Impact of upper-plate splay faults on accreting-sediment stress state and on megathrust strength and fluid budgets

上板块张开断层对增生沉积物应力状态以及巨型逆冲断层强度和流体收支的影响

• 批准号: 2041496
• 负责人: Maria Nikolinakou
• 金额: $ 29.24万
• 依托单位: University of Texas at Austin
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-09-01 至 2024-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2041496&HistoricalAwards=false
• 关键词: Impact; upper; plate; splay; faults

Earth’s subduction plate boundaries form where two tectonic plates converge. This creates a shallowly dipping interface as one plate (the subducting plate) descends beneath the other (overriding plate). This interface, also known as the subduction megathrust, produces some of the largest, most destructive and tsunamigenic earthquakes. Global examples include offshore Oregon-Washington states, offshore Alaska (Aleutians), Japan, New Zealand, and Costa-Rica–Nicaragua. The strength of the megathrust depends on the pore-fluid pressure in the Earth. Forces associated with subduction tend to increase fluid pressures at the megathrust. However, when a fault in the upper plate links the megathrust to the seafloor (splay fault), it can establish a path that drains excess pressures. This locally increases the megathrust strength, potentially promoting earthquakes. A number of subduction zones with splay faults have hosted tsunamigenic earthquakes. Additionally, splay faults transmit fluids from the plate interface to the seafloor, facilitating the transport of chemical species and cycling of elements. This process can potentially sustain biological communities at the seafloor. Here, the researchers study the effect of splay faults on the megathrust and sediments. They use numerical models that simulate the spatial and temporal evolution of a subduction zone. The models employ sediment-behavior laws that quantify the coupled interactions between fluid flow, deformation, and strength. The study outcomes provide insights on the mechanisms that govern the transition from aseismic (no earthquake) to seismogenic (earthquake) behaviors. They help improving earthquake and tsunami hazard assessment near subduction zones. They improve the understanding of volatile cycling and fluid flow. This project supports an early-career female scientist and one graduate student trained in an interdisciplinary context between geoscience and engineering geomechanics. It integrates field measurements, experiments, and modeling. It is funded by both the Geophysics program and the Marine Geology and Geophysics program.More specifically, the researchers develop large-strain, forward geomechanical models. They investigate whether splay faults limit the upper plate’s strength and lead to low differential stress and lateral heterogeneity in both horizontal stresses and sediment strength. They test the hypothesis that drainage along splay faults leads to heterogenous megathrust strength and mechanical properties and enhances dewatering rates. Their modeling approach 1) captures the large-strain evolution of a subduction system, 2) generates discrete faults, 3) couples the full stress tensor to deformation and porous fluid flow, and 4) simulates transient flow both along faults and in the sediment matrix. The models are constrained using published data from field measurements and laboratory experiments. Expected results include the full stress tensor, pore pressure, and porosity of sediments, as they are consumed into the subduction zone. Model outputs provide quantitative predictions of permeability, porosity and density, elastic moduli, strength, and seismic velocity/impedance at the plate interface, as well as seepage rates at the seafloor. Overall, the project results provide quantitative insights on the coupled processes of faulting, deformation, and fluid flow. More broadly, the project provides the foundation for a technical approach to address geological systems where large strains, pore fluids, sedimentation, and faulting interact.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

地球的俯冲板块边界形成于两个构造板块交汇的地方。当一个板块（俯冲板块）下降到另一个板块（上覆板块）之下时，这就形成了一个浅倾斜的界面。这个界面，也被称为俯冲大逆冲，产生了一些最大的，最具破坏性的海啸性地震。全球的例子包括俄勒冈-华盛顿州近海、阿拉斯加（阿留申群岛）近海、日本、新西兰和哥斯达黎加-尼加拉瓜。巨型逆冲断层的强度取决于地球上的孔隙流体压力。与俯冲有关的力倾向于增加大逆冲构造的流体压力。然而，当上部板块的断层将巨型逆冲断层与海底连接起来（展断层）时，它可以建立一条排出过量压力的路径。这在局部增加了巨型逆冲的强度，潜在地促进了地震。许多带展断层的俯冲带都曾发生过海啸性地震。此外，展断层将流体从板块界面输送到海底，促进了化学物质的输送和元素的循环。这一过程有可能维持海底的生物群落。在这里，研究人员研究了展断层对巨型逆冲断层和沉积物的影响。他们使用数值模型来模拟俯冲带的时空演化。这些模型采用沉积行为定律，量化流体流动、变形和强度之间的耦合相互作用。研究结果对控制从地震（无地震）行为向发震（地震）行为转变的机制提供了见解。它们有助于改善俯冲带附近的地震和海啸危险评估。它们提高了对挥发性循环和流体流动的理解。该项目支持一名早期职业女性科学家和一名在地球科学和工程地质力学跨学科背景下接受培训的研究生。它集成了现场测量，实验和建模。它由地球物理项目和海洋地质与地球物理项目共同资助。更具体地说，研究人员开发了大应变的正演地质力学模型。他们研究了展断层是否限制了上板块的强度，并导致了低差应力和水平应力和沉积物强度的横向非均质性。他们验证了这样一个假设，即沿展断层的排水导致了非均质大逆冲强度和力学性质，并提高了脱水速率。他们的建模方法1)捕捉俯冲系统的大应变演化，2)生成离散断层，3)将全应力张量与变形和多孔流体流动耦合，4)模拟沿断层和沉积物基质中的瞬态流动。这些模型使用来自现场测量和实验室实验的公开数据进行约束。预期结果包括全应力张量、孔隙压力和沉积物的孔隙度，因为它们被消耗到俯冲带中。模型输出提供了渗透率、孔隙度和密度、弹性模量、强度、板块界面地震速度/阻抗以及海底渗透率的定量预测。总体而言，该项目的结果为断层、变形和流体流动的耦合过程提供了定量的见解。更广泛地说，该项目为解决大应变、孔隙流体、沉积和断层相互作用的地质系统的技术方法提供了基础。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。