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The AAPG/Datapages Combined Publications Database

Pacific Section of AAPG

Abstract


San Andreas Fault - Cajon Pass to Wrightwood, 1984
Pages 23-26

Prehistoric Large Earthquakes Produced by Slip on the San Andreas Fault at Pallett Creek, California

Kerry E. Sieh

Abstract

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 relationships along the main fault. The effects upon the marsh deposits of the eight prehistoric events are comparable to those of the great (MS=8.25+) 1857 event, which is the youngest of the nine events that disturbed the strata and is associated with about 4.5 m of right-lateral slip nearby. 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 frac12.gif (855 bytes) century to about 3 centuries. The dates may indicate a fairly systematic pattern of occurrence of large earthquakes.


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