Exploring an Ominous Bulge

Scientists study puzzling earth movements in California

Nearly 300 scientists this week are beginning one of the most extraordinary surveys in the annals of U.S. geology. Under a $1.4 million program conducted jointly by the National Geodetic Survey, the U.S. Geological Survey and local authorities, 36 teams of specialists are fanning across Southern California with levels, gravity meters and other exotic scientific gear for three months of intensive measuring. Their mission: to study the gradual--and extremely puzzling--elevation of a large region centered around the small town of Palmdale (pop. 12,800) that has taken place recently in this seismically active area.

The survey is no casual exercise in abstract research. Palmdale sits atop the San Andreas Fault, the great crack that marks the boundary between two of the earth's shifting tectonic plates. Scientists fear that the ominous rise of the 83,000-sq.-km. (32,000-sq.-mi.) region, nicknamed the Palmdale Bulge, could be the first hint of a future major earthquake along that section of the fault, which lies only some 56 km. (35 miles) north of downtown Los Angeles.

Their efforts should produce, in effect, a stop-action picture of the rapidly changing features of the great land swell. From this picture, they hope to obtain a more definitive view of the strange events around Palmdale. That knowledge, in turn, could eventually enable them to predict if--and possibly when and where --an earthquake will strike. "'Uplifts have been observed before several major earthquakes," notes Seismologist Peter Ward, chief of earthquake mechanics and predictions for the U.S.G.S. Among these quakes is California's last large temblor, the one that shook the San Fernando Valley in 1971, taking 58 lives.

Initially, seismologists thought that the Palmdale uplifting was the result of tiny fissuring that occurs in rocks when they are subjected to great stress, expanding the volume of the rock. But the bulge appears much too large to be explained only by this effect, which is known as dilatancy and has already been used to make experimental earthquake predictions. Instead, scientists are leaning increasingly to the idea that other factors may be involved, notably a concept called elastic deformation, in which moving land masses snag against each other and force some of the earth's crust to roll up like a rug pushed against a wall. In this particular case, the snag is apparently occurring along a dogleg bend in the San Andreas Fault in the vicinity of the Palmdale Bulge. There the two huge plates, which normally grind past each other in opposite directions, appear to be locked together, causing a buildup of tremendous strain that eventually must be released in one or more quakes.

Whatever the cause of the perplexing uplift around Palmdale, scientists have plenty to study in the region. Though the bulge has been rising in most areas, it fell almost 15 cm. (6 in.) in at least one section between Palmdale and Pasadena from 1974 to 1976, a phenomenon that baffles experts. In addition, scientists are disturbed by a flurry of minor tremors, measuring up to 3 on the Richter scale, that have occurred along the fault southeast of Palmdale, since November 1976. Says Don L. Anderson, director of Caltech's seismological laboratory: "Until these recent swarms there has been very little action along this section of the San Andreas Fault. It could be significant."

Finally, while parts of the fault appear to be jammed, recent work by Caltech scientists--using ultraprecise radio telescopes as measuring instruments and signals from distant quasars (see following story) as benchmarks--shows that there has been relative motion of up to 20 cm. (8 in.) in only three years between the opposing plates just south of the San Gabriel mountains. That motion, which may be rapid by San Andreas standards, also mystifies the researchers. Says Caltech's Peter MacDoran, who has been directing these measurements. "What we need is a nice, big nondestructive earthquake that we can intensely study. Then maybe all these puzzling pieces would fit together "

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