Reproducibility and variability of earthquake subsidence estimates from saltmarshes of a Cascadia estuary

Abstract

We examine fossil foraminiferal assemblages from 20 sediment cores to assess sudden relative sealevel (RSL) changes across three mud-over-peat contacts at three salt marshes in northern Humboldt Bay, California (~44.8°N, −124.2°W). We use a validated foraminiferal-based Bayesian transfer function to evaluate the variability of subsidence stratigraphy at a range of 30-6000 m across an estuary. We use the consistency in RSL reconstructions to support estimates of coseismic subsidence from megathrust earthquakes. To assess the variability of subsidence estimates, we analyzed: nine examples of the 1700 CE earthquake (average of 0.64 ±0.14 m subsidence; range of 0.24 ± 0.27 to 1.00 ± 0.44 m), five examples of the ca. 875 cal a BP earthquake (average of 0.43 ±0.16 m; range of 0.41 ± 0.36 to 0.48 ± 0.39 m), and six examples of the ca. 1120 cal a BP earthquake (average of 0.70±0.18 m; range of 0.47 ± 0.36 to 0.80 ± 0.49 m). Our subsidence estimates suggest ~±0.3 m of within-site (intrasite) variability, which is consistent with previous research. We also identify inconsistencies between sites (intersite) at northern Humboldt Bay greater than one-sigma uncertainties, driven by variable foraminiferal assemblages in the mud overlying the 1700 CE subsidence contact. Therefore, we recommend at least two quantitative microfossil reconstructions across the same stratigraphic sequence from different marsh sites within an estuary to account for estimate variability and provide increased confidence in vertical coseismic deformation estimates. Our results have broad implications for quantitative, microfossil-based reconstructions of coseismic subsidence at temperate coastlines globally.

Testing Megathrust Rupture Models Using Tsunami Deposits

“…Without hindcast tides for pre‐1700‐CE Cascadia events, tsunami models should be run at high and low tides along with a variety of DEMs that incorporate adjustments to modern topography based on sea‐level reconstructions and evidence of geomorphological change. Despite these limitations, even rough estimates of pre‐1700‐CE tsunami magnitude or coseismic subsidence could help characterize past rupture variability (e.g., Padgett et al., 2022).…”

“…(2018) and Padgett et al. (2022) are annotated. Note that at Zeballos, British Columbia, and northern Humboldt Bay, California, tsunami deposits were not observed on top of 1700 subsidence contacts.…”

“…Source models of the past Cascadia events have generally relied on along-strike coastal subsidence estimates to validate modeled deformation (Gao et al, 2018;Leonard et al, 2010;Li & Liu, 2021;Melgar, 2021;Ramos et al, 2021;Wang et al, 2013) and the ca. 1700 CE event is most commonly considered due to the breadth of sites where foraminifera-based coseismic subsidence estimates exist (Kemp et al, 2018;Padgett et al, 2021Padgett et al, , 2022 (Figure 1). Tsunami deposits in Cascadia have been used before to test earthquake source models of a ca.…”

Integrating onshore and offshore paleoseismic evidence to reveal past large earthquakes on the weakly coupled central Hikurangi margin, New Zealand

“…Periods of non-tectonic sea level change also impact the preservation potential of coastal sites, making it necessary to characterize these trends upfront to assess the probability of recording or eroding earthquake evidence (Dura et al, 2016a). If the presence of coseismic deformation has been established with confidence using the criteria outlined in Table 2.1, the analysis of microfossil assemblages across horizons of paleoenvironmental change can yield estimates of the amount of vertical deformation that occurred during past earthquakes (Cochran, 2002;Padgett et al, 2022;Shennan et al, 1999;Wang et al, 2013). This is particularly useful for distinguishing vertical displacements where sedimentary evidence is more subtle (Hayward et al, 2016).…”