Investigating what causes the morphology of Large low-shear Velocity Provinces (LLSVPs)

当地震发生时，能量以地震波的形式在地球内部传播。人们发现，地震波在非洲和太平洋下面的最下层地幔的两个区域传播得比周围地区慢得多。这些区域被称为“大低剪切速度省”，或llsvp，覆盖了地核约30%的面积。在不同的位置，LLSVP的高度从100公里到~ 2000公里不等，其中太平洋LLSVP比非洲LLSVP高。LLSVP边缘的陡峭度也随位置的变化而显著变化。llsvp的形状受深部地幔密度和黏度结构的控制，并提供了深部地幔密度和黏度结构的重要信息，这些信息与llsvp的起源、深部地幔动力学和地球演化有关。因此，了解深部地幔的密度和粘度结构如何控制llsvp的形状是至关重要的。人类无法进入地球深处详细研究llsvp，但llsvp与周围地幔之间的相互作用应该遵循质量、动量和能量守恒的基本物理定律。在这项研究中，团队将遵循这些物理定律，并进行数值模拟，以量化llsvp的形状与深部地幔的密度和粘度结构之间的关系。研究结果不仅对llsvp复杂形状的成因提供了新的认识，而且有助于约束深部地幔的密度和粘度结构。该项目还将有助于了解地球深部地幔的基本物理学。该项目为一名早期职业科学家和一名二年级研究生提供支持。它还提供将用于本科和研究生课程的教材，并通过外展活动和通过互联网向公众推广教育。最下层地幔的结构和动力学在地球的演化中起着至关重要的作用。地震研究表明，在地幔的最底部有两个大的低剪切速度省（llsvp）。地震观测表明，LLSVP具有复杂的地形、内部结构和边缘陡度等特征。LLSVPs的形态主要是LLSVPs与周围地幔物理相互作用的结果，而这种相互作用又在很大程度上受最下层地幔的粘度和密度的控制。然而，最下层地幔的粘度和密度仍然没有得到很好的约束。有人认为，llsvp是由组成不同的热化学桩引起的。在这个项目中，该团队将使用数值模拟来量化热化学桩的形态与深部地幔粘度和密度结构之间的关系，并探索导致深部地幔结构具有与地震观测到的llsvp相似特征的条件。这些结果将有助于了解llsvp的形态，并为深部地幔的密度和粘度提供约束条件。它们还将导致对控制llsvp与周围地幔之间相互作用的基本物理过程和机制的理解。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。

When earthquakes happen, the energy propagates through the Earth's interior in the form of seismic waves. It is found that seismic waves propagate remarkably slower in two regions of the lowermost mantle beneath Africa and the Pacific Ocean than the surroundings. These regions are called the "large lower shear velocity provinces", or LLSVPs, which cover ~30% area of the Earth's core. The height of the LLSVPs ranges from 100s of km to ~2,000 km at different locations, with the Pacific LLSVP taller than the African LLSVP. The steepness of the LLSVP edges also changes significantly from place to place. The shape of the LLSVPs is controlled by, and provides important information on the density and viscosity structures of the deep mantle, which are linked to the origin of the LLSVPs, the dynamics of the deep mantle and the evolution of the Earth. It is thus critical to understand how density and viscosity structures of the deep mantle control the shape of the LLSVPs. Human beings are not able to go into the deep Earth to examine the LLSVPs in detail, but the interaction between the LLSVPs and the surrounding mantle should be governed by the fundamental physics laws of conservation of mass, momentum and energy. In this research, the team will follow these physical laws and perform numerical simulations to quantify the relationship between the shape of the LLSVPs and the density and viscosity structures of the deep mantle. The results should not only provide new understanding on the causes of the complex shapes of LLSVPs, but also help constrain the density and viscosity structures of the deep mantle. The project will also lead to understanding about the fundamental physics in the Earth's deep mantle. This project provides support to an early-career scientist and a 2nd year graduate student. It also provides teaching materials that will be used in undergraduate and graduate classes, and promotes educational outreach to the public at outreach events and via the internet.The structure and dynamics of the lowermost mantle play a critical role in the Earth's evolution. Seismic studies have revealed two large low-shear velocity provinces (LLSVPs) in the lowermost mantle. Complex features of the LLSVPs have been suggested by seismic observations, with variable topography, internal structure and steepness of the LLSVP edges. The morphology of the LLSVPs is mostly a result of the physical interaction between the LLSVPs and the surrounding mantle, which is in turn largely controlled by the viscosity and density of the lowermost mantle. However, the viscosity and density of the lowermost mantle remain not well constrained. It has been suggested that the LLSVPs are caused by compositionally distinct thermochemical piles. In this project, the team will use numerical modeling to quantify the relationship between the morphology of thermochemical piles and the deep mantle viscosity and density structures, and to explore conditions that lead to structures in the deep mantle with similar features to the seismically observed LLSVPs. The results will help understand the morphology of the LLSVPs, and provide constraints on the density and viscosity of the deep mantle. They will also lead to understanding about the fundamental physical processes and mechanisms controlling the interaction between the LLSVPs and the surrounding mantle.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.