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

地震层析成像利用地震产生的各种体波的旅行时间提供地球下地幔内快速和缓慢区域的图像，并由全球地震仪记录。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。其目的是发现快速异常是否与俯冲物质一致相关，并解决温度和成分在产生这些异常时可能产生的影响。最新的地震层析成像模型在中下地幔具有良好的分辨率，并揭示了快速异常的特征随深度的变化。在中地幔，快速异常类似于与古俯冲带有关的线状特征。在下地幔，快速异常呈现出环太平洋模式，基本上包围了中太平洋和非洲从核幔边界升起的大型低速省份。在全地幔对流的经典观点中，地震快速俯冲的大洋岩石圈下降到核幔边界，在那里它驱动被动的、地震缓慢的物质的流动。虽然对这些慢速特征的详细研究已经引起了很大的关注，但快速特征的地震特征却鲜有文献记载。我们计划使用最新的S、P、Sdiff、Pdiff、ScS、Sks和Pkp相的长周期走时目录来更好地定义这些异常在下地幔中的剪切、压缩和整体声速。研究人员正在为这些地震波的相对变化开发一种公式，这种公式可以从矿物物理框架中解释，并适用于研究3D结构。目前，玄武岩在下地幔的地震行为尚不清楚，但研究人员的初步计算表明，玄武岩可能是产生快速地震速度的候选者。研究人员正在使用迭代方法来辨别快速异常是否可以简单地解释为热异常，或者它们是否需要海洋玄武岩等快速组分的贡献，如果是，数量是多少。调查人员还在利用对FAST特征进行采样的区域阵列的短周期地震数据来探测异常。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约束。在核-地幔边界附近，调查人员预计，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。该项目正在资助C.Reif博士的博士后工作，她为在小学和普通社区推广地球科学做出了重大贡献，并正在继续工作，以满足圣克鲁斯地区地球科学K-12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.