We derived slip rates on the San Andreas fault from displacement of alluvial fans and from approximate dating of the fans by the degree of soil development. Distinctive red-purple sedimentary rocks of the Miocene(?) Potato Sandstone exposed on Yucaipa Ridge in the San Bernardino Mountains were transported down Wilson Creek and shed across the modern trace of the San Andreas fault. Soils developed in alluvial-fan deposits contain red-purple clasts of the Potato Sandstone and are progressively more developed toward the north-west as a result of fault displacement.
We used soil development to estimate the ages of these alluvial fans and underlying deposits. Using dated soils from Cajon Pass and from the Central Valley of California, we estimated ages of Wilson Creek deposits, using a statistical method of maximum likelihood. These methods also utilize Monte Carlo simulations of larger data populations and account for soil variability, uncertainty of calibration dates, and the limited number of soils described and sampled. Resulting age estimates have considerable (as much as 80%) uncertainty, resulting largely from quality of calibration dates.
Slip rates for three time intervals were obtained by restoring the medial axis of each displaced fan to Wilson Creek. Maximum slip rates were also obtained by restoring the westernmost extent of each fan to Wilson Creek. Best-estimate rates averaged to the present for 0 to 14,000 yr are 14 to 25 mm/yr; for 0 to 30,000 yr, 22 to 34 mm/yr, and for 0 to 65,000 or 90,000 yr, 12 to 16 mm/yr. Maximum rates averaged to present for 0 to 14,000 yr are as high as 65 mm/yr; older rates are as high as 80 mm/yr. When Holocene rates are accounted for, actual or incremental slip rates for the interval 30,000 to 65,000 or 90,000 yr B.P. are probably 6 to 13 mm/yr. These data tentatively suggest that slip rates on the modern trace have increased during the late Quaternary. Uncertainties in slip and age data, however, allow for rates to have been nearly constant or even decreasing toward the present.