Probing the Earth's Outer Core for a Stratified Layer

探测地球外核的分层层

• 批准号: 0944283
• 负责人: Edward Garnero
• 金额: $ 18.43万
• 依托单位: Arizona State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-12-01 至 2013-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0944283&HistoricalAwards=false
• 关键词: Probing; Earth; Outer; Core; Stratified

Core-mantle boundary science spans the deep Earth disciplines of seismology, dynamics, geochemistry, geomagnetism, and mineral physics. An outstanding question in deep Earth science is whether or not the iron core and silicate mantle chemically react with each other. These chemical reactions at the core-mantle boundary (CMB) would result in fine-scale outermost core and lowermost mantle layering that could be detected using seismic waves. Recent geochemical, dynamical, and geomagnetism considerations have supported core-side layering, but such a layer has not been unambiguously imaged. Better resolving outermost core structure has the potential of (a) spawning research activities in a number of fields that depend on accurate knowledge of the core, and (b) helping geoscientists better resolve issues relating to the current chemical state of the planet.Seismic studies of the outermost core rely on SmKS waves: mantle S-waves that convert to P-waves upon entering the core, reflect m-1 times from the underside of the CMB, then convert back to an S-wave for the final mantle leg. SmKS phases are the only seismic waves that have turning depths in outermost core. Past work indicates reduced P velocities at the top of the outer core, however, SmKS phases can be significantly contaminated by lower mantle heterogeneities. Also, a stably stratified outer core layer must be less dense, and hence predicts that the velocity of such a layer will be elevated, not decreased. Thus, the investigators seek to better image the outermost core, while accounting for anomalous mantle structure. Our goal is to seismically study the fine scale structure of Earth?s outermost core in unprecedented detail, to detect and characterize (if present) any outer core layering (or establish its absence). We first seek to better characterize the heterogeneous lowermost mantle that affects SmKS. Building on an existing whole mantle tomographic model, we seek to refine our understanding of the lowermost mantle by including a vastly increased data set of SmKS arrivals from (a) new SmKS data from recent regional and global seismic deployments obtained using a clustering algorithm optimized for processing large data volumes, and (b) existing data sets of two other seismologists. We will simultaneously invert for 3D mantle structure and 1D outer core velocity to assess the need for an updated 1D reference model while accounting for deep mantle heterogeneity. We will also forward model aspects of the broadband SmKS wavefield that depend strongly on the P-wave velocity and density in the outermost 10?s of km of the core. These experiments will include modeling S4KS-S3KS and S3KS-S2KS times, and the ?birth? of the S2KS and S3KS waves. These waveform-modeling experiments will include 1- and 2-D synthetic seismogram predictions, array analyses, and deconvolution algorithms to sharpen arrivals ? each of which will permit us to map any fine scale structure just beneath the CMB. This project will reveal properties of core-mantle reactions which are fundamental to understanding the evolution of the Earth. A unique feature of this proposal is combining the tools of tomography with those of detailed waveform modeling which offers cross-training for most of the proposal participants.

地核-地幔边界科学横跨地震学、动力学、地球化学、地磁和矿物物理学等深部地球学科。地球深部科学中的一个突出问题是铁核和硅酸盐地幔是否会相互发生化学反应。这些在地核-地幔边界（CMB）的化学反应将导致精细尺度的最外层地核和最下层地核分层，可以用地震波探测到。最近的地球化学、动力学和地磁方面的考虑支持了地核侧分层，但这样的层还没有得到明确的成像。更好地解析最外层地核结构具有以下潜力：(a)在许多依赖于地核准确知识的领域催生研究活动；(b)帮助地球科学家更好地解决与地球当前化学状态有关的问题。最外层地核的地震研究依赖于SmKS波：地幔s波在进入地核后转换为纵波，从CMB底部反射m-1次，然后转换回s波，用于最后的地幔分支。SmKS相位是唯一在最外层核有旋转深度的地震波。过去的工作表明，外核顶部的P速度降低，然而，SmKS相可能受到下地幔非均质性的严重污染。此外，一个稳定分层的外核层必须密度较低，因此预测这样一层的速度将会提高，而不是降低。因此，研究人员在考虑异常地幔结构的同时，寻求更好地描绘最外层核的图像。我们的目标是用地震研究地球的精细结构。以探测和表征（如果存在）任何外核层（或确定其不存在）。我们首先寻求更好地表征影响SmKS的异质性最下层地幔。在现有的完整地幔层析模型的基础上，我们试图通过包括大量增加的SmKS数据集来完善我们对最下层地幔的理解，这些数据集来自：(a)使用优化处理大数据量的聚类算法获得的最近区域和全球地震部署的新SmKS数据，以及(b)其他两位地震学家的现有数据集。我们将同时反演三维地幔结构和一维外核速度，以评估在考虑深部地幔非均质性的同时更新一维参考模型的必要性。我们还将提出宽带SmKS波场的模型方面，这些方面强烈依赖于最外层10？S （km）的核心。这些实验将包括S4KS-S3KS和S3KS-S2KS时间的建模，以及出生？S2KS和S3KS波这些波形建模实验将包括1- d和2-D合成地震记录预测、阵列分析和反褶积算法，以锐化到达？每一个都能让我们绘制出宇宙微波背景下的精细结构。这个项目将揭示核心-地幔反应的性质，这是理解地球演化的基础。该提案的一个独特之处在于将断层扫描工具与详细波形建模工具相结合，为大多数提案参与者提供了交叉培训。