Evolving work budget of fault initiation, linkage and growth within accretionary systems

下一次破坏性的地震会发生在一个已经有很长滑动历史的断层上，还是会发生在一个地质学家还没有认识到的新形成的断层上？在世界地震活跃地区，新断层的增长对准确评估地震危险性提出了重大挑战。对新断层生长的预测将有助于更好地了解活动断层系统的发育。通过多次地震的积累，每一次地震都会逐渐推动断层系统的发展，最初分布的1厘米裂缝可以演变成具有破坏性地震的100公里长的断层。由于断层的生长发生在广阔的空间尺度和地下，地质学家无法直接观察新断层的生长。本项目采用物理实验室实验和模拟断层生长的计算机模型相结合的方法，在没有直接观测的情况下探索这一问题。计算机模型建立在这样一种思想的基础上，即新故障的生长是为了使功最小化并达到较低的能量状态。计算机预测将与实验室实验进行比较，将模拟日本东京南部南开海槽增生楔前新断层的发育。这一极好的俯冲带成像为检验地壳断层生长模型提供了良好的约束条件。南开海槽产生了几次毁灭性的地震，从这个俯冲带中获得的见解将指导我们对其他俯冲带的理解，如西雅图的卡斯卡迪亚边缘和阿拉斯加的阿留申群岛。其他期望的社会成果包括妇女和残疾人充分参与STEM，通过研究生和博士后研究人员培训发展多样化的有竞争力的STEM劳动力，国际合作，以及通过开发和分发新的计算机建模代码来增强研究基础设施。增生棱镜是研究断层发育的理想位置，因为在棱镜的脚趾处合并的材料相对没有断层。野外观察表明，贯穿式棱镜级断层是通过增生系统内较小裂缝的相互作用和联系而形成的。这种结合导致了应变从最初沿许多小裂缝的扩散应变到沿离散断层的局部化。本项目在规模物理实验和南开棱镜中研究裂缝合并到断层形成的演变。增生系统将使用一种创新的新数值代码GROW进行建模，该代码通过优化外部功来预测裂缝的扩展。裂缝合并将在四个不同的系统中进行模拟：(1)双轴和三轴系统，通过在挪威奥斯陆大学进行的类似负载的实验室测试进行验证，该测试使用断层扫描来跟踪裂缝合并；(2)法国Cergy-Pontoise大学进行的规模物理吸积实验，提供了已知的边界条件、材料特性以及断层演化和相关变形的详细定量观测；(3)中等复杂性和规模的吸积模拟，提供了(2)和(4)之间的关键步骤，并允许研究越来越现实的参数；(4)对横跨南开增生三棱镜的熊野样带进行了模拟，为序内逆冲和非序外逆冲的发育提供了良好的约束实例。对于后者，将在几个连续的时间间隔中模拟过去2 Ma的巨型逆冲发育。所有四组模型都将考虑故障增长的前提来研究故障的发展，以优化系统的总体工作。许多模型将从初始分布的种子点开始断层生长和合并，这代表了实验和地壳的弱非均质性。对于所有的模型套件，系统内不断变化的工作预算揭示了不同变形机制的竞争影响。通过研究每个系统中应变局部化、断裂相互作用和各种能量消耗过程之间的关系，研究人员将能够比较cm尺度的实验室物理实验模型和km尺度的自然造山带模型之间这些关系的差异。

Will the next damaging earthquake occur on a fault that has had a long history of slip or will it happen on a newly formed fault that is not yet recognized by geologists? The growth of new faults presents a significant challenge for accurate assessments of earthquake hazards in seismically active regions of the world. Prediction of new fault growth would allow better understanding of the development of active fault systems. Through the accumulation of many earthquakes that each incrementally advance the fault system development, an initial distribution of 1 cm cracks can evolve into a 100-km long fault capable of damaging earthquakes. Because fault growth happens across a wide range of spatial scales and in the subsurface, geologists are unable to directly observe the growth of a new fault. This project uses a combination of physical laboratory experiments and computer models that simulate fault growth to explore this problem in the absence of direct observation. The computer models build on the idea that new faults grow to minimize work and achieve a lower energy state. The computer predictions, which will be compared with laboratory experiments, will simulate the development of new faults in front of an accretionary wedge, the Nankai trough south of Tokyo, Japan. This extremely well imaged subduction zone provides good constraints for testing the crustal fault growth models. The Nankai trough has produced several devastating earthquakes and insights learned from this subduction zone will guide our understanding of other subduction zones such as Cascadia margin off Seattle and the Aleutians in Alaska. Additional desired societal outcomes include full participation of women and persons with disabilities in STEM, development of a diverse competitive STEM workforce through graduate student and post-doctoral researcher training, international collaboration, and enhanced infrastructure for research by development and distribution of new computer modeling code.Accretionary prisms are ideal locations for the study of fault development because the material incorporated at the toe of the prism is relatively un-faulted. Field observations suggest that through-going, prism-scale faults develop via the interaction and linkage of smaller fractures within accretionary systems. This coalescence results in the localization of strain along discrete faults from initially diffuse strain along many small fractures. This project examines the evolution of crack coalescence to fault formation within both scaled physical experiments and the Nankai prism. Accretionary systems will be modeled with an innovative new numerical code, GROW, which predicts fracture propagation through the optimization of external work. Crack coalescence will be simulated within four distinct systems: (1) biaxial and triaxial systems for validation with similarly loaded laboratory tests run at the University of Oslo, Norway, that use tomography to track crack coalescence; (2) scaled physical experiments of accretion run at the University of Cergy-Pontoise, France, that provide known boundary conditions, material properties and detailed quantitative observations of the evolution of faulting and associated deformation; (3) accretion simulations of intermediate complexity and scale that provide a critical step between (2) and (4) and allow investigation of increasingly realistic parameters; and (4) simulation of the Kumano transect across the Nankai accretionary prism, which provides a well-constrained example of the development of in-sequence and out-of-sequence crustal thrusts. For the latter, the proposed megathrust development will be simulated over the past 2 Ma in several successive time intervals. All four suites of models will investigate fault development considering the premise that faults grow to optimize the total work on the system. Many of the models will initiate fault growth and coalescence from initial distributed seed points, that represent weak heterogeneities in the experiment and crust. For all suites of models, the evolving work budget within the systems reveals the competing influences of different deformation mechanisms. By investigating the relationship between strain localization, fracture interaction and various energy consuming processes within each system, the researchers will be able to compare the differences in these relationships between models of cm-scale, laboratory physical experiments and km-scale, natural orogens.