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TitleMicrofossil measures of rapid sea-level rise: Timing of response of two microfossil groups to a sudden tidal-flooding experiment in Cascadia
AuthorHorton, B P; Milker, Y; Dura, T; Wang, KORCID logo; Bridgeland, W T; Brophy, L; Ewald, M; Khan, N S; Engelhart, S E; Nelson, A R; Witter, R C
SourceGeology vol. 45, no. 6, 2017 p. 535-538,
Alt SeriesEarth Sciences Sector, Contribution Series 20160264
PublisherGeological Society of America
Mediapaper; on-line; digital
File formatpdf
ProvinceWestern offshore region; British Columbia
AreaCalifornia; Washington; Oregon; Vancouver Island; Cascadia Subduction Zone; United States of America
Lat/Long WENS-133.0000 -120.0000 50.0000 40.0000
Subjectsmarine geology; surficial geology/geomorphology; subduction zones; tidal environments; subsidence; coastal environment; earthquakes; fossil assemblages; fossils; fossil descriptions; diatoms
Illustrationslocation maps; graphs; photographs; histograms
ProgramPublic Safety Geoscience Western Canada Geohazards Project
Released2017 03 27
AbstractComparisons of pre- and post-earthquake microfossils in tidal sequences are the most widely applicable and accurate means to measure subsidence during past plate-boundary earthquakes at subduction zones, but measurement errors are uncertain because the response times of fossil taxa to coseismic relative sea-level (RSL) rise are unknown. We measured the response of diatoms and foraminifera during restoration of a former salt marsh in southern Oregon. Tidal flooding following dike removal caused a RSL rise of ~1 m, as typically occurs through coseismic subsidence caused by Mw > 8.1 earthquakes on the Cascadia subduction zone. Less than two weeks after dike removal diatoms colonized low marsh and tidal flats, showing that they can record seismically induced subsidence soon after earthquakes. In contrast, low marsh foraminifera waited at least ten months to colonize the restored marsh. Where subsidence measured with diatoms and foraminifera differs, their different response times can provide an estimate of post-seismic vertical deformation in the months following past megathrust earthquakes.
Summary(Plain Language Summary, not published)
The only method of estimating crustal deformation and fault slip in past great Cascadia megathrust earthquakes is the study of sudden coastal subsidence induced by these earthquakes. The most accurate tool for such studies is the analysis of tidal-zone microfossils that are extremely sensitive to the relative sea level rise due to the subsidence, but the response times of the microfossils to the sudden change have been poorly known, adding uncertainties regarding to what degree the inferred coastal subsidence is co-seismic. A controlled flooding operation in an Oregon tidal zone provided a rare opportunity to investigate the response times. Results reported in this paper show that diatoms responded in less than two weeks after the sudden water level rise in the flooding event, but foraminifera responded ten months later. Therefore, diatoms can constrain coseismic subsidence more accurately. However, the difference between the two provides a new way of estimating post-seismic crustal deformation following pre-instrumental earthquakes.

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