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

在世界主要俯冲带中，卡斯卡迪亚在历史时期没有经历过破裂。例如，在此期间，阿拉斯加、智利、苏门答腊岛、堪察加和日本/千岛俯冲带中的每一个都经历了超过8.5级的多次大型逆冲断裂。要了解卡斯卡迪亚-S破裂模式，关键的一步是重建其过去几千年的陆地历史，这一历史与过去的地震周期有关。该项目使用一种新的基于统计的微化石(有孔虫和硅藻)分析结合计算机模拟来量化卡斯卡迪亚潮汐沉积物中的同震沉降，以确定卡斯卡迪亚俯冲的破裂模式。该项目将提供对评估北美太平洋沿岸的地震和海啸危害以及该地区遭受远震海啸影响的地点十分重要的数据。该项目有很大潜力造福社会或通过以下方式推动预期的社会成果：1)妇女充分参与STEM；2)通过公众宣传努力，提高公众的科学素养和公众对科学技术的参与；3)通过更好地了解卡斯卡迪亚的地震灾害，以及有计划的外联资源管理人员、决策者和规划者，改善社会个人的福祉；4)通过发展早期职业研究人员，指导博士后学者，让研究生和本科生参与研究，以及为高中和社区大学生开展活动，发展一支多样化的、具有全球竞争力的STEM劳动力队伍。卡斯卡迪亚俯冲带河口边缘的湿地沉积物是过去5000年来板块边界地震的记录。这些都是从潮汐湿地下泥炭和泥质沉积物互层的地层学证据中推断出来的，这些沉积物被用来重建陆地平面的变化。然而，卡斯卡迪亚过去对土地水平变化的测量精度很低，而且测量结果在空间上是有限的。这使得过去的测量不足以确定哪些板块边界变形的假设是最有效的。该项目将通过将最近开发的统计传递函数应用于微化石来重建卡斯卡迪亚的破裂模式以及数千年来应变释放的时间和大小，从而重新解决这一不足。这项技术将被用来检验关于公元1700年和更早的三次大地震期间破裂性质的三个假说：1)在过去的卡斯卡迪亚板块边界地震中，同震沉降在空间和时间上发生变化；2)同震沉降的估计可以区分宽破裂宽度和窄破裂宽度；以及3)对地震证据进行更精确的年代测定，可以更直接地评估大破裂分段。现场、实验室、计算和理论调查将集中在从俄勒冈州南部到华盛顿州北部六个河口的四次地震事件上。这些精心挑选的地点还包括走向法线横断面。埋藏土壤的地层学描述、AMS 14C测年和多指标微化石转移函数的组合方法，在遗迹变形虫和地球化学的支持下，将导致陆地水平的变化。一个具有3D巨型逆冲断层几何形状的3D位错模型将被用来比较同震形变和古地震估计。

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.