A MICROFOSSIL-BASED APPROACH TO CONSTRAINING MEGATHRUST-INDUCED COSEISMIC LAND DISPLACEMENT IN THE PACIFIC NORTHWEST

It has long been established that the interchange between terrestrial and marine sediments can provide information on relative sea level (RSL) change and plate boundary earthquakes. Paleoseismologists infer the amount of coseismic subsidence during plate-boundary earthquakes from stratigraphic changes in microfossils across sharp peat-mud and peat-sand contacts. However, the use of lithostratigraphic-based reconstructions is associated with a number of limitations. To address these limitations, paleoecologists working in the coastal zone have adopted a transfer-function approach to environmental reconstruction. The timing of the last great earthquake along the Cascadia subduction zone (1700AD) is now well defined by Japanese records. Further, modeling of the tsunami waveform and coastal subsidence data estimate the magnitude to be approximately M 9.0, likely rupturing over 1000km of the subduction boundary, and causing up to 20m of slip along the seaward plate. RSL changes accompanying these earthquakes are described by an earthquake cycle of rapid coseismic deformation during plate-boundary rupture followed by gradual interseismic strain accumulation typically lasting hundreds of years. We will apply the transfer function to modern foraminiferal datasets along coastal Oregon to analyze the fossil record and quantitatively determine the amount of vertical land movement associated with the 1700AD earthquake event. To date, we have collected 9 modern transects that run from the intertidal zone into the upland coastal forest totaling 170 samples. We have also collected 9 cores recording the 1700AD earthquake. Furthermore, a 4m vibracore was collected at each of Nehalem and Siuslaw Rivers contains between 3 and 5 potential earthquake horizons. The 1700AD earthquake in the vibracores shows a distinct litho- and biostratigraphical change representing an instantaneous episode of subsidence between 0.5m and 1.5m. However, development and application of the transfer function to such events will provide quantitative constrained estimates of vertical coseismic land movement. Measurements that are more accurate are necessary to help modelers develop simulations that are more realistic in order to better assess earthquake and tsunami hazards.