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

地幔占地球内部面积的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。坚固的地幔岩石会在很长的时间尺度上变形和流动，而地幔岩石的粘度（抗流动）会影响地幔的流动速度、地球深处的能量收支以及构造板块相互移动的速度。对地幔黏度结构的最佳约束之一来自于对地球长波长重力场变化的建模。该团队将结合地幔流动模型、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的地幔物质的行为；(3)地球地幔地震图像的不确定性如何影响我们在粘度剖面中的不确定性？除了这项研究之外，该项目还有助于培养一名研究生和一名博士后研究员。此外，该团队将与波特兰地区的一名高中教师合作，开发课程材料，教授《下一代科学标准》中与地幔流动有关的概念。全波形全地幔层析成像最近提供了改进的下地幔横波速度异常测量方法。这些图像为地幔上升流和下升流的行为提供了新的线索。从地核地幔边界上方到岩石圈底部的宽羽流是连续的，在地球上许多活跃的火山热点之下，在1000公里的深度，羽流经常出现横向偏转，在过渡带之下，在已知的地震不连续的深度，但在这个深度，板块停滞，羽流偏转，长波径向相关结构的变化出现在许多层析模型中。层析成像研究结合了全地球自由振荡和各种其他地震学观测（如体波传播时间、表面波色散、全波形），最近改进了对过渡带和中地幔长波速度变化的限制。最近的地震数据集也表明密度和剪切速度之间的简单标度关系存在偏差，这表明在最下层的地幔中存在大规模的化学非均质性。研究人员将确定最近层析模型的稳健方面，估计与密度变化相关的不确定性，并采用一种新的反演技术，以概率方式将这些结果结合起来，以获得对地幔粘度结构的改进约束。然后，他们将使用这些解作为数值地幔对流模拟的基础，以评估推断出的粘度结构在多大程度上与对流类型和地幔热传输速率的可用约束相兼容。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-ESS2-3相关的课程材料。

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.