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Published September 10, 1984 | Published
Journal Article Open

Lateral Offsets and Revised Dates of Large Prehistoric Earthquakes at Pallett Creek, Southern California


Recent excavation and new radiocarbon dates of sediments at Pallett Creek are the basis for new conclusions regarding the late Holocene history of the San Andreas fault. Systematic dissection of a 50-m-long, 15-m-wide, 5-m-deep volume of earth, centered on the fault, enables documentation in three dimensions of fault patterns, lateral offsets, and vertical deformation associated with large earthquakes of the past. The excavations expose evidence for 12 earthquakes that occurred between about 260 and 1857 A.D., with an average recurrence interval of about 145 years. Prehistoric slip events that occurred in 1720±50, 1550±70, 1350±50, 1080±65, and 845±75 A.D. have lateral offsets that are comparable to those of the most recent great earthquake of 1857. Thus all of these events represent earthquakes of large magnitude. The lateral offsets of two other events, in 935±85 and 1015±100 A.D., are an order of magnitude smaller and may be interpreted in several ways with regard to the size of these events. The new data constrain the average recurrence interval for large earthquakes at this site to between 145 and 200 years but suggest a monotonic decrease in individual intervals to below this range during the past 900 years. On the basis of these data, the probability of a large earthquake with surficial fault rupture at this site is between 0.2 and 5% during 1984 and 7 and 60% by the year 2000.

Additional Information

© 1984 by the American Geophysical Union. Received November 7, 1983; revised March 29, 1984; accepted April 17, 1984. This paper is based on field studies conducted during the summers of 1979 and 1980. Assisting me in the field were Ari Fuad, Christopher Finch, Kristian Meisling, Ronald Miller, Christopher Sanders, Laurie Sieh, and Stuart Stephens. Most of the excavations were dug by equipment operators Jerry Goodman and Chuck Pattison. Access to the property was graciously granted by the owners George and Adele Nickel. Very helpful criticisms of earlier manuscripts were given by Clarence Allen, David Brillinger, Malcolm Clark, Art Darrow, Jim Lienkemper, Art McGarr, Carol Prentice, and Pat Williams. Most of the drafting was done by Kathi Kronenfield. This study was supported by the National Earthquake Hazards Reduction Program, U.S. Geological Survey contract 14-08-0001-16774, 18385, and 19756. Contribution 3823, Division of Geological and Planetary Sciences, California Institute of Technology.

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