Late Holocene paleoseismicity, tsunamis, and relative sea-level changes in Yaquina Bay, central coastal Oregon

Graduation Date

2012

Document Type

Thesis

Program

Other

Program

Thesis (M.S.)--Humboldt State University, Environmental Systems: Geology, 2012

Committee Chair Name

Harvey Kelsey

Committee Chair Affiliation

HSU Faculty or Staff

Keywords

Earthquakes, Cascadia, Tsunamis, Paleoseismology, Tectonics, Subduction zone, Humboldt State University -- Theses -- Geology

Abstract

Wetland sediments in Yaquina Bay on the central Oregon coast record paleoearthquakes on the Cascadia subduction zone (CSZ). Great earthquakes on the CSZ caused widespread subsidence at Sallys Slough in Yaquina Bay twelve times during the last 4,400 yrs BP. Nine of the twelve soils subsided during subduction zone earthquakes were buried by overlying sandy silt that gradually fined up section into estuarine mud. It is inferred that the mechanism transporting sandy silt to the site is tsunami. Diatoms within the sandy silt material indicate that the sandy silt was sourced from a tidal flat (Yaquina Bay). Diatoms also indicate that buried soils, which were developed in a high to upland freshwater marsh environment, were abruptly subsided to low salt-marsh conditions. Subsidence during two of the subduction zone earthquakes, on the basis of evaluation of diatom data across buried soil contacts, was on the order of one meter or more. The chronology of subduction related earthquakes recorded at Sallys Slough is consistent with paleoseismological investigations along other parts of the CSZ. The Yaquina Bay paleoseismic record provides support for an instance when only a segment of the CSZ ruptured. Based on comparing subduction zone earthquake chronologies in adjacent estuaries, a north-central segment of the CSZ (Yaquina Bay to Cannon Beach) may have ruptured between approximately 500-700 yrs BP.

https://scholarworks.calstate.edu/concern/theses/k3569669v

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