Microfossil Measures of Subsidence During Past Plate-Boundary Earthquakes: Their Accuracy Revealed by a Sudden Tidal-Flooding Experiment in Cascadia

Date: 4/24/2019

Time: 05:15 PM

Room: Vashon

Comparison of pre- and post-earthquake microfossil assemblages in intertidal sediment is the most widely applicable and accurate means to measure coseismic subsidence from past plate-boundary earthquakes at subduction zones. Quantitative methods typically state only the analytical uncertainty associated with predicted subsidence. However, uncertainty on subsidence estimates may not be 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. Tidal flooding following dike removal caused an increase in inundation equivalent to 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, depending on the rupture length along strike. 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, it took at least 11 months for notable numbers of low-marsh foraminifera to colonize the restored marsh.

A comparison of 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—reflects the delayed foraminifera response. This suggests that postseismic uplift or subsidence may cause foraminiferal-based transfer functions to underestimate or overestimate, respectively, coseismic subsidence if sedimentation rates post-earthquake exceed 1-2cm/yr given a typical fossil core/outcrop sampling resolution of 1cm. Our results suggest that different response times of diatoms and foraminifera may provide useful information on post-seismic vertical deformation in the months following past megathrust earthquakes.

Presenting Author: Tina Dura

Authors