Collaborative Research: Detecting Seismic Anisotropy in the Upper Mantle and Upper Mantle Transition Zone

从地核逸出的热量通过对流被带到地表，这一过程导致地幔中的固体岩石在地质时间尺度上流动和变形。热的物质上升到表面，而冷的物质下沉到底部。通过对流的地幔翻转被认为是分隔地壳的刚性板块运动背后的驱动机制，而刚性板块运动反过来又会产生地震和火山。关于将地表的刚性板块与下方更易变形的对流地幔分开的边界的性质，仍然存在一些基本问题。特别是410 ~ 670 km深度之间的地幔过渡带的性质对决定地球对流的性质起着重要的作用。地幔中岩石的流动或变形会使矿物沿流动方向排列，这可以用地震波通过观察地震各向异性来探测。在这里，波传播的速度成为传播路径方向的函数。在这个项目中，pi将模拟地幔上部800公里地震各向异性的三维变化。通过结合多种类型的地震数据，研究人员将大大提高他们的模型的准确性，特别是在地幔过渡带。他们的数值正演模拟技术使研究小组能够定量地评估模型的不确定性。这一关键因素对于解释他们在矿物物理学、地球动力学或地幔地球化学方面的模型，并指导未来的研究是必要的。研究结果将通过改进地幔变形和板块构造模型、公共宣传和培养深部地球研究科学的研究生，使整个地球科学界受益。提出的工作将解决三个主要问题：(1)岩石圈-软流圈边界（LAB）的地震特征是什么？(2)深部上地幔和地幔过渡带是否存在可探测的地震各向异性？(3) MTZ的性质及其在对流中的作用是什么？为了回答这些问题，研究人员将利用联合正演方法模拟地幔上部800km区域径向和方位地震各向异性的全球三维（3-D）变化，包括基本模态和更高模态的面波频散测量、面波到达角测量、SS前兆传播时间和SKS分裂数据。该研究将产生(1)一个新的表面波到达角数据集，将大大增强上地幔小尺度各向异性的成像。这将使我们对海洋和大陆实验室的性质获得新的、更好的见解；(2)建立了地幔上800km区域方位角和径向各向异性的新三维模型。这将使我们能够测试上地幔深部径向各向异性的存在和标志，这可以对这些深度的主要剪切方向和地幔流动施加约束。它还将测试解决250公里以下径向和方位角各向异性横向变化的能力，以及这些结构与地幔动力学的关系；(3)整合一个新的全球SS前驱旅行时间数据集，提供MTZ边界的地形。这将减少MTZ边界地形和MTZ三维结构变化之间的权衡，并为地球深部壳层的热与成分性质提供新的限制。本研究的一个重要方面是使用数值正演模拟来统计识别新模型的良好约束特征。通过前向建模，团队将能够通过量化参数权衡和不确定性来评估模型分辨率，这是确定哪些模型参数是稳健的关键。它还将指导未来的研究，以确定需要哪些其他类型的数据来进一步提高分辨率。

The heat that escapes from Earth's core is brought towards the surface through convection, a process that causes solid rocks in the mantle to flow and deform over geological time scales. Hot materials rise to the surface, while cold materials sink to the bottom. The overturn of the mantle through convection is thought to be the driving mechanism behind the motion of the rigid plates that divide the Earth's crust, which in turn generates earthquakes and volcanoes. Fundamental questions remain regarding the nature of the boundary that separates the rigid plates at the surface from the underlying, more deformable convecting mantle. In particular, the nature of the mantle transition zone between 410 and 670 km depth plays an important role in determining the nature of convection in the Earth. Flow or deformation of the rocks in the mantle will align minerals with the flow direction, which can be detected with seismic waves through the observation of seismic anisotropy. Here, the velocity with which waves travel becomes a function of the orientation of the travel path. In this project, the PIs will model three-dimensional variations in seismic anisotropy in the upper 800 km of the mantle. By combining multiple types of seismic data, the investigators will greatly enhance the accuracy of their model, particularly in the mantle transition zone. Their numerical forward modeling technique allows the team to quantitatively assess model uncertainties. This key element is necessary to interpret their models in terms of mineral physics, geodynamics, or mantle geochemistry, and to guide future research. The results will benefit the geoscience community as a whole through improved models of mantle deformation and plate tectonics, public outreach presentation, and training of graduate students in deep earth research science.The proposed work will address three major questions: (1) What is the seismological character of the lithosphere-asthenosphere boundary (LAB)? (2) Is there detectable seismic anisotropy in the deep upper mantle and mantle transition zone (MTZ)? (3) What is the nature of the MTZ and it's role in convection? To answer these questions, the investigators will model global, three-dimensional (3-D) variations in radial and azimuthal seismic anisotropy in the upper 800km of the mantle using a joint forward modeling approach for fundamental and higher mode surface wave dispersion measurements, surface wave arrival angle measurements, SS precursor travel times, and SKS splitting data. The proposed research will produce (1) a new surface wave arrival angle dataset that will greatly enhance the imaging of small-scale anisotropy in the uppermost mantle. This will allow us to obtain new, improved insight on the nature of the oceanic and continental LAB; (2) a new 3-D model of azimuthal and radial anisotropy in the upper 800km of the mantle. It will enable us to test for the presence and sign of radial anisotropy in the deep upper mantle, which can impose constraints on the dominant shear direction and mantle flow at these depths. It will also test the ability to resolve lateral variations in radial and azimuthal anisotropy below 250km and how such structures are related to mantle dynamics; (3) the integration of a new global dataset of SS precursor travel times providing topography at the MTZ boundaries. This will reduce trade-offs between MTZ boundaries topography and 3-D structural variations in the MTZ, as well as provide new constraints on the thermal versus compositional nature of this depth shell of the Earth. An important facet of this research is the use of numerical forward modeling to statistically identify well-constrained features of the new models. With forward modeling the team will be able to assess model resolution by quantifying parameter trade-offs and uncertainties, which is key to determining which model parameters are robust. It will also guide future research in determining what other type of data is needed to further improve resolution.