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

在全球主要的俯冲带中，卡斯卡迪亚在历史时期没有经历过破裂。例如，阿拉斯加、智利、苏门答腊、堪察加和日本/千岛群岛俯冲带在这段时间内都经历了多次大于8.5级的巨型逆冲断层破裂。了解卡斯卡迪亚的关键一步？地震的破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。该项目使用一种新的基于化石的微体化石（有孔虫和硅藻）分析，再加上计算机建模，以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的破裂模式。该项目将产生对评估北美太平洋沿岸沿着地震和海啸灾害以及该地区遭受海啸灾害的地点十分重要的数据。该项目具有很大的潜力，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危害，加上计划的宣传资源管理者，决策者，规划者，改善社会中个人的福祉; 4）通过培养早期职业研究人员，指导博士后学者，让研究生和本科生参与研究，以及为高中和社区大学学生举办活动，培养多元化的、具有全球竞争力的STEM劳动力。卡斯卡迪亚俯冲带河口边缘的湿地沉积物记录了过去5,000年的板块边界地震。这些都是推断从层间泥炭和泥质沉积物下的潮汐湿地，用于重建土地水平变化的地层证据。然而，过去在卡斯卡迪亚的土地水平变化的测量精度很低，测量空间有限。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体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幅度来弥补这一不足。这种技术将被用来检验关于公元1700年和更早的三次大规模逆冲断层地震期间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期间，同震沉降在空间和时间上变化：2）同震沉降的估计可以区分宽和窄的破裂宽度; 3）更精确的地震测年证据可以更直接地评价巨型逆冲断层的分段。野外、实验室、计算和理论研究将集中于从俄勒冈州南部到北方华盛顿的六个河口的四次地震事件。这些精心挑选的地点还包括一个走向正常的样带。一个综合的方法，地层描述的埋藏土壤，AMS 14 C测年和多代理微体化石转换功能，支持有遗嘱的变形虫和地球化学，将导致建设的土地水平变化。一个三维位错模型与三维巨型逆冲断层几何形状将被用来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.