FORAMINIFERAL ESTIMATE OF COSEISMIC SUBSIDENCE DURING THE AD 1700 MEGATHRUST EARTHQUAKE FROM SOUTHERN WASHINGTON

From the mid- to late-Holocene, there is turbidite and coastal evidence of up to twelve megathrust earthquakes along the Cascadia subduction zone, the most recent of which occurred in AD 1700 and resulted in ~0.2 to >0.8m of coastal subsidence. Due to the absence of instrumentally recorded large (>8.0M) seismic events, intertidal foraminifera are used to reconstruct paleoenvironments and estimate coseismic subsidence. Importantly, the AD 1700 earthquake estimates are used to refine plate-rupture models to describe rupture width and length (constraints on magnitude), in order to understand the impact of past and future megathrust earthquakes. While rupture length appears variable based on the distribution of archived paleoseismic indicators (e.g. buried soils, tsunami sands), rupture width is assumed to be limited by the 350⁰C or 450⁰C isotherms and determines the position of maximum subsidence. To further constrain the rupture width for the AD 1700 earthquake, two sites (western Willapa Bay and eastern Grays Harbor) separated longitudinally by 20km, were selected in Washington. A modern foraminiferal training set from Oregon, collected by Hawkes et al. 2010, Engelhart et al. 2013, and Milker et al. 2016, was used to develop a transfer function relating modern marsh assemblages to paleo-assemblages found within sediment cores to determine the paleoenvironment. Subsidence estimates for replicate cores within a single site will be used to demonstrate the reproducibility of intra-site reconstructions using the transfer function technique and inter-site (western vs. eastern) subsidence estimates will be compared to constrain the down-dip rupture extent.