RELATIVE SEA-LEVEL CHANGE DURING THE LAST FEW HUNDREDS YEARS AT DISCOVERY BAY, WASHINGTON STATE

Numerous studies show that the Pacific Ocean shoreline subsided in the Pacific Northwest during a great Cascadia subduction zone earthquake in AD1700. An outstanding question, important in determining accurate models of the subduction zone, is how far inland the subsidence reached. We hypothesize that no vertical deformation accompanied or followed the AD1700 at Discovery Bay, located at the northern end of Puget Sound, Washington State, USA. Discovery Bay is an important site because the marshes contain a record of nine possible tsunami deposits, yet little to no evidence for coastal deformation. To test this hypothesis, we reconstructed relative sea-level changes using fossil diatoms extracted form intertidal deposits. Relative sea-level reconstructions were based on quantitative comparisons between modern and fossil diatom assemblages. Modern diatom datasets collected from 110 surface sediment samples from Puget Sound are related to modern tidal elevations. From these datasets, we developed transfer functions quantifying the relationship between diatoms and a standardized water level index (SWLI) using weighted averaging partial least squares (WA-PLS). We applied the transfer functions to fossil diatom assemblages extracted from core samples from Discovery Bay. The reconstructed SWLIs were converted to continuous change in elevation by subtracting the recent eustatic sea-level rise (2 mm/year in the past 50 years) and steady sedimentation components. The calibrated elevation changes show that there was no detectable vertical deformation at Discovery Bay just before and after the AD1700 earthquake. Deposits younger than the AD 1700 earthquake, probably deposited in the last 150 years, record rapid rise in elevation and this may represent accelerated sedimentation due to increasing river flow.

This work is collaboration with Brian L. Sherrod of USGS. This is supported partly by JSPS.