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 14C-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.
Highlights
Coastal wetlands of the Cascadia subduction zone, from British Columbia to northern California (Figure 1), host one of the longest and best-preserved onshore records of great megathrust earthquakes and accompanying tsunamis (e.g., Atwater 1992; Nelson, Shennan & Long 1996; Clague 1997; Witter, Kelsey & Hemphill-Haley 2003; Dura et al 2016a), debate continues as to the rupture lengths, locations, magnitudes, and timing of Cascadia’s megathrust earthquakes (Witter et al 2012; Atwater et al 2014; Goldfinger et al 2016; Hutchinson and Clague 2017)
In the laboratory we described the lithostratigraphy of core V1 and recorded its lithology with the locations of microfossil and 14C samples on a color-corrected, photo mosaic of the core
Botts marsh – stratigraphy, ages, and earthquake subsidence To determine the lateral extent of the four peat-mud contacts in the East Bank outcrop to the east, we studied the stratigraphy of Botts marsh, a 270-m by 700-m wetland that hosts the only laterally extensive record of relative sea-level (RSL) changes over thousands of years that we mapped in the Nehalem River valley lowlands (Figures 2 and 3)
Summary
Coastal wetlands of the Cascadia subduction zone, from British Columbia to northern California (Figure 1), host one of the longest and best-preserved onshore records of great (magnitude 8–9) megathrust earthquakes and accompanying tsunamis (e.g., Atwater 1992; Nelson, Shennan & Long 1996; Clague 1997; Witter, Kelsey & Hemphill-Haley 2003; Dura et al 2016a), debate continues as to the rupture lengths, locations, magnitudes, and timing of Cascadia’s megathrust earthquakes (Witter et al 2012; Atwater et al 2014; Goldfinger et al 2016; Hutchinson and Clague 2017).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
