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Unearthing the earth's history.

When and where California's next big earthquake will occur and how bad it will be are questions geologists Kerry E. Sieh of Caltech in Pasadena has been trying to answer with some pioneering techniques in the past few years. His most recent study, a collaborative effort with Ray Weldon of Occidental College in Los Angeles, published in the June GEOLOGICAL SOCIETY OF AMERICA BULLETING, supports earlier predictions that the event will occur in the southernmost 300 kilometers of the San Andreas fault, the geologic seam running northwest/southeast along the length of the state. But it also suggests that the quake may actually be overdue, by about 115 years.

Sieh's approach is to gather deatiled information about past earthquakes at specific sites in order to establish probabilities about future quakes (SN: 12/24 & 31/83, p. 404). Sieh and Weldon excavated a site on the San Andreas fault at Cajon Pass, 60 miles northeast of Los Angeles, and pieved together the site's earthquake history over the last 1,500 years. "Our techniques," Sieh told SCIENCE NEWS, "were not unlike those archaeologists use."

The displacement of sedimentary layers of peat, silt and sand at the excavation site yielded "tentative evidence," says Sieh, that since the late 13th century, four "really big" earthquakes with an average recurrence of 15 years had occurred there. The last large earthquake, the researcher says, was in 1720 or 1725. The excavation also revealed that the overall rupture length of a well-known 1857 earthquake in Los Angeles was perhaps shorter than those caused by the earlier four, a finding suggesting that the 1720 quake should perhaps be viewed as the most recent in a regular pattern of large earthquakes.

Weldon told SCEINCE NEWS, however, that the findings should be viewed cautiously. The recurrence rates were based on averages, he notes, "and averages don't make earthquakes -- it could still wait another 50 to 100 years."
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Title Annotation:earthquake prediction
Author:Mathewson, Judith
Publication:Science News
Date:Jul 6, 1985
Words:317
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