Collaborative Research: Bayesian Estimation of Mantle Viscosity Structure and Geodynamic Implications

合作研究：地幔粘度结构的贝叶斯估计及其地球动力学意义

• 批准号: 1622464
• 负责人: Maxwell Rudolph
• 金额: $ 19.3万
• 依托单位: Portland State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-09-01 至 2018-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1622464&HistoricalAwards=false
• 关键词: Collaborative; Research; Bayesian; Estimation; Mantle

Earth's mantle comprises more than 80% of our planet's interior, and convection in the mantle is linked to plate tectonics, the magnetic field, volcanic activity, and the gases in our atmosphere. Solid mantle rocks deform and flow over long time scales, and the viscosity (resistance to flow) of mantle rocks affects the rate of flow in the mantle, the energy budget of Earth's deep interior, and the speed with which tectonic plates move past one another. One of the best constraints on the mantle viscosity structure comes from modeling variations in Earth's long-wavelength gravity field. The team will combine mantle flow models, seismic images of the Earth's interior, and results from mineral physics to better constrain the depth variation of viscosity and its influence on mantle convection. The project will address the following scientific questions: (1) How does viscosity vary with depth, (2) How does viscosity structure affect behavior of buoyant, upwelling mantle material, and (3) How does uncertainty in seismic images of Earth's mantle affect our uncertainty in the viscosity profile? In addition to this research, the project contributes to the training and professional development of a graduate student and a postdoctoral researcher. Additionally, the team will work with a Portland-area high school teacher to develop curricular materials to teach concepts related to flow in the mantle from the Next Generation Science Standards.Full waveform whole-mantle tomography has recently provided improved measurements of lower mantle shear wave velocity anomalies. These images shed new light on the behavior of mantle upwellings and downwellings. Wide plumes, continuous from just above the core mantle boundary to the base of the lithosphere, are resolved beneath many of Earth's active volcanic hot spots, and plumes frequently appear to be deflected laterally below the transition zone, at a depth of 1000 km, a depth not coincident with known seismic discontinuities, but at which slabs stagnate, plumes are deflected, and changes in long-wavelength radial correlation structure appear in many tomographic models. Tomographic studies combining whole-Earth free oscillations with various other seismological observations (e.g. body wave travel times, surface wave dispersion, full waveforms) have recently improved constraints on the long-wavelength variations in wavespeed in the transition zone and mid mantle. Recent seismological data sets also suggest a deviation from simple scaling relationships between density and shear velocity, indicative of large-scale chemical heterogeneity in the lowermost mantle. The investigators will identify robust aspects of recent tomographic models, estimate uncertainties associated with their translation to density variations and employ a new inversion technique that incorporates these results in a probabilistic way to obtain improved constraints on the mantle viscosity structure. They will then use the solutions as the basis for numerical mantle convection simulations to evaluate the extent to which the inferred viscosity structures are compatible with available constraints on the style of convection and rate of heat transport in the mantle. In addition to training and mentoring of a graduate student and a postdoctoral researcher, the investigating team will work with a high school educator to develop curricular materials relevant to the Next Generation Science Standard HS-ESS2-3.

地幔占地球内部的80%以上，地幔中的对流与板块构造、磁场、火山活动和大气中的气体有关。固体地幔岩石在很长一段时间内变形和流动，地幔岩石的粘性（流动阻力）影响地幔的流动速率、地球内部深处的能量收支以及构造板块相互移动的速度。对地幔粘性结构的最佳约束之一来自地球长波重力场的模拟变化。该团队将结合联合收割机地幔流模型、地球内部的地震图像和矿物物理学的结果，以更好地约束粘度的深度变化及其对地幔对流的影响。该项目将解决以下科学问题：（1）粘度如何随深度变化，（2）粘度结构如何影响浮力，上涌地幔物质的行为，以及（3）地球地幔地震图像的不确定性如何影响我们的粘度分布的不确定性？除了这项研究之外，该项目还有助于一名研究生和一名博士后研究员的培训和专业发展。此外，该小组将与波特兰地区的高中教师开发课程材料，从下一代科学Standards.Full波形全地幔层析成像提供了改进的测量下地幔剪切波速度异常的地幔流动相关的概念。这些图像为地幔的升降行为提供了新的线索。宽的地幔柱，从核幔边界上方一直延续到岩石圈底部，在地球上许多活火山热点的下方被分辨出来，并且在1000公里的深度，地幔柱经常出现在过渡带下方横向偏转，这个深度与已知的地震不连续性不一致，但是在这个深度，板片停滞，地幔柱偏转，并且在许多层析成像模型中出现长波长径向相关结构的变化。层析成像研究结合全地球自由振荡与其他各种地震观测（如体波旅行时间，表面波色散，全波形）最近改善了在过渡带和中地幔的波速的长波长变化的约束。最近的地震学数据集也表明，密度和剪切速度之间的简单比例关系的偏差，表明大规模的化学不均匀性在最低地幔。研究人员将确定最近的层析成像模型的鲁棒性方面，估计与其转换为密度变化相关的不确定性，并采用一种新的反演技术，将这些结果以概率的方式结合起来，以获得对地幔粘度结构的改进约束。然后，他们将使用解决方案作为数值地幔对流模拟的基础，以评估推断的粘度结构与对流类型和地幔热传输速率的现有约束条件的兼容程度。除了培训和指导一名研究生和一名博士后研究人员外，调查小组还将与一名高中教育工作者合作，开发与下一代科学标准HS-ESS 2 -3相关的课程材料。