Earth's Density and Inner Core Rotation after the great Sumatra-Andaman Earthquake

苏门答腊-安达曼大地震后地球的密度和内核旋转

• 批准号: 0635587
• 负责人: Gabriele Laske
• 金额: $ 25.36万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-04-01 至 2012-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0635587&HistoricalAwards=false
• 关键词: Earth; Density; Inner; Core; Rotation

The observation and analysis of Earth's free oscillations help seismologists to image Earth internal structure. In particular, free oscillation analysis provides the unique opportunity to study variations of density, a parameter that remains somewhat elusive to other seismological applications. On the other hand, density constraints play a key role in understanding processes proposed by neighboring disciplines in the Earth. For example, the transition of Earth's solid outer core to the inner core is associated with an abrupt increase in density. This density jump is an important constraint in discussions of the maintenance of Earth's geodynamo. Another area of interest is the "hot abyssal layer" in the lowermost few 100km of the mantle that has been proposed a few years ago. It carries the seismic signature of "hot" material with low seismic velocities, but it is nevertheless not buoyant. This layer is thought to be anomalously dense and its thickness must vary significantly laterally because seismology has been unable to detect an associated global discontinuity. Such a layer is thought to be the ultimate origin of lavas found on ocean islands whose chemical composition is very different from that found along mid-ocean ridges. Unfortunately, Earth's free oscillations have not been known precisely enough to prove or disprove with great confidence whether the "hot abyssal layer" is really dense. Another area for which free oscillation analyses contribute significantly is the structure and dynamics of Earth's core. It has been proposed in the 1990s that the inner core spins independently of the rest of the planet and a super-rotation of 6 degrees per year was initially published as being consistent with body waves that graze Earth's inner core beneath South America. Such a super-rotation has profound implications for Earth's geodynamo and the gravitational coupling of the mantle and core. Subsequent analyses of Earth's free oscillations have disproved such high rotation rates but the fidelity of free oscillation observations have so far not allowed seismologists to reduce uncertainties below 0.15 degrees/year.The great December 6, 2004 Sumatra-Andaman earthquake excited Earth's free oscillations to a level not seen since the 1964 Good Friday Earthquake in Alaska. In fact, it nearly rivals the great May 22, 1960 Chile earthquake for which free oscillations were observed for the first time. This time, numerous high-quality digital seismic stations recorded the earthquake, with an unprecedented fidelity. PI Laske and her team measure free oscillation parameters for Earth's average and laterally varying internal structure. The team developed an analysis technique in which details of the earthquake process do not have to be known. This allows the analysis of events with relatively complicated source mechanisms, such as the Sumatra-Andaman earthquake whose shaking lasted for nearly 10 min. Laske essentially measures globally varying mode frequencies and attenuation rates. Earth's deviation from a non-rotating, uniformly layered planet removes the degeneracy of normal modes much like electron energy levels are split when an atom encounters a magnetic field. The measurement of this splitting allows Laske to image lateral heterogeneity that is symmetric. Earth structure that is not symmetric causes coupling between modes, hence the analysis of coupling coefficients allows her to fully image Earth's 3D heterogeneity. Prior to the Sumatra-Andaman earthquake, Earth's attenuating structure has been particularly difficult to assess because the seismic signal it causes is relatively small. It usually takes very deep earthquakes, such as the great 1994 Bolivia earthquake to excited modes that are sensitive to inner core structure. Due to its very large rupture area, the Sumatra-Andaman earthquake also excited these modes to a level that was not observed since the Bolivia earthquake. Though more recent, smaller earthquakes have been used to constrain inner core rotation rates, the Sumatra-Andaman earthquake adds an important, high-precision data point a decade after the Bolivia earthquake. Laske can now test inner core rotation rates over a timespan covering almost 30 years. Among the broader impacts of this project are the analysis of Earth's free oscillations provides key constraints on Earth structure to neighboring disciplines of the Earth sciences. Especially constraints on density are extremely difficult to obtain using other seismic methods, if not impossible. Furthermore, the project would contribute to the training of a graduate and an undergraduate student.

对地球自由振荡的观察和分析有助于地震学家对地球内部结构进行图像。特别是，自由振荡分析为研究密度变化提供了独特的机会，该参数在其他地震学应用中仍然难以捉摸。另一方面，密度约束在理解地球中相邻学科提出的过程中起着关键作用。例如，地球固体外核向内芯的过渡与密度的突然增加有关。在讨论地球地球地球的维持方面，这种密度跳跃是一个重要的限制。另一个感兴趣的领域是几年前提出的最低100公里地幔中最低的100公里处的“热点层”。它具有低地震速度的“热”材料的地震签名，但它并不浮动。该层被认为是异常的致密，由于地震学无法检测到相关的全局不连续性，因此其厚度必须横向变化。这样一层被认为是海洋岛上发现的熔岩的最终起源，其化学成分与沿着海中山脊沿线的熔岩截然不同。不幸的是，地球的自由振荡尚未得到足够的众所周知，无法充分证明或反驳“热的深渊层”是否真的很稠密。自由振荡分析显着贡献的另一个领域是地球核心的结构和动力学。在1990年代，人们提出，内部核心独立于行星的其余部分，每年的超级旋转最初发表，这与掠过南美下内部核心的体波一致。这样的超级旋转对地球的地球人的地球人物和地幔和核心的重力耦合具有深远的影响。随后对地球自由振荡的分析已经驳回了如此高的旋转率，但是自由振荡观察的忠诚度到目前为止，不允许地震学家将不确定性降低到/年以下。实际上，它几乎与1960年5月22日的智利地震竞争，首次观察到自由振荡。这次，许多高质量的数字地震电台记录了地震，并以前所未有的忠诚度。 Pi Laske和她的团队测量了地球平均值和横向变化的内部结构的自由振荡参数。该团队开发了一种分析技术，其中不必知道地震过程的细节。这允许分析具有相对复杂的源机制的事件，例如苏门答腊 - 安达曼地震持续了将近10分钟。 Laske基本上测量了全球变化的模式频率和衰减率。当原子遇到磁场时，地球与非旋转，均匀分层的行星的偏离消除了像电子能级一样的正常模式的退化。这种分裂的测量使Laske可以对称的侧面异质性成像。不是对称的地球结构会导致模式之间的耦合，因此对耦合系数的分析使她能够完全对地球的3D异质性进行图像。在发生苏门答腊 - 安达曼地震之前，地球的衰减结构尤其难以评估，因为它引起的地震信号相对较小。它通常需要非常深的地震，例如1994年的玻利维亚大地震对激发模式，对内部核心结构敏感。由于其非常大的破裂区域，苏门答腊 - 安达曼地震也将这些模式激发到自玻利维亚地震以来未观察到的水平。尽管最近的地震已被用来限制内部核心旋转率，但苏门答腊 - 安达曼地震在玻利维亚地震十年后增加了一个重要的高精度数据点。 Laske现在可以在覆盖近30年的时间板上测试内部核心旋转率。 该项目的更广泛的影响是对地球自由振荡的分析为地球结构对地球科学的邻近学科提供了关键的限制。尤其是使用其他地震方法（即使不是不可能），对密度的限制极为难以获得。 此外，该项目将有助于培训研究生和本科生。