Bayesian Diatom-Based Estimates of Coastal Deformation During Megathrust Earthquakes at the Cascadia Subduction Zone

Date: 4/24/2019

Time: 05:00 PM

Room: Vashon

Reconstructing the magnitude and frequency of megathrust earthquakes at Cascadia, as well as at other subduction zones, requires accurate measures of upper-plate deformation during great earthquakes. At Cascadia, coastal subsidence during successive earthquakes is commonly recorded by stratigraphic sequences of mud-over-peat contacts beneath tidal wetlands. The application of diatom-based transfer functions in relative sea-level reconstruction studies yields quantitative estimates of coseismic land-level change, but the estimates are limited by the high species diversity of tidal floras, the common absence of modern analogues for fossil assemblages, and the complex and variable species distributions in salt marsh and estuarine environments.

Using a newly developed diatom Bayesian transfer function (BTF) we quantitatively reconstruct coseismic coastal subsidence at Willapa Bay, WA. We address the problem of high species diversity and improve computational efficiency by employing a statistical pre-treatment, grouping species that indicate similar responses to elevation. The diatom BTF is used to calculate flexible species-response curves for the grouped taxons, capturing the complex relationships between species and elevation. The modern diatom training dataset (calibrations) consists of 60 samples from three tidal marshes in Willapa Bay with elevations tied to local tidal benchmarks. A 10-fold cross-validation provides an assessment of the predictive performance of the diatom BTF showing a strong relationship between observed and predicted elevations (mean absolute residual = 0.33 m) with a root-mean square error of prediction of 0.42 m. We apply the diatom BTF to fossil assemblages from a core along the Niawiakum River, Willapa Bay to estimate subsidence during four prehistoric earthquakes. Our new subsidence estimates will help constrain models of coseismic and interseismic deformation that are key to assessing the hazards from Cascadia’s megathrust earthquakes.

Presenting Author: Isabel Hong

Authors