| Title | Microfossil measures of rapid sea-level rise: Timing of response of two microfossil groups to a sudden tidal-flooding experiment in Cascadia |
| Author | Horton, B P; Milker, Y; Dura, T; Wang, K; Bridgeland, W T; Brophy, L; Ewald, M; Khan, N S; Engelhart, S E; Nelson, A R; Witter, R C |
| Source | Geology vol. 45, no. 6, 2017 p. 535-538, https://doi.org/10.1130/g38832.1 |
| Year | 2017 |
| Alt Series | Earth Sciences Sector, Contribution Series 20160264 |
| Publisher | Geological Society of America |
| Document | serial |
| Lang. | English |
| Media | paper; on-line; digital |
| File format | pdf |
| Province | Western offshore region; British Columbia |
| Area | California; Washington; Oregon; Vancouver Island; Cascadia Subduction Zone; United States |
| Lat/Long WENS | -133.0000 -120.0000 50.0000 40.0000 |
| Subjects | marine geology; surficial geology/geomorphology; subduction zones; tidal environments; subsidence; coastal environment; earthquakes; fossil assemblages; fossils; fossil descriptions; diatoms |
| Illustrations | location maps; graphs; photographs; histograms |
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| Program | Western Canada Geohazards Project, Public Safety Geoscience |
| Abstract | Comparisons 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. |
| GEOSCAN ID | 299422 |
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