CAREER: Seismic Imaging of the Earth's Mid-Mantle, the Deep Inner Core and Stress Transients

职业：地球中地幔、深层内核和应力瞬变的地震成像

• 批准号: 0748455
• 负责人: Fenglin Niu
• 金额: $ 54.87万
• 依托单位: William Marsh Rice University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-06-01 至 2014-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0748455&HistoricalAwards=false
• 关键词: CAREER; Seismic; Imaging; Earth; Mid

This proposal seeks to make significant progress in our basic understanding of deep processes that are related to the formation and evolution of the Earth's inner core and mantle heterogeneities. Another successful outcome of the proposed work could also constitute a major step towards monitoring subsurface stress transients that accompany and perhaps precede seismic activity. This proposal will promote seismology studies for undergraduates and raise public awareness and readiness for large earthquakes and tsunamis.The Earth's mantle and core are characterized by thermal and chemical heterogeneities at all length scales. Seismology provides a powerful means of exploring these heterogeneities. The PI intends to take advantage of recent developments in passive seismic observations and imaging techniques to map out seismic heterogeneities. This will enhance our understanding of deep Earth processes that are related to formation and evolution of the Earth's interior. The research will focus on the structure of the middle mantle and the innermost part of the inner core, as they are relatively less well studied than the rest of the Earth's interior. Yet, they are very important for understanding mantle convective circulation as well as the formation and evolution of the core. Travel time tomography has been the most efficient tool to image 3D velocity variations in the mantle. Many tomographic images reveal a mantle that can be in general divided into three domains. A relatively homogeneous middle mantle is sandwiched by two strong heterogeneous layers, the uppermost and lowermost mantle. Using converted waves from deep earthquakes several studies, including the PI's, have found the existence of mid-mantle reflectors associated with subduction zones. In general seismic reflectors in the mantle result from abrupt changes in composition, mineral phase, anisotropic structure, and or partial melt accumulation. Their distribution closely reflects the compositional, thermal and dynamic state of the mantle, providing critical complementary information to tomography. I plan to apply and reformulate existing imaging techniques, such as Kirchhoff migration and generalized radon transform methods to SS reflections, P to S, and S to P conversion data to improve images of Earth's middle mantle. In addition to studying the mantle structure, I will also research the nature of the innermost inner core. Compared to the top ~400 km of the inner core, the deep part of the inner core is less well known because of the inapplicability of the reference phase method commonly used in inner core studies. I have found a new reference phase, PKIIKP, which is observable at antipodal distances and can be used to study seismic structure in the center of the Earth. I propose to extend the search for PKIIKP to all the available array data. In particular I will start to analyze array data of deep earthquakes occurring in South America recorded by regional networks of the China Earthquake Administration. Another focus of my research involves understanding the time-varying stress field at seismogenic depths. This is perhaps the single most crucial parameter for understanding the earthquake triggering process. Measuring stress changes within seismically active fault zones has been a long-sought goal of seismology. It is well known from laboratory experiments that seismic velocities vary with the level of the applied stress. In principle, this dependence constitutes a stress meter, provided that the induced velocity changes can be measured precisely and continuously. In collaborating with scientists from Carnegie Institution of Washington (CIW) and Lawrence Berkeley National Laboratory (LBNL), I have conducted several continuous active source cross-well experiments to measure in situ seismic velocity changes along fixed baselines at Earth's surface and seismogenic depths. In either case we have demonstrated that stress changes such as variations in barometric pressure are detectable. Especially at the SAFOD drill site (San Andreas Fault Observatory at Depth) we observed co-seismic velocity changes from two earthquakes and preseismic velocity changes that might be related to pre-rupture dilatancy. In order to verify these observations, I propose to conduct a series of controlled source experiments at SAFOD and other segments of the San Andreas Fault. I also plan to develop time-lapse seismic imaging (4D) techniques for the detection of seismic and magmatic crustal stress changes.In terms of education, I propose four major activities: (1) utilize an on-campus seismograph to promote students' appreciation to seismology and Earth science; (2) promote seismology in local community colleges and displaying modern seismograph at local science museum to raise public awareness and readiness for large earthquakes and tsunamis; (3) develop a new introductory geophysics course "An Introduction of Plate tectonics, Earthquakes and Volcanoes" for major and non-major undergraduates, (4) provide research activities for undergraduate students.

这一建议旨在使我们对与地球内核和地幔非均质性的形成和演化有关的深层过程的基本认识取得重大进展。拟议工作的另一个成功成果也可能是朝着监测伴随地震活动或在地震活动之前的地下应力瞬变迈出的重要一步。这一建议将促进大学生的地震学研究，提高公众对大地震和海啸的认识和准备。 地震学为探索这些非均匀性提供了强有力的手段。PI打算利用被动地震观测和成像技术的最新发展来绘制地震非均匀性。 这将增强我们对与地球内部形成和演化有关的地球深部过程的理解。这项研究将集中在中地幔和内核最里面的部分，因为它们比地球内部的其他部分研究得相对较少。它们对于理解地幔对流循环和地核的形成与演化具有重要意义。 走时层析成像是研究地幔三维速度变化最有效的方法。许多层析成像图像显示，地幔一般可分为三个领域。一个相对均匀的中地幔夹在两个强烈的不均匀层，最上地幔和最下地幔之间。利用深源地震的转换波，包括PI在内的几项研究发现了与俯冲带有关的中地幔反射体的存在。一般地说，地幔中的地震反射层是由成分、矿物相态、各向异性结构和（或）部分熔体堆积的突变引起的。它们的分布密切地反映了地幔的成分、热状态和动力学状态，为层析成像提供了重要的补充信息。我计划应用和重新制定现有的成像技术，如基尔霍夫迁移和广义氡变换方法SS反射，P到S，S到P转换数据，以改善地球中地幔的图像。 除了研究地幔结构，我还将研究最里面的内核的性质。 与内核顶部~400 km相比，内核的深部不太为人所知，因为内核研究中常用的参考相位法不适用。我已经找到了一个新的参考相位，PKIIKP，这是可观察到的对映距离，并可用于研究地震结构在地球的中心。我建议将PKIIKP的搜索扩展到所有可用的阵列数据。特别是，我将开始分析中国地震局区域台网记录的南美洲发生的深源地震的台阵数据。我研究的另一个重点是了解孕震深度随时间变化的应力场。这也许是理解地震触发过程的唯一最关键的参数。测量地震活动断层带内的应力变化一直是地震学长期追求的目标。从实验室实验中可以知道，地震波速度随所施加的应力水平而变化。原则上，这种依赖性构成了应力计，只要可以精确和连续地测量诱导的速度变化。我与华盛顿卡内基研究所（CIW）和劳伦斯伯克利国家实验室（LBNL）的科学家合作，进行了几次连续的有源井间实验，以测量地球表面和孕震深度沿固定基线的原位地震速度变化。在这两种情况下，我们已经证明，压力变化，如大气压的变化是可检测的。特别是在SAFOD钻探现场（圣安德烈亚斯断层观测站在深度），我们观察到同震速度变化，从两个地震和preseepage速度变化，可能与破裂前的不稳定性。为了验证这些观测结果，我建议在SAFOD和圣安德烈亚斯断层的其他部分进行一系列受控源实验。在教育方面，我提出了四项主要活动：（1）利用校内地震仪，提高学生对地震学和地球科学的认识;（二）在本地社区学院推广地震学，并在本地科学馆展出现代地震仪，以提高市民对地震的认识，地震和海啸;（3）为本科生和非本科生开设新的物理学导论课程"板块构造、地震和火山概论";（4）为本科生提供研究活动。