Anisotropic structure of Earth's inner core from noise correlations

地球的固体内核是在地球冷却时由液体的外核结晶形成的。内核具有轴对称各向异性，由于内核铁晶体倾向于朝南北方向排列，内核的快轴几乎朝南北方向排列。以往对地核结构的研究显示出极大的复杂性。因此，了解这种突出的各向异性的起源是当今地球科学的基本问题之一；成像复杂的三维各向异性结构的内核是至关重要的在这一努力。不幸的是，地球深处的地震成像受到地震分布和台站位置的限制。该项目旨在探索一种新技术，基于地震噪声（来自周围环境和地震尾波）的相关性，提取出一套截然不同的地球深部观测数据。我们期望这种改进的内核地震模型将有利于矿物学和地球动力学界，因为它为他们的模型提供了约束。该项目对理解地球各向异性的起源、组成、动力学和演化具有重要意义。S核心，尤其是内核。深穿透波的反演克服了传统地震方法的一些缺点，将对揭示地球结构产生根本性的影响。内心深处。该项目建立在最近从环境噪声或地震尾波的相关性中提取体波的成功基础上。近十年来，环境噪声相关技术被广泛用于研究地球的结构，特别是岩石圈结构的表面波层析成像。最近的研究表明，从环境噪声或地震尾波的相关性中提取体波是可行的。我们最近成功地检索了清晰的三次PKP阶段和内核到达的PKIKP2和PKIIKP2同时使用经验Green？S函数来自于单站的自相关或不同站记录的数据之间的相互相关。这些观测结果使我们能够使用不同相位之间的差分旅行时间来消除或减少地幔非均质性和源误差的影响。新的PKIKP2和PKIIKP2数据表明，地球内核的地震各向异性与外核的南北各向异性不同，在赤道附近有一个快速的轴线。在这个项目中，我们试图探索这些新的数据类型来约束内核的三维各向异性结构。从噪声互相关得到的不同分支间的差分PKP传播时间提供了与现有地震数据截然不同的数据覆盖范围。PKIKP2 - PKIKP2旅行时间的差异提供了内核最深处的全新样本。具体来说，我们将系统地检索PKIKP2和PKIIKP2，同时对世界各地所有可用的阵列应用自相关和互相关，并通过对距离大于145度的站点阵列对应用互相关，系统地检索三倍PKP分支。然后，我们将新的基于噪声的数据与地震数据相结合，对三维内核各向异性结构进行建模和反演。

The Earth's solid inner core is formed from the crystallization of the liquid outer core as the Earth cools. The inner core possesses axisymmetric anisotropy with its fast axis aligned nearly North-South due to the preferred alignment of the inner core iron crystals in this direction. Previous studies of the inner core structure have shown great complexities. Thus, understanding the origin of this prominent anisotropy is one of the fundamental questions of earth science today; and imaging the complex three-dimensional anisotropic structure of the inner core is crucial in this endeavor. Unfortunately, seismic imaging of the deep Earth has been limited by the distribution of earthquake and station locations. This project seeks to explore a new technique that extracts a vastly different set of observations for the deep Earth based on correlations of seismic noise (from ambient environment and from earthquake coda). We expect that this improved inner core seismic model will benefit the mineralogical and the geodynamical communities, as it provides constraints on their models. The project has implications for understanding the origin of the anisotropy, composition, dynamics, and evolution of Earth?s core and the inner core in particular. The retrieval of the deep penetrating waves overcomes some of disadvantages of traditional earthquake-based methods, which will have a fundamental impact on revealing the structure of the Earth?s deep interior.This project builds on recent successes in extracting body waves from correlations of either ambient noise or earthquake coda. During the last decade, ambient noise correlation technique has been widely used to study the structure of our planet, in particular surface wave tomography of lithosphere structure. More recent studies have shown feasibility to extract body waves from correlations of ambient noise or earthquake coda. We have recently succeeded in retrieving clear triplicated PKP phases and inner-core arrivals PKIKP2 and PKIIKP2 simultaneously using stacks of empirical Green?s functions from autocorrelations of single station or cross-correlations between data recorded at different stations. These observations allow us to use differential travel times between different phases to eliminate or reduce the influence of mantle heterogeneity and source errors. The new PKIKP2 and PKIIKP2 data suggest a seismic anisotropy in the inner-inner core of the Earth that has a fast axis aligned near the equator and a different form of anisotropy from the North-South aligned anisotropy observed in the outer inner core. In this project, we seek to explore these new types of data to constrain the 3D anisotropic structure of the inner core. The differential PKP travel times between different branches from noise cross-correlations provide vastly different data coverage from existing earthquake data. The differential PKIIKP2- PKIKP2 travel times provide brand new samples of the deepest part of the inner core. Specifically, we will systematically retrieve PKIKP2 and PKIIKP2 simultaneously applying autocorrelations and cross-correlations to all available arrays around the world and systematically retrieve triplicated PKP branches by applying cross-correlations to pairs of station arrays at distances greater than 145 degrees. We will then combine the new noise-based data with earthquake data to model and invert for 3D inner-core anisotropic structure.