Applications of foraminifera to detecting land level change associated with great earthquakes along the west coast of North America.

摘要

Microfossils such as foraminifera, diatoms and pollen serve as proxy indicators of environmental change in tidal marsh environments. In this study, marsh foraminifera were used to estimate the magnitude of land-level change at Willapa Bay, Washington during repeated great (M > 8) earthquakes at the Cascadia subduction zone. Tidal muds abruptly overlie buried high marsh soils in estuarine wetlands along the Pacific coast from northern California to central Vancouver Island. This stratigraphy records sudden submergence during great earthquakes.; A large brackish marsh along Niawiakum River at Willapa Bay was selected for this study because it contains buried soils that record the last seven great Cascadia earthquakes. The zonation of marsh foraminiferal species at Willapa Bay is closely related to elevation. Foraminiferal zonation in the modern marsh was determined by visually analysing, and applying cluster analysis to, surface samples collected along a transect across the marsh. Three zones were delimited: low marsh (1.30--2.23 m above MLLW), middle marsh (2.23--2.86 m above MLLW), and high marsh (2.86--3.06 m above MLLW). Modern foraminiferal assemblages were compared to fossil assemblages recovered from a vertical sediment monolith to estimate the paleoelevation of fossil samples. Amounts of coseismic subsidence of ∼1 m were calculated using these estimates and a transfer function derived from partial least squares. Due to a limited sampling range for the modern analogue of the lowest marsh zone relative, these values are considered as minima. The values are similar to estimates of coseismic subsidence in other parts of coastal Washington, Oregon and British Columbia. This research complements similar diatom- and pollen-based studies at Willapa Bay and contributes to a multi-proxy assessment of the earthquake history of the area.