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

这一提议寻求在我们对与地球内核和地幔不均质性的形成和演化有关的深层过程的基本理解方面取得重大进展。拟议工作的另一个成功成果也可能成为监测伴随地震活动的地下应力瞬变的重要一步，或许在地震活动之前也是如此。这项建议将促进本科生的地震学学习，提高公众对大地震和海啸的认识和准备。地球的地幔和地核在所有长度尺度上都具有热和化学不均匀的特征。地震学为探索这些非均质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质性。这将加强我们对与地球内部形成和演化相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内部的结构上，因为与地球内部的其他部分相比，对它们的研究相对较少。然而，它们对于了解地幔对流环流以及地核的形成和演化都是非常重要的。走时层析成像是成像地幔三维速度变化的最有效工具。许多断层图像显示，地幔大体上可以分为三个区域。一个相对均匀的中地幔被两个强烈的非均质层夹在中间，即最高和最低的地幔。利用深部地震的转换波，包括圆周率在内的几项研究发现，存在与俯冲带有关的地幔中反射体。一般来说，地幔中的地震反射体是由成分、矿物相、各向异性结构和或部分熔体堆积的突然变化造成的。它们的分布密切反映了地幔的成分、热和动力学状态，为层析成像提供了关键的补充信息。我计划应用和重新制定现有的成像技术，如基尔霍夫偏移和广义氡变换方法到SS反射，P到S，S到P转换数据，以改善地球中地幔的图像。除了研究地幔结构外，我还将研究最内侧内核的性质。与内核顶部~400公里相比，由于内核研究中常用的参考相位法不适用，内核的深部就不那么为人所知了。我发现了一个新的参考震相，PKIIKP，它可以在对极距离观察到，并可用于研究地球中心的地震结构。我建议将对PKIIKP的搜索扩大到所有可用阵列数据。特别是，我将开始分析中国地震局区域台网记录的南美深部地震的阵列数据。我研究的另一个重点是了解孕震深度的时变应力场。这可能是理解地震触发过程的最关键的一个参数。测量地震活动断裂带内的应力变化一直是地震学追求的目标。众所周知，实验室实验表明，地震速度随施加应力水平的不同而变化。原则上，只要能够精确和连续地测量诱导速度的变化，这种相关性就构成了应力计。在与华盛顿卡内基研究所(CIW)和劳伦斯伯克利国家实验室(LBNL)的科学家合作下，我进行了几个连续的活跃震源井间实验，以测量地球表面和孕震深度沿固定基线的现场地震速度变化。在这两种情况下，我们都证明了压力变化，例如大气压的变化是可以检测到的。特别是在SAFOD钻探现场(深部的圣安德烈亚斯断层观测站)，我们观察到了两次地震的同震速度变化和可能与破裂前扩张有关的震前速度变化。为了验证这些观察结果，我提议在SAFOD和圣安德烈亚斯断层的其他部分进行一系列受控震源实验。在教育方面，我提出了四项主要活动：(1)在校园内使用地震仪，提高学生对地震学和地球科学的认识；(2)在当地社区大学推广地震学，并在当地科学馆展示现代地震仪，以提高公众对大地震和海啸的认识和准备；(3)为主修和非主修本科生开发一门新的地球物理学入门课程《板块构造、地震和火山概论》，(4)为本科生提供研究活动。

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.