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

在世界上主要的俯冲带中，卡斯卡迪亚在历史上没有经历过断裂。例如，在这段时间内，阿拉斯加、智利、苏门答腊、堪察加和日本/千岛群岛的每个俯冲带都经历了多次大于8.5级的大型逆冲断裂。这是了解卡斯卡迪亚的关键一步吗？美国的断裂模式正在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。该项目采用了一种新颖的基于统计的微化石（有孔虫和硅藻）分析方法，结合计算机建模来量化卡斯卡迪亚潮汐沉积物的同震沉降，以确定卡斯卡迪亚俯冲带的断裂模式。该项目将产生对评估北美太平洋沿岸的地震和海啸灾害以及该地区易发生远震海啸的地点具有重要意义的数据。该项目具有很高的潜力，可以通过以下方式造福社会或促进预期的社会成果：1)妇女充分参与STEM；2)通过公共宣传工作提高公众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，以及有计划的外展资源管理人员、决策者和规划者，提高社会个人的福祉；4)通过培养早期职业研究人员，指导博士后学者，研究生和本科生参与研究，以及为高中和社区大学生举办活动，发展一支多元化的、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记录了过去5000年来的板块边界地震。这些都是从潮汐湿地下的互层泥炭质和泥质沉积物的地层证据中推断出来的，这些证据被用来重建陆地水平变化。然而，过去对卡斯卡迪亚地区陆面变化的测量精度较低，而且测量结果在空间上受到限制。这使得过去的测量不足以确定哪种关于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，以及几千年来应变释放的时间和大小，从而弥补这一缺陷。该技术将用于验证关于公元1700年和之前三次大逆冲地震破裂性质的三个假设：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都是不同的；2)同震沉降估算可以区分宽破裂宽度和窄破裂宽度；3)更精确的地震证据定年可以更直接地评价大逆冲分割。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的六个河口的四个地震事件上。这些精心挑选的地点还包括一个走向正常样带。埋藏土壤的地层描述、AMS 14C测年和多代微化石传递函数相结合的方法，将在地球化学和遗存变形虫的支持下构建陆面变化。三维断层位错模型与三维大逆冲断层几何形状将用于比较同震变形与古地震估计。

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.