QUANTITATIVE PALEOENVIRONMENTAL RECONSTRUCTIONS OF PAST COASTAL SUBSIDENCE ALONG THE NORTHERN MARGIN OF THE CASCADIA SUBDUCTION ZONE

Coastal estuaries archive evidence of large megathrust (>8.0M) earthquakes occurring along the Cascadia Subduction Zone (CSZ) over the last 7,000 years. The most recent of these events occurred in AD 1700 with considerable coseismic subsidence ranging from 0.2-0.8 meters. Although relatively seismically quiescent over the last 300 years, evidence of coastal subsidence is archived in saltmarsh stratigraphy showing abruptly buried high marsh and upland forest sediments overlain by intertidal muds and occasional landward tapering tsunami sands. Microfossils within these sediment sequences, such as foraminifera, are used to estimate the former elevation of stratigraphic marsh horizons. Fossil foraminiferal assemblages can therefore be used to estimate coastal subsidence along the CSZ by comparing pre- and post-seismic paleo-marsh elevations by way of a transfer function that quantifies the relationship between species distribution and elevation. These coseismic estimates are critical to elastic dislocation models which have proposed heterogenous coseismic slip during the AD 1700 earthquake. While precise estimates have been produced in Oregon the best fit earthquake segmentation models are hampered by a lack of quantitative coseismic subsidence estimates in the northern part of the CSZ towards Vancouver Island, BC. Previous subsidence estimates of 0.71 ± 0.3m produced by a microfossil-based transfer function on Vancouver Island need updating to reduce inconsistencies with current taxonomy, increase the size of modern training set to reduce no analogue issues and increase statistical robustness, and make use of new transfer function techniques. A new regional transfer function will be developed for the northern margin, using foraminifera assemblages found in 43 sediment samples collected across four transects on Vancouver Island. This data set, along with two sediment cores showing the AD 1700 contact, will be used to estimate coseismic subsidence in the area during the AD 1700 earthquake.