Collaborative Research: Heterogeneous Rupture of Great Cascadia Earthquakes Inferred from Coastal Subsidence Estimates

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Of the major subduction zones worldwide, Cascadia has not experienced rupture in the historical period. For example, each of the Alaska, Chile, Sumatra, Kamchatka, and Japan/Kurils subduction zones experienced multiple megathrust ruptures greater than magnitude 8.5 during this time. A critical step toward understanding Cascadia?s rupture patterns is reconstructing its land-level history over the past few thousands of years, a history that is linked to past earthquake cycles. This project uses a novel statistically-based microfossil (foraminifera and diatoms) analysis coupled with computer modeling to quantify coseismic subsidence in Cascadia tidal sediments to determine the rupture patterns of the Cascadia subduction. This project will produce data that is important to the assessment of seismic and tsunami hazards along the Pacific coast of North America, as well as for sites subject to teleseismic tsunamis produced by this region. The project has high potential to benefit society or advance desired societal outcomes through: 1) full participation of women in STEM; 2) increased public scientific literacy and public engagement with science and technology through public outreach efforts; 3) improved well-being of individuals in society through a better understanding of earthquake hazards in Cascadia coupled with planned outreach resource managers, decision makers, planners; 4) development of a diverse, globally competitive STEM workforce through development of early career researchers, mentoring of a post-doctoral scholar, involvement of graduate and undergraduate students in research, and activities for high school and community college students.Wetland sediments fringing estuaries at the Cascadia subduction zone harbor a record of plate-boundary earthquakes during the past 5,000 years. These are inferred from stratigraphic evidence of interbedded peaty and muddy sediment beneath tidal wetlands that are used to reconstruct land-level changes. However, the precision of past measurements of land-level changes at Cascadia is low and the measurements are spatially limited. This makes past measurements insufficient for determining which hypotheses of plate-boundary deformation are most valid. This project will re-dress this deficiency by applying recently developed statistical transfer functions to microfossils to reconstruct Cascadia's rupture patterns and timing and magnitude of strain release over several thousands of years. This technique will be employed to test three hypotheses regarding the nature of rupture during the AD 1700 and three earlier megathrust earthquakes: 1) Coseismic subsidence varied spatially and temporally during past Cascadia plate-boundary earthquakes; 2) Estimates of coseismic subsidence can differentiate between wide and narrow rupture widths; and 3) More precise dating of earthquake evidence allows more direct evaluation of megathrust segmentation. Field, laboratory, computational, and theoretical investigations will focus on four earthquake events from six estuaries from southern Oregon to northern Washington. These carefully selected sites also include a strike-normal transect. A combined approach of stratigraphic description of buried soils, AMS 14C dating and multi-proxy microfossil transfer functions, supported by testate amoebae and geochemistry, will result in the construction of land-level changes. A 3D dislocation model with the 3D megathrust fault geometry will be used to compare coseismic deformation to with paleoseismic estimates.