Serial ruptures of the San Andreas fault, Carrizo Plain, California, revealed by three-dimensional excavations

摘要

It is poorly known if fault slip repeats regularly through many earthquake cycles. Well-documented measurements of successive slips rarely span more than three earthquake cycles. In this paper, we present evidence of six sequential offsets across the San Andreas fault at a site in the Carrizo Plain, using stream channels as piercing lines. We opened a latticework of trenches across the offset channels on both sides of the fault to expose their subsurface stratigraphy. We can correlate the channels across the fault on the basis of their elevations, shapes, stratigraphy, and ages. The three-dimensional excavations allow us to locate accurately the offset channel pairs and to determine the amounts of motion for each pair. We find that the dextral slips associated with the six events in the last millennium are, from oldest to youngest, ≥5.4 ± 0.6, 8.0 ± 0.5, 1.4 ± 0.5, 5.2 ± 0.6, 7.6 ± 0.4 and 7.9 ± 0.1 m. In this series, three and possibly four of the six offset values are between 7 and 8 m. The common occurrence of 7–8 m offsets suggests remarkably regular, but not strictly uniform, slip behavior. Age constraints for these events at our site, combined with previous paleoseismic investigations within a few kilometers, allow a construction of offset history and a preliminary evaluation of slip- and time-predictable models. The average slip rate over the span of the past five events (between A.D. 1210 and A.D. 1857.) has been 34 mm/yr, not resolvably different from the previously determined late Holocene slip rate and the modern geodetic strain accumulation rate. We find that the slip-predictable model is a better fit than the time-predictable model. In general, earthquake slip is positively correlated with the time interval preceding the event. Smaller offsets coincide with shorter prior intervals and larger offset with longer prior intervals.