ASSESSING TEMPORAL VARIABILITY OF COASTAL DEFORMATION DURING MEGATHRUST EARTHQUAKES AT THE CASCADIA SUBDUCTION ZONE USING A DIATOM BAYESIAN TRANSFER FUNCTION

Estimates of coastal subsidence derived from microfossils (e.g., diatoms, foraminifera) within stratigraphic records in temperate settings are the most precise means to reconstruct deformation during past megathrust ruptures. At the Cascadia subduction zone, coastal subsidence during successive megathrust earthquakes is commonly recorded by stratigraphic sequences of mud-over-peat or mud-over-soil contacts in tidal wetlands. However, coseismic subsidence reconstructions of prehistoric Cascadia earthquakes using diatom-based transfer functions are often hampered by the high species diversity of diatoms and the common absence of modern analogs for fossil assemblages.

We developed a new, diatom-based Bayesian transfer function (BTF) to quantitatively reconstruct coseismic subsidence. The modern diatom dataset consists of 366 species from 94 samples collected from the subtidal to forested upland environments of Willapa Bay, WA, USA. To improve the dissimilarity between modern and fossil samples, we used a hierarchical clustering method to identify diatom groups with similar abundance and distributions with elevation. The diatom BTF integrates the grouped species response curves (positive and negative linear, unimodal, bimodal and multimodal) and prior information (lithologic context) about each sample to improve the precision of reconstructed earthquake subsidence. 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. We apply the BTF to fossil diatom assemblages from a stratigraphic sequence at the Redtail locality of Willapa Bay where six, mud-over-peat contacts have been inferred to record subsidence during megathrust earthquakes over the past 3500 years. These new estimates will aid in constraining models of earthquake deformation as part of earthquake hazard assessments for the Cascadia subduction zone.