CSEDI Collaborative Research: Joint seismic-geodynamic constraints on deep Earth structure - Implications for mantle convection and Earth rotation

由轨道卫星精确绘制的地球重力场有“起伏和山谷”，在这些地方，重力场有强有弱。这个场在最长波长上的变化描述了地球的椭圆形状。这个椭圆的图形很重要，因为它决定了穿过地球中心的旋转轴的位置，也控制了地球相对于太阳的倾斜度的变化，这是由于来自月球、太阳和其他行星的引力“拉”。实际上，地球就像一个旋转的陀螺，可以以复杂的方式振荡和摆动。轴向旋转和倾角的这些变化影响到地球上不同纬度，特别是极地地区接收到的太阳能（“日照量”）的多少，因此对气候及其长时间间隔内的变化产生重大影响。地球物理学的一个主要问题是确定地球的椭圆形状是如何由地球内部深处的力产生的。这些力还驱动地球构造板块的水平运动和大陆的地质“漂移”，以及随着时间的推移，垂直地推动和拉动大陆和海洋的上下运动。尽管地球物理学家在利用地震波绘制地球内部结构横向变化方面取得了三十多年的进展，计算地球物理学家也取得了类似的进展，他们利用这些地图来模拟内力，以解释构造板块的运动和地球重力场中的“颠簸”，但在正确解释卫星所看到的地球椭圆图形的扁平化方面仍然存在困难。本提案的主要研究人员（pi）将通过使用地震和重力数据，以及描述地球地形和构造板块运动的数据，直接解决这一突出的挑战，以获得地球内部结构的新地图和可以同时解释所有这些数据的力的新确定。这项工作的结果将使pi能够专门解决当前的挑战，即计算地球的长期椭圆图，它如何随时间变化，以及它如何影响高纬度日照，从而长期影响气候。未来三年，将部署强大的计算机资源来开展这项研究，并聘请来自德克萨斯大学和佛罗里达大学的研究人员组成合作小组。这项工作将为培训新的研究生提供机会，他们将学习最先进的计算机建模技术，并将在地球物理科学方面发展先进的专业知识。这项工作的成果将通过网络平台、会议和研讨会中的交流以及外联活动与广大科学家、学生和公众分享。尽管地球内部的全球地震成像已经取得了三十年的进展，但关于地幔大尺度结构和动力学的关键问题仍然悬而未决。长期以来，在令人满意地解释地球重力场中最长波长异常方面存在困难，这与地球的椭圆图形有关，对地球随时间变化的旋转动力学具有重要意义。关于过渡带的非均质性和下地幔“大低剪切速度省”（LLSVP）的性质的问题也仍然难以捉摸。这些关于异常地幔结构的不确定性直接影响了我们对地球全球尺度动力学的理解。为了解决这些问题，主要研究人员提出了一项多学科的努力，以获得新一代的断层扫描模型，该模型与与现今地幔结构和对流流动相关的大量地表数据相匹配，并且还可以约束整个新生代地幔随时间的演化。为了实现这些目标，pi将：(i)联合反演全球地震和对流相关数据，极大地提高了3-D地幔结构的覆盖范围，将横向粘度变化和过渡带顶部和底部不连续面地形的地球动力学响应结合起来；（ii）将数据同化方法应用于新的节理层析成像模型，重建三维地幔结构的新生代演化；（iii）计算地球转动惯量的相应变化，用于重建与长期气候变化有关的真极移和米兰科维奇轨道周期。预计这些新的联合断层扫描模型将更好地约束过渡区非均质性对关键区域对流质量和热输运的影响，以及对LLSVP内异常密度结构的空间分布的影响。这项工作还将有助于解决一个长期存在的难题，即地球异常椭圆图形的起源，以及对过去地质时期米兰科维奇气候周期的相位和幅度的影响。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。

The Earth's gravity field, which is precisely mapped by orbiting satellites, has 'bumps and valleys' where the field is stronger and weaker. The variation of this field on the very longest wavelengths describes the elliptical shape of the Earth. This elliptical figure is important because it determines the location of the axis of rotation that passes through the Earth's center and also controls changes in inclination of the Earth relative to the sun, owing to the gravitational 'pull' from the moon, the sun, and other planets. The Earth, in effect, behaves as a spinning top that can oscillate and wobble in complex ways. These changes in axial rotation and inclination affect how much solar energy ('insolation') is received at different latitudes on Earth, especially in the polar regions, and therefore has a major impact on climate and how it changes over long time intervals. A major problem in geophysics is to determine how the elliptical form of the Earth is generated by forces deep inside our planet. These forces also drive the horizontal motions of Earth's tectonic plates and the geological 'drift' of continents, as well as vertically pushing and pulling the continents and oceans up and down over time. Despite more than thirty years of progress by geophysicists who map lateral changes in Earth structure deep inside our planet using seismic waves, and similar progress by computational geophysicists who use these maps to model the internal forces needed to explain the motions of tectonic plates and the 'bumps' in Earth's gravity field, there remains a difficulty in properly accounting for the flattening of the elliptical figure of the Earth that is seen by satellites. The principal investigators (PIs) in this proposal will directly tackle this outstanding challenge by using both earthquake and gravity data, and data describing the topography of the Earth and the movements of tectonic plates, to obtain new maps of Earth's internal structure and new determinations of the forces that can explain all these data simultaneously. The outcome of this work will allow the PIs to specifically tackle the current challenge of accounting for Earth's long-term elliptical figure, how it changes with time, and how it influences high-latitude insolation and hence climate over long times. Powerful computer resources will be deployed to carry out this study over the next three years, employing a collaborative team of researchers from the Universities of Texas and Florida. This work will provide opportunities for training new graduate students who will learn state-of-the-art techniques in computer modelling and will develop advanced expertise in the geophysical sciences. The results of this work will be shared with a broad community of scientists, students and the general public with web-based platforms, communications in conferences and workshop, and in outreach activities.Key questions regarding the large-scale structure and dynamics of the mantle remain outstanding, despite three decades of progress in global seismic imaging of Earth's interior. There are longstanding difficulties in satisfactorily accounting for the very longest wavelength anomalies in Earth's gravity field, which are related to Earth's elliptical figure, with significant implications for Earth's time-dependent rotational dynamics. Questions concerning heterogeneity in the transition zone and the nature of "large low shear velocity provinces" (LLSVP) in the lower mantle, also continue to be elusive. These uncertainties regarding anomalous mantle structures directly impact our understanding of the global-scale dynamics of the Earth. To address the questions, the principal investigators propose a multidisciplinary effort to derive a new generation of tomography models that match a wide suite of surface data related to present-day structure and convective flow in the mantle, and can also constrain the time-dependent evolution of the mantle throughout the Cenozoic. To pursue these objectives the PIs will: (i) jointly invert global seismic and convection-related data with greatly improved coverage of 3-D mantle structure, incorporating geodynamic responses to lateral viscosity variations and topography on the discontinuities at the top and bottom of the transition zone; (ii) apply data assimilation methods to the new joint tomography models to reconstruct the Cenozoic evolution of 3-D mantle structure; (iii) calculate corresponding changes of Earth's moment of inertia, to be used in reconstructions of true polar wander and Milankovitch orbital cycles that are implicated in long-term climate variations. It is anticipated these new joint tomography models will provide improved constraints on the impact of transition-zone heterogeneity on convective mass and heat transport across this key region and on the spatial distribution of anomalous density structures within the LLSVP. This work will also contribute to resolving a longstanding difficulty concerning the origin of Earth's anomalous elliptical figure and the implications for the phase and amplitude of Milankovitch climate cycles in the geological past.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.