The displacement histories of the San Jacinto and southernmost San Andreas fault zones are constrained by offset data with ages in the range of 5 Ma to 5 ka. Apparent discrepancies between long- and short-term average displacement rates can be reconciled with a time-variable rate model. In this model, the displacement rate on the San Andreas decelerated from ∼ 35 mm/yr at 1.5 Ma to as low as 9 ± 4 mm/yr by 90 ka. Over this same time period, the rate on the San Jacinto fault zone accelerated from an initial value of zero to a rate of 26 ± 4 mm/yr. The data also imply that the rate of the San Andreas fault accelerated since ca. 90 ka, from ∼9 mm/yr to the modern rate of 27 ± 4 mm/yr, whereas the San Jacinto decelerated from 26 ± 4 mm/yr to the modern rate of 8 ± 4 mm/yr. The time scale of these changes is significantly longer than the earthquake cycle, but shorter than time scales characteristic of lithospheric-scale dynamics. The emergence of the San Jacinto fault zone ca. 1.5 Ma coincided with the development of a major restraining bend in the San Andreas fault zone, suggesting that the formation of new subparallel faults could be driven by conditions that inhibit displacement on preexisting faults.
All Science Journal Classification (ASJC) codes