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Published August 10, 1978 | public
Journal Article

Prehistoric large earthquakes produced by slip on the San Andreas Fault at Pallett Creek, California


Late Holocene marsh deposits composing a terrace about 55 km northeast of Los Angeles, California, contain geologic evidence of many large seismic events produced by slip on the San Andreas fault since the sixth century A.D. I excavated several trenches into the deposits in order to study this evidence. The principal indicators of past events are (1) sandblows and other effects of liquefaction, (2) the termination of secondary faults at distinct levels within the stratigraphic section, and (3) sedimentary deposits and faulted relationships along the main fault. The effects upon the marsh deposits of six of the eight prehistoric events are comparable to those of the great (Ms = 8¼+) 1857 event, which is the youngest of the nine events disturbing the strata and is associated with about 4½ m of right lateral slip nearby. Two large events may be smaller than this. Radiocarbon dates indicate that the events occurred in the nineteenth, eighteenth, fifteenth, thirteenth, late twelfth, tenth, ninth, seventh, and sixth centuries A.D. Recurrence intervals average 160 years but vary from ½ century to about 3 centuries. The dates may indicate a fairly systematic pattern of occurrence of large earthquakes.

Additional Information

The advice, support, and insight of many friends and colleagues were critical to this effort. I am especially grateful to my Ph.D. advisor, Richard Jahns. Clarence Allen, Malcolm Clark, Tom Hanks, and Peter Molnar also suggested many improvements in the manuscript. In the field I was assisted principally by my brother, Rodger; my wife, Laurie; and my friend, David Drake. W. A. Crocket kindly allowed us to excavate on his property. U.S. Geological Survey contract 14-08-0001-15225 supported this work, which is contribution 2989 of the Division of Geological and Planetary Sciences, California Institute of Technology.

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