National Geographic : 1973 Jan
TO TAKE THE MEASURE of the San Andreas system, Jim Blair and I flew it in hops, skips, and jumps from its origins in sweltering Mexican sands to its northernmost springboard into the Pacific, only a hundred miles south of Oregon. As it unfolded, we were astounded, almost shocked, by the viv idness and violence of its mark upon the land. Circling the head of the Gulf of California with Caltech seismologist Thomas C. Hanks, we looked down on one of the fault's most recent works. During the past four million years it has torn the rocky rib of Baja Cali fornia from the breast of mainland Mexico, giving birth to the gulf. Here in the south, Tom Hanks explained, the fault is splayed, taking the form of several parallel fractures. Two of these loosened a great slab of desert floor and let it slowly drop between them-the Imperial Valley. Hot springs, symptoms of the fever afflict ing this fitful region, steamed below as we skirted the Salton Sea. Ahead, a low escarp ment took shape-the southernmost trace of the San Andreas Fault itself. Where no scarps or sheared hills marked the fault, I learned to follow its path by the contrasting rock colors on each side, or by streams whose beds had been offset by earth movements across them (page 40). Often a thin line of gray marked the fracture. This was "gouge," rock ground into fine flour by the millstones of the plates. North of Indio the fault bends abruptly west, a jog that here brings the plate edges into head-on collision. Struggling to pass, they gradually compress and thrust up the towering Transverse Ranges. For the hundred miles between Cholame and San Juan Bautista, truly great quakes are unknown, although lesser ones abound. This is the realm of creep, where the fault's sides slide past each other at about an inch a year. South of Hollister this steady creep is slowly ripping apart an old winery building. Streets of the city itself are being offset. Pursuing the fault north along the San Francisco peninsula, we flew over glistening San Andreas Lake, which lies within the fault trough (pages 48-9). It is for this lake and its elongated valley that the fault is named. North of the Golden Gate the fault follows the coast, skipping at several points into the sea. At Shelter Cove only a gentle crease marks the quiet headland where it dives for the last time into the ocean. We landed at the northern California town of Garberville. There were no cabs at the When the earth twitched: Collapsed overpasses lie in ruin and toppled cans strew a supermarket after an earthquake along a minor fault shook the San Fernando Valley on February 9, 1971. Measuring a moderate 6.5 on the Richter scale, it left 64 persons dead and caused half a billion dollars' damage in suburban Los Angeles.