Microfossil Measures of Subsidence During Past Plate-Boundary Earthquakes: Their Accuracy Revealed By a Sudden Tidal-Flooding Experiment in Cascadia
Comparison of pre- and post-earthquake microfossil assemblages in tidal sequences is the most widely applicable and accurate means to measure coseismic subsidence at coastal sites during past plate-boundary earthquakes at
Comparison of pre- and post-earthquake microfossil assemblages in tidal sequences is the most widely applicable and accurate means to measure coseismic subsidence at coastal sites during past plate-boundary earthquakes at subduction zones. However, the uncertainty on subsidence estimates is not well constrained because the response times of fossil taxa to coseismic relative sea-level (RSL) rise are unknown. We explored the response of diatoms and foraminifera to a sudden increase in tidal inundation following dike removal during restoration of a former salt marsh in southern Oregon. We also applied diatom and foraminifera-based transfer functions, which use the empirical relationship between modern microfossil assemblages and elevation within the tidal frame to convert fossil assemblages into quantitative estimates of past RSL, to pre-and post-restoration microfossil assemblages to validate the technique.
Tidal flooding following dike removal caused a RSL rise of ~1 m, as might occur by coseismic subsidence during Mw 8.1-8.8 earthquakes on this section of 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. Despite issues with modern analogs in the diatom transfer function, quantitative estimates of RSL rise reflect the sudden increase in tidal inundation following dike removal. In contrast, it took at least 10 months for notable numbers of low-marsh foraminifera to colonize the restored marsh. A comparison of diatom and foraminifera-based transfer function estimates of RSL rise following restoration reflects the delayed foraminifera response, suggesting that postseismic uplift or subsidence may cause foraminifera-based transfer functions to underestimate or overestimate, respectively, coseismic subsidence. The delayed foraminifera response to a sudden RSL rise may be responsible for observed differences between foraminifera- and diatom-based coseismic subsidence reconstructions at various coastal sites along the Cascadia subduction zone. However, the different response times of diatoms and foraminifera may provide useful information on post-seismic vertical deformation in the months following past megathrust earthquakes.