Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 20-10
Presentation Time: 9:00 AM-6:00 PM


STREIG, Ashley R., Department of Geology, Portland State University, 1721 SW Broadway Ave, Portland, OR 97201, WELDON II, Ray J., Department of Earth Sciences, University of Oregon, Eugene, OR 97403-1272, DAWSON, Timothy E., California Geological Survey, 345 Middlefield Road, MS 520, Menlo Park, CA 94025, BIASI, Glenn P., Nevada Seismological Laboratory, Univ Nevada - Reno, MS 174, Reno, NV 89557-0138, GAVIN, Daniel, Department of Geography, University of Oregon, 1251 University of Oregon, Eugene, OR 97403 and GUILDERSON, Thomas P., Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, P.O. Box 808, L397, LIvermore, CA 94550

High-precision, dendrochronologically-constrained ages for three sedimentary layers at the Hazel Dell paleoseismic site on the San Andreas Fault in the Santa Cruz Mountains provide an opportunity to evaluate how well materials typically collected for 14C analysis agree with the known age of the layer containing them. A wiggle-matched 274-year dendrochronological record from a buried redwood tree stump at the site stratigraphically corresponds with deposits faulted in the ante-penultimate earthquake. This layer contains axe-cut wood chips, indicating it is an historic earthquake. Wood chips and detrital charcoal collected from faulted stratigraphy yield broad 14C age ranges extending into the historic period. The outermost ring of the stump has a calendar age of 1789 – 1797 AD, a considerably tighter constraint than from 14C ages alone. To estimate in-built 14C ages, we use a lognormal fit to all charcoal sample dates after removing the mean apparent layer date, then compare to the known value. For Hazel Dell the most likely sample (mode) is ~ 220 yrs older than the actual age of the deposit it came from. Determining a single scatter parameter for a site produces more consistent results than trying to adjust individual layers. We use this correction method to broaden date distributions and improve correlations with other Northern San Andreas Fault sites that also depend on detrital charcoal. We combine historical observations of 19th century earthquakes to strengthen our earlier findings that the Bay Area San Andreas fault ruptured in 1838 (~80 km), 1890 (30+ km) and 1906 (~490 km). The fourth paleoearthquake identified at Hazel Dell may have been 1838-like in rupture length, or alternatively could have been a longer 1906-like rupture length based on along-fault paleoseismic record correlations.