New Insights into Paleoseismic Age Models on the Northern San Andreas Fault: Charcoal Inbuilt Ages and Updated Earthquake Correlations

Abstract

ABSTRACT Dendrochronological age constraints at the Hazel Dell (HD) paleoseismic site provide a means to explore the potential for systematic bias in age estimates of earthquakes in forested settings including the Santa Cruz Mountains section of the San Andreas fault, California. Age constraints developed from detrital charcoal are compared with absolute dates from dendrochronology. We develop a new method for estimating inbuilt layer ages that makes use of more than just the youngest radiocarbon dates in a given layer. The improved age model relates likely layer deposition dates with observed C14 ages. We find that for HD, the most likely charcoal sample (mean) is ∼322  yr older than the actual age of the deposit that contains it. With this correction, two historical Bay area earthquakes are confirmed to have ruptured the surface in 1838 and 1890. Earlier work based on similar charcoal dates proposed these ruptures occurred in the mid-1700s or earlier, but historical ages of these events are unequivocal because they rupture layers containing wood chips and a redwood stump from European logging at the site. This charcoal correction method also shifts the fourth earthquake, which is constrained only by detrital charcoal, ∼300  yr younger to A.D. ∼1266. Considering the full site record at HD, recurrence of ground rupture has averaged ∼150  yr for the last four events, not ∼250  yr as inferred from unadjusted C14. Our research shows that reasonable estimates of inbuilt age can be made for charcoal-dominated paleoseismic sites and that correcting for the inbuilt age can make a significant difference for earthquake ages and potential correlations with other sites on the fault. We also examine other common inputs to Bayesian age models and find when adequate uncertainty is not incorporated, modeled ages may exclude the actual age of the event of interest.