1855 and 1991 Surveys of the San Andreas Fault : Implications for Fault Mechanics

Two monuments from an 1855 cadastral survey that span the San Andreas fault in the Carrizo Plain have been right-laterally displaced 11.0 --2.5 m by the 1857 Fort Tejon earthquake and associated seismicity and afterslip. This measurement confirms that at least 9.5 --0.5 m of slip occurred along the main fault trace, as suggested by measurements of offset channels near Wallace Creek. The slip varied by 2 to 3 m along a 2.6-km section of the main fault trace. Using radiocarbon dates of the penultimate large earthquake and measurements of slip from the 1857 earthquake, we calculate an apparent slip rate for the last complete earthquake cycle that is at least 25% lower than the lateHolocene slip rate on the main fault trace. Comparison of short-term broadaperture strain accumulation rates with the narrow-aperture late-Holocene slip rate indicates that the fault behaves nearly elastically over a time scale of several earthquake cycles. Therefore, slip in future earthquakes should compensate the slip-rate deficit from the 1857 earthquake.