Revising Estimates of Spatially Variable Subsidence during the A.D. 1700 Cascadia Earthquake Using a Bayesian Foraminiferal Transfer Function

Abstract

Coseismic subsidence along the Cascadia subduction zone causes abrupt relative sea-level (RSL) rise that is recorded in coastal stratigraphy and foraminiferal assemblages. RSL reconstructions therefore provide insight into the magnitude, nature, and frequency of great earthquakes that can constrain deformation models and quantify the seismic risk faced by coastal populations. These reconstructions are commonly generated using transfer functions that are calibrated from counts of modern (surface) foraminifera and corresponding elevation measurements. We developed four transfer functions of increasing complexity to explore how and why the composition of the modern dataset and the choice of transfer-function type affects subsidence reconstructions. Application of these four models to stratigraphic contacts (mud abruptly overlying peat or soil) representing the A.D. 1700 Cascadia earthquake and a field experiment that simulated subsidence show that a Bayesian transfer function (BTF) calibrated using a large modern dataset (19 sites from California to Vancouver Island) and incorporating prior information from stratigraphic context produces systematically larger subsidence estimates than a weighted-averaging transfer function calibrated using a smaller modern dataset (8 sites in Oregon) that does not leverage stratigraphic context. This difference arises from ( 1) training set composition, (2) taxa-elevation relationships in the BTF that are not assumed to be unimodal, and(3) stratigraphic prior information that compensates for postdepositional, downward mixing of postearthquake foraminifera into pre-earthquake sediment, which biases reconstructions at some sites toward smaller subsidence. Our reconstructions support a heterogeneous rupture model for the A.D. 1700 earthquake, but indicate that slip estimates in patches from Alsea Bay to Netarts Bay (Oregon) and from Netarts Bay to Vancouver Island should be increased. Electronic Supplement:Table listing counts of foraminifera and sample elevations used to construct the West Coast modern training set.

Controls on Sediment Accretion and Blue Carbon Burial in Tidal Saline Wetlands: Insights From the Oregon Coast, USA

“…Secondary variables, including tidal range, water temperature and salinity patterns, and tectonic history (Hickey & Banas, 2003;Kemp et al, 2018), are similar among these estuaries.…”

“…Independently, there exists a roughly 30‐fold range in fluvial sediment flux to Oregon estuaries due primarily to variations in river basin area (Wheatcroft & Sommerfield, ). Secondary variables, including tidal range, water temperature and salinity patterns, and tectonic history (Hickey & Banas, ; Kemp et al, ), are similar among these estuaries.…”

Identifying the Greatest Earthquakes of the Past 2000 Years at the Nehalem River Estuary, Northern Oregon Coast, USA

“…Transfer functions use the relations among modern assemblages and their respective elevations in modern tidal environments as analogs to hindcast past tidal elevations from fossil assemblages in stratigraphic sequences (Kemp and Telford 2015). The most recent development are Bayesian foraminiferal transfer functions (Cahill et al 2016;Kemp et al 2018;Hong 2019;Padgett 2019) that, unlike previous non-Bayesian transfer functions, allow species response curves to deviate from a pre-defined form (commonly unimodal) and may incorporate prior information about sampled sediment (i.e., stratigraphy, lithology, paleoecologic information from other types of fossils) to help constrain estimates of past RSL change.…”

“…The north-to-south contrast in numbers and recurrence of great megathrust earthquakes in land and marine records suggests differences in earthquake history critical to the assessment of earthquake hazard in central western North America, and of tsunami hazard assessment in the Pacific basin. But studies of earthquake or tsunami stratigraphy at many sites at Cascadia are reconnaissance investigations completed 15-30 years ago; few address thresholds for creating and preserving stratigraphic evidence of earthquakes and their tsunamis (e.g., Nelson, Jennings & Kashima 1996;Atwater and Hemphill-Haley 1997;Shennan et al 1998;Hutchinson et al 2000;Nelson, Kelsey & Witter 2006;Graehl et al 2014;Shennan, Garrett & Barlow 2016); and even fewer quantitatively assess deformation during individual earthquakes (e.g., Nelson et al 2008;Hawkes et al 2011;Wang et al 2013;Kemp et al 2018;Padgett 2019), or use rigorous sample evaluation criteria with statistically based models Figure 1: Physiography and major features of the Cascadia subduction zone showing the location of the Nehalem River estuary on the northern Oregon coast (base map data source: GEBCO Compilation Group (2019) GEBCO 2019 Grid, doi:10.5285/836f016a-33be-6ddc-e053-6c86abc0788e). The deformation front of the subduction-zone megathrust fault on the ocean floor (red barbed line) is near the bathymetric boundary between the continental slope and abyssal plain.…”

“…In this paper, we compile and interpret extensive but unintegrated stratigraphic data-collected intermittently over three decades-to reconstruct earthquake history for a key site in the northern Oregon part of the central Cascadia subduction zone. The Nehalem River estuary lies between coastal wetland sites in southwest Washington with thick-mud, thin-peat stratigraphy and fossils suggesting >1-1.5 m of sudden subsidence during great earthquakes (e.g., Hemphill-Haley 1995;Shennan et al 1996;Atwater and Hemphill-Haley 1997;Kemp et al 2018), and estuarine sites to the south where thin-mud, thick-peat stratigraphy and microfossil reconstructions suggest a history of substantially less earthquake subsidence (e.g., Nelson 1992;Nelson and Personius 1996;Shennan et al 1998;Nelson et al 2008;Wang et al 2013;Kemp et al 2018;Padgett 2019) (Figures 1 and 2). Based on the initial work of Grant (unpublished 1994 report in Supplementary Files) describing evidence for four megathrust earthquakes in river outcrops, we hoped that Nehalem River estuary stratigraphy would help show how these apparent differences in deformation history transition from north to south.…”