Collaborative Research: Towards improved imaging of the outermost core through determination of the effects of lowermost mantle heterogeneity and anisotropy

地球的外核是熔融的铁，加上大约10%的较轻的合金材料。其半径略大于火星，核心位于地球表面以下约2900公里处。流体外核具有很高的重要性，原因有很多，包括地球磁场的产生，以及将热量传递到地幔的重要机制，这有助于驱动负责板块构造的对流引擎。 在过去的几十年里，地球物理学家已经在外核的顶部发现了一个薄壳，地震波似乎在那里减慢，这表明流体中存在某种形式的分层。 然而，这种壳的地震模型并不完全一致，这激发了进一步的研究。对外核进行地震成像的一个主要挑战是，用于研究核的地震波必须穿过地球的整个地幔两次（向下然后返回），并且地幔非常不均匀，地震速度随位置而变化。 因此，地震学家如何确定测量信号的模式是由于异常的最外层核结构还是地幔的不均匀性？ 这个项目的目的是记录地幔的不均匀性对用于绘制地核图的数据的影响程度。由四名地震学家组成的团队将收集来自世界各地的地震和地震传感器的地震数据，使用最先进的地震波场计算预测观测结果，并对对地球深部地幔和外核部分敏感的信号进行精细测量。 预期的结果是更好地了解地球的异质地幔影响数据测量的程度，以研究核心，并通过使用被证明受地幔影响最小的最佳数据来制作最外层核心的改进模型。 更好地绘制地球最外层核心的地震图将为研究提供信息，旨在了解磁场的神秘性质，从核心到地幔的热流，以及地幔和核心之间可能的化学交换，这对了解地球的化学演化很重要。 团队中的所有四位地震学家将在各种场所（包括教室，公共演讲和科学博览会）与公众分享结果，以提高人们对地球和行星内部在塑造地表现象方面的重要性的认识。该项目将培训若干名研究生和本科生，地震成像的P波速度（Vp）在最外面的50-400公里的核心减少意味着存在一个稳定的分层覆盖在更深的，单独的对流内部。减速（和密度）层的精确厚度和性质对于解决地球磁场形成的地球发电机模型以及理解核心成分和矿物学的能力具有至关重要的意义。然而，在30多年的地震研究中，地震学家们对该层的厚度或速度结构没有达成共识，这可能是由于地幔非均匀性和各向异性对用于探测最上层外核的地震数据的影响。该项目将调查在最外层地核传播的地震波的旅行时间、波径、波形和横波分裂异常，包括来自核-幔边界下侧的多次反射（“SmKS”波），这比任何其他地震波对外核结构更敏感，以便查明和减轻地幔结构对外核模型的影响。为了实现这一目标，将采用一种方法，对地理上局部化的地震子阵列的地震图进行求和，以提高弱信号相对于噪声的清晰度，这可以改进对地幔异质性和各向异性污染的识别。3D波场合成将被用来衡量1D外核成像工具如何受到地幔不均匀性的影响。新的最外层岩心地震模型随后将根据最高质量的数据，在正演和反演实验中确定，这些数据已根据地幔不均匀性和各向异性的影响进行了校正。因此，该项目将直接测试长期存在的（但可能不完善）假设，即差分SmKS走时可用于可靠地检索外核结构，而无需明确考虑地幔异质性和各向异性的影响。除了基于高质量的校正数据生成外核结构的新模型外，该项目还将生成补充数据产品，其中包含对下地幔速度不均匀性和各向异性的新见解。该奖项反映了NSF的法定使命，并被认为值得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来支持。

Earth’s outer core is molten iron, plus roughly 10% of a lighter alloying material. With a radius that is slightly larger than the planet Mars, the core sits roughly ~2900 km below Earth’s surface. The fluid outer core holds high importance for a number of reasons, including the generation of Earth’s magnetic field, and being an important mechanism to transfer heat to the mantle, which helps to drive the convective engine responsible for plate tectonics. In the past few decades, geophysicists have detected a thin shell at the top of the outer core where seismic waves appear to slow down, suggesting some form of stratification in the fluid. However, the seismic models for this shell do not all agree, which motivates further study. A principle challenge in seismically imaging the outer core is that seismic waves from earthquakes that are used to study the core have to travel through the entire mantle of the Earth twice (down and then back), and the mantle is very heterogeneous, with seismic velocities that vary with position. Thus, how can seismologists determine if patterns in measured signals are due to anomalous outermost core structure versus heterogeneities in Earth’s mantle? The purpose of this project is to document the degree to which the heterogeneous mantle affects the data which are used to map the core. The team of four seismologists will collect seismic data from earthquakes and seismic sensors from all over the world, predict the observations using state of the art seismic wavefield computations, and conduct refined measurements on the signals that are sensitive to deep mantle and outer core parts of the planet. The expected outcome is a better understanding of the degree to which Earth’s heterogeneous mantle affects measurements of data to study the core, and to produce an improved model of the outermost core by using the best data which are demonstrated to be minimally affected by the mantle. Better seismic mapping of Earth’s outermost core will inform research that aims to understand the enigmatic nature of the magnetic field, heat flow from the core to the mantle, as well as possible chemical exchange between the mantle and core which is important for understanding the chemical evolution of the planet. All four seismologists on the team will share results with the public in a variety of venues (including the classroom, public presentations, and science fairs) to promote awareness of the importance of Earth and planetary interiors in shaping phenomena experienced at the surface. The project will train several graduate and undergraduate students.Seismically imaged P wave velocity (Vp) reductions in the outermost 50-400 km of the core imply the presence of a stably stratified layer overlying the deeper, separately convecting interior. The precise thickness and nature of the reduced velocity (and density) layer holds critical significance for geodynamo models that address making Earth’s magnetic field, as well as the ability to understand core composition and mineralogy. However, in over ~30 years of seismic studies, no consensus has emerged among seismologists on either the thickness of the layer or the velocity structure within it. This is likely due to the effects of mantle heterogeneity and anisotropy on the seismic data used to probe the uppermost outer core. This project will investigate travel time, wave path, waveform, and shear wave splitting anomalies of seismic waves that travel in the outermost core, including multiple reflections from the underside of the core-mantle boundary (“SmKS” waves), which are more sensitive to outer core structure than any other seismic wave, in order to identify and mitigate the effects of mantle structure on outer core models. To accomplish this, a method that sums seismograms at geographically localized seismic sub-arrays will be used to improve the clarity of weak signals relative to noise, which can improve upon identification of contamination from mantle heterogeneity and anisotropy. 3D wavefield synthetics will be used to benchmark how 1D outer core imaging tools are affected by mantle heterogeneity. New outermost core seismic models will subsequently be determined in forward and inverse experiments based on highest-quality data that have been corrected for the effects of mantle heterogeneity and anisotropy. Thus, this project will thus directly test the longstanding (but likely imperfect) assumption that differential SmKS travel times can be used to reliably retrieve outer core structure without explicitly considering the effects of mantle heterogeneity and anisotropy. In addition to producing new models of outer core structure based on high-quality, corrected data, this project will produce complementary data products that contain new insights into lower mantle velocity heterogeneity and anisotropy.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.