CSEDI: A Joint Seismological and Mineral Physics Approach to Unravel the Origin of Lower Mantle Heterogeneity

地震层析成像技术利用地震产生的各种体波的传播时间，并在全球地震仪上记录下来，提供了地球下地幔内快速和缓慢区域的图像。研究人员正在检查下地幔层析成像最快区域的地震和矿物学行为。目标是发现快速异常是否与俯冲物质一致相关，并解决产生这些异常的温度和成分的可能影响。最近的地震层析模型在中、下地幔具有良好的分辨率，并揭示了快速异常随深度的变化特征。在中地幔，快速异常类似于与古俯冲带相关的线性特征。在下地幔中，快速异常呈现出环太平洋模式，基本上包裹着太平洋中部和非洲下方从核心-地幔边界上升起来的大型慢速省份。在全地幔对流的标准观点中，地震快速俯冲的海洋岩石圈下降到核心-地幔边界，在那里它驱动被动的、地震缓慢的物质流动。虽然对这些慢震特征的详细研究得到了很多关注，但对快震特征的研究却很少。我们计划使用最新的下地幔S、P、Sdiff、Pdiff、ScS、SKS和PKP相的长周期传播时间目录来更好地定义下地幔这些异常的剪切、压缩和整体声速。研究人员正在开发一种公式来解释这些地震波的相对变化，这种公式可以从矿物物理框架中解释，并且适用于研究三维结构。目前，下地幔中玄武岩的地震行为还不是很清楚，但研究人员的初步计算表明，它可能是产生快速地震速度的候选者。研究人员正在使用一种迭代方法来辨别快速异常是否可以简单地解释为热异常，或者它们是否需要快速成分（如海洋玄武岩）的贡献，如果是，其数量是多少。研究人员还利用区域阵列的短周期地震数据来探测这些异常，这些数据对快速特征进行了采样。如果存在反射器或速度梯度，它们将对材料的热化学状态提供额外的限制。在核心-地幔边界附近，研究人员预计一些快速异常可能不是由温度和/或化学异常产生的，可能需要将钙钛矿后相变的地震特征纳入他们的热化学模型中。该项目资助C. Reif博士的博士后工作，她为小学和普通社区的地球科学推广做出了重大贡献，并继续她的工作，以满足圣克鲁斯地区K-12地球科学教育工作者的需求。

Seismic tomography provides images of the fast and slow regions within the Earth's lower mantle using the travel times of a variety of body waves produced by earthquakes and recorded at seismometers around the globe. The investigators are examining the seismic and mineralogical behavior of the tomographically fastest regions of the lower mantle. The goal is to discover whether or not fast anomalies are uniformly associated with subducted material, and to resolve the likely effects of temperature and composition in generating these anomalies. Recent seismic tomographic models have excellent resolution in the mid and lower mantle and reveal a change in character with depth of fast anomalies. In the mid mantle, fast anomalies resemble linear features that can be associated with paleo-subduction zones. In the lower mantle, the fast anomalies take on a circum-Pacific pattern that essentially wraps around the large slow velocity provinces under the central Pacific and Africa that rise from the core-mantle boundary. In the canonical view of whole-mantle convection, seismically fast subducting oceanic lithosphere descends to the core-mantle boundary, where it drives the flow of the passive, seismically slow material. While much attention has been paid to detailed investigations of these slow features, the seismic characteristics of the fast features are poorly documented. We plan to use the most current catalog of long-period travel times of lower mantle S, P, Sdiff, Pdiff, ScS, SKS, and PKP phases to better-define the shear, compressional, and bulk sound speed of these anomalies in the lower mantle. The investigators are developing a formulation for the relative variations of these seismic waves that is interpretable from a mineral physics framework, and which is suitable for investigating 3D structure. Presently, the seismic behavior of basalt is not well understood in the lower mantle, but the investigators' preliminary calculations indicate that it is a likely candidate for producing fast seismic velocities. The investigators are using an iterative approach to discern whether the fast anomalies can be explained simply as thermal anomalies, or it they require a contribution from a fast component such as oceanic basalt, and if so, in what quantity. The investigators are also probing the anomalies with short-period seismic data from regional arrays that sample the fast features. If reflectors or velocity gradients are present, they will provide additional constraints on the thermo-chemical state of the material. Near the core-mantle boundary, the investigators anticipate that some of the fast anomalies may not be generated by temperature and/or chemical anomalies, and may require incorporation of the seismic characteristics of the post-perovskite phase transition into their thermo-chemical models. This project is funding the postdoctoral work of Dr. C. Reif who has made major contributions to Earth science outreach in elementary schools and the general community, and who is continuing her work to meet the needs of Earth science K-12 educators in the Santa Cruz area.