California's Fate

A big quake in the offing?

Though California is plagued by frequent temblors, it has suffered only two major earthquakes in recorded history: the fabled and destructive San Francisco quake of 1906 and an even bigger shock in 1857, which rocked the then sparsely populated southern and central parts of the state. Now that California is the nation's most populous state, it could suffer incalculable damage and thousands of deaths in a major quake. Such a quake will almost certainly happen and, says a young California scientist, probably within the next half-century.

Kerry Sieh, 27, a Caltech geologist, bases his prediction on Southern California's earthquake history, which until recently was quite sketchy; the earliest reported quake, an apparently minor temblor described by a Spanish explorer, was chronicled in 1769. Seeking evidence of earlier quakes, Sieh in 1974 began a painstaking tour of hundreds of miles of the San Andreas Fault in central and southern California. The following year, under an ancient marsh that straddles the fault 88 km (55 miles) northeast of downtown Los Angeles, he struck pay dirt.

Bulldozing a trench four meters (13 ft.) deep, he found several distinct breaks in the strata of sand, silt, gravel and peat that had been deposited on the bottom of the marsh over the centuries. Each break represented a sudden shift of at least a meter or two between the land masses on opposite sides of the fault--unmistakable signs of a major earthquake. Using radioactive-carbon dating techniques to determine the age of the dead organic material in the peat layers, he has now determined that the quakes occurred around A.D. 575, 665, 860, 965, 1190, 1245, 1470 and 1745. Thus the intervals between the quakes varied from 55 to 275 years, and the average interval was 167 years. That was the information Sieh had been looking for.

Says he: "The important thing to remember is that prior to this research, we didn't know if major earthquakes in Southern California occurred every 1,000 years or every 100. Now we know." Because the last big quake hit the area in 1857, Sieh concludes, "a major earthquake can be expected there within the next 50 years or so rather than, say, the next 500." Sieh may well be right. His telltale marsh is only 24 km (15 miles) from Palmdale, the center of a region that has been bulging upward for 17 years in what some seismologists feel is a prelude to a major earthquake. qed

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