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TitleTesting the use of microfossils to reconstruct great earthquakes at Cascadia
AuthorEngelhart, S E; Horton, B P; Nelson, A R; Hawkes, A D; Witter, R C; Wang, KORCID logo; Wang, P -L; Vane, C H
SourceGeology vol. 41, no. 10, 2013 p. 1067-1070, Open Access logo Open Access
Alt SeriesEarth Sciences Sector, Contribution Series 20130047
PublisherGeological Society of America
Mediapaper; on-line; digital
File formatpdf
ProvinceBritish Columbia
NTS103; 102; 92B; 92C; 92E; 92F; 92K; 92L; 92M
AreaWashington; California; Canada; United States of America
Lat/Long WENS-130.0000 -120.0000 52.0000 40.0000
Subjectspaleontology; geophysics; stratigraphy; fossils; fossil distribution; fossil assemblages; microfossils; earthquakes; earthquake studies; earthquake foci; biostratigraphy; earthquake magnitudes; Cascadia subduction zone; foraminifera
Illustrationslocation maps; stratigraphic columns
ProgramPublic Safety Geoscience Targeted Hazard Assessments in Western Canada
Released2013 10 01
AbstractCoastal stratigraphy from the Pacific Northwest of the United States contains evidence of sudden subsidence during ruptures of the Cascadia subduction zone. Transfer functions (empirical relationships between assemblages and elevation) can convert microfossil data into coastal subsidence estimates. Coseismic deformation models use the subsidence values to constrain earthquake magnitudes. To test the response of foraminifera, the accuracy of the transfer function method, and the presence of a pre-seismic signal, we simulated a great earthquake near Coos Bay, Oregon, by transplanting a bed of modern high salt-marsh sediment into the tidal flat, an elevation change that mimics a coseismic subsidence of 0.64 m. The transplanted bed was quickly buried by mud; after 12 mo and 5 yr, we sampled it for foraminifera. Reconstruction of the simulated coseismic subsidence using our transfer function was 0.61 m, nearly identical to the actual elevation change. Our transplant experiment, and additional analyses spanning the A.D. 1700 earthquake contact at the nearby Coquille River 15 km to the south, show that sediment mixing may explain assemblage changes previously interpreted as evidence of pre-seismic land-level change in Cascadia and elsewhere.
Summary(Plain Language Summary, not published)
For subduction zones where witten history is short such as the Cascadia subduction zone, knowledge of past great earthquakes is based almost entirely on geological evidence. One most widely used evidence is sudden subsidence of the coast inferred from microfossil analyses, for which comparison with the modern living environments of the involved species is critical. In this work, we simulate the sudden subsidence due to a great earthquake by transplanting a stratigraphic section on the Oregon coast intertidal zone to a nearby lower elevation (by 0.64 m). Five years later, we returned to the same site to study the species in the section of the "man made coseismic subsidence". The analyses yielded a subsidence value of 0.61 m, very similar to the actual value. This is the first direct validation of miscrofossil analysis method in the study of past subduction earthquakes. The results from this type of analysis can be used to assess errors in other types of geological evidence and therefore improve the accuracy of seismic hazard assessment using geological information.

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