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

Abstract

We infer a history of three great megathrust earthquakes during the past 2000 years at the Nehalem River estuary based on the lateral extent of sharp (≤3 mm) peat-mud stratigraphic contacts in cores and outcrops, coseismic subsidence as interpreted from fossil diatom assemblages and reconstructed with foraminiferal assemblages using a Bayesian transfer function, and regional correlation of 14 C-modeled ages for the times of subsidence. A subsidence contact from 1700 CE (contact A), sometimes overlain by tsunami-deposited sand, can be traced over distances of 7 km. Contacts B and D, which record subsidence during two earlier megathrust earthquakes, are much less extensive but are traced across a 700-m by 270-m tidal marsh. Although some other Cascadia studies report evidence for an earthquake between contacts B and D, our lack of extensive evidence for such an earthquake may result from the complexities of preserving identifiable evidence of it in the rapidly shifting shoreline environments of the lower river and bay. Ages (95% intervals) and subsidence for contacts are: A, 1700 CE (1.1 ± 0.5 m); B, 942-764 cal a BP (0.7 ± 0.4 m and 1.0 m ± 0.4 m); and D, 1568-1361 cal a BP (1.0 m ± 0.4 m). Comparisons of contact subsidence and the degree of overlap of their modeled ages with ages for other Cascadia sites are consistent with megathrust ruptures many hundreds of kilometers long. But these data cannot conclusively distinguish among different types or lengths of ruptures recorded by the three great earthquake contacts at the Nehalem River estuary.

Timing and amount of southern Cascadia earthquake subsidence over the past 1700 years at northern Humboldt Bay, California, USA

“…However, little work has considered the potential differences in the age estimate results that could be imposed by the numerical age-model of choice. Moreover, often only the modeling program is cited without the specific type of model identified and/or explained (Milker et al, 2016;Nelson et al, 2020). We attempt to address this gap by comparing useful Bayesian age-depth models, in order to assess the variability in age estimates that may be imposed by model choice.…”

Minimal stratigraphic evidence for coseismic coastal subsidence during 2000 yr of megathrust earthquakes at the central Cascadia subduction zone

“…dating, we sought to identify sudden changes in RSL that might correlate with evidence of earthquakes and tsunamis at more recently studied coastal sites and earthquake-generated turbidites offshore (e.g., Graehl et al, 2014;Goldfinger et al, 2016;Milker et al, 2016;Nelson et al, 2020). We examined 61 gouge cores (100-cm-long segments, 25 mm diameter), describing 38 of them in the field along with stratigraphic sections at outcrops on the north side of Cox Island and 200 m upriver from its northeast corner using the Troels-Smith (1955) system for describing organic-rich sediment (methods described by Nelson, 2015;see Figs.…”

“…that were obvious outliers or that we interpreted to be less accurate minimum or maximum estimates of the times contacts formed (e.g., Milker et al, 2016;Witter et al, 2019;Nelson et al, 2020). Our inferred closest maximum and minimum ages for each contact are marked in bold on Table 2.…”

“…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 developments are Bayesian transfer functions (Cahill et al, 2016;Kemp et al, 2018;Hong, 2019;Padgett, 2019;Nelson et al, 2020), which allow species response curves to deviate from a predefined 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 reconstructions of past RSL change.…”

“…This tidal stratigraphy of interbedded lithologies reflecting jerky RSL rise was contrasted with the 1-to 4-m-thick sections of largely peaty wetland sediment common on temperate North American coasts, which were interpreted as the product of gradual late Holocene sea-level rise (e.g., Bloom and Stuiver, 1963;Redfield, 1972). Although some cautioned that vertical tectonic deformation is only one of many factors that influence tidal sedimentation at Cascadia (Darienzo and Peterson, 1990;Nelson, 1992b;Long and Shennan, 1994;Nelson et al, 1996b;Allen, 2000), the model of jerky late Holocene RSL rise remained the basis for interpretations of repeated subsidence of tidal wetlands during as many as 12 great earthquakes at tens of sites along the subduction zone (Atwater et al, 1995;Nelson and Personius, 1996;Clague, 1997;Shennan et al, 1998;Kelsey et al, 2002;Witter et al, 2003;Nelson et al, 2004;Schlichting and Peterson, 2006;McCalpin and Carver, 2009;Valentine et al, 2012;Graehl et al, 2014;Hutchinson and Clague, 2017;Hong, 2019;Padgett, 2019;Nelson et al, 2020). Similar assumptions were used to infer that a record of megathrust earthquake deformation is preserved in tidal sequences on other subduction-zone coasts (Nelson, 2013;Dura et al, 2016a;Shennan et al, 2016).…”