An offset Holocene stream channel and the rate of slip along the northern reach of the San Jacinto fault zone, San Bernardino Valley, California

The three-dimensional geometry of a buried channel-fill deposit that crosses, and is offset by, a strand of the San Jacinto fault zone in San Bernardino, California, is reconstructed from an extensive suite of trench logs. The reconstruction shows that the original course of the channel was deflected for a short distance along strike of the fault before it was offset. As a result, apparent offset of the channel deposit across the fault zone is about 12 m in a right-lateral sense, but only 3.5 to 6 m of the apparent offset is due to fault slip. The age of the deposit is ≤1931 ± 109 yr B.P. based on radiometric dating of detrital charcoal collected from the unit. Our excavations do not encompass all active strands of the fault at this site. Hence, our study points to a minimum slip rate of 1.7 to 3.3 mm/yr along the northern San Jacinto fault zone. Historical records show that it has been at least 64 to 90 yr since the last large earthquake occurred along the northern San Jacinto fault. It can thus be inferred from our estimate of fault slip rate that at least 10 to 30 cm of slip is currently stored along the northern segment of the San Jacinto fault. Because prior studies along the same fault zone to the southeast indicate much higher slip rates (on the order of 10 mm/yr), and coseismic offsets for the largest historical earthquakes along the San Jacinto fault zone have been on the order of 30 cm, it is inferred that the northern reach of the San Jacinto fault is now in the later stages of the strain accumulation cycle prior to a moderate to large M 6-7 earthquake. Additionally, the complex geometry of the reconstructed channel serves to illustrate the caution that must be exercised when determining the true offset of buried channel deposits.