973 Jan
calIornilas San Andreas Fault By THOMAS Y. CANBY Photographs by JAMES P. BLAIR BOTH NATIONAL GEOGRAPHIC STAFF Raw wound across a tortured land, the San Andreas Fault slashes the desolate Carrizo Plain 100 miles north of Los Angeles. A ranch fence banked with blown tumbleweed borders the escarpment at right. Slicing for 700 miles (page 42), the fault system marks the boundary of two gigantic slabs of earth's crust. The eastern plate carries most of California and the rest of North America; the western bears a sliver of coastal California and the Pacific Ocean. As the Pacific plate pushes northwest in relation to the American plate, friction binds the edges. Periodically the rock breaks along the fault and shifts, unleashing earthquakes. Such a rupture caused the disastrous San Francisco earth quake of 1906. Many scientists predict another major quake somewhere along the fault before the end of the century. FROM FIRST LIGHT OF DAY we have occupied our strategic hilltop, overlooking a long, shadowy crease in the land that marks the San Andreas Fault. Now we deploy around a concrete-and bronze geodetic marker, one of many that pepper this quake-prone area south of San Francisco. Our detachment is small-four fieldmen of the United States Geological Survey, a helicopter crew, photographer Jim Blair, and myself. Our armament is a laser gun, which we now train across the San An dreas toward a reflector ten miles away. Our mission: To measure within a hundredth of an inch the amount the markers have shifted in the past year-and thus how much the State of California is being wrenched and warped by the great fracture zone that scars most of its length and part of northern Mexico. With a sudden flash our laser beam strikes the re flector across the fault and bounces back, giving us the distance. Later a computer will scrub out atmospheric distortions detected by instruments on the copter. Ul timately our measurements will join thousands of others made along the fault to advance a goal that both the Federal and California Governments regard as urgent: that of earthquake prediction. In the not-so -distant future, scientists believe, the seismic forecaster will join the weatherman, able to give timely warning of danger to the one American in ten who lives in earthquake country. A dramatic understanding of why California is earthquake country has emerged only in the past decade. It is based on the newly accepted theories of continental drift and sea-floor spreading described in the article preceding this one. These theories de pict the San Andreas Fault as the meeting place of two of the many segments of crustal rock that form the planet's surface. Pushed or pulled by titanic forces still not clearly understood, these segments-geologists call them "plates"-slowly drift. In some regions they collide, buckle, and lift up mountains. Along the San Andreas they grind past each other, scraping edge to edge. The Pacific plate carries California's coastal edge northwest in relation to the rest of the state, which rides the American plate (map, page 42). Some scientists say the plates grind past each other at only an inch a year; others believe they gallop along at three times that speed. At an in-between rate of two inches, Los Angeles, riding the Pacific plate, would majestically draw abreast of San Francisco in only ten million years, on its way toward oblivion in the Aleutian Trench (page 6). If the plates would slide smoothly past each other, as indeed they seem to do along part of the fault, most Californians would happily ride along. This smooth motion is what geologists know as "creep." But stupendous pressures along most of the plate boundary create friction; the edges lock.