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.

对地球自由振动的观测和分析有助于地震学家对地球内部结构的认识。特别是，自由振荡分析提供了独特的机会，研究密度的变化，一个参数，仍然有些难以捉摸的其他地震应用。另一方面，密度约束在理解地球邻近学科提出的过程中发挥着关键作用。例如，地球固体外核向内核的转变与密度的突然增加有关。这种密度跳跃是讨论地球发电机维持的一个重要限制。另一个令人感兴趣的领域是几年前提出的地幔最低几百公里处的“热深海层”。它携带着低地震速度的“热”物质的地震特征，但它却没有浮力。这一层被认为是异常致密的，其厚度在横向上变化很大，因为地震学无法检测到相关的全球不连续性。这一层被认为是在海洋岛屿上发现的熔岩的最终来源，这些岛屿的化学成分与沿着大洋中脊发现的熔岩非常不同。不幸的是，地球的自由振荡还没有被精确地了解到足以证明或反驳与非常有信心的“热深海层”是否真的密集。自由振荡分析的另一个重要贡献是地球核心的结构和动力学。在1990年代，有人提出内核独立于地球的其他部分旋转，并且每年6度的超旋转最初被发表为与南美洲下方掠过地球内核的体波一致。这种超自转对地球的地球发电机和地幔与地核的引力耦合有着深远的影响。随后对地球自由振荡的分析已经否定了如此高的自转速率，但自由振荡观测的精确度至今还不允许地震学家将不确定性降低到0.15度/年以下。2004年12月6日苏门答腊-安达曼大地震激发了自1964年阿拉斯加耶稣受难日地震以来从未见过的地球自由振荡。事实上，它几乎可以与1960年5月22日智利大地震相媲美，当时首次观察到自由振荡。这一次，众多高质量的数字地震台站以前所未有的保真度记录了地震。PI Laske和她的团队测量地球平均和横向变化内部结构的自由振荡参数。该团队开发了一种分析技术，其中地震过程的细节不必知道。这使得事件的分析相对复杂的源机制，如苏门答腊-安达曼地震的震动持续了近10分钟。拉斯克基本上措施全球变化的模式频率和衰减率。地球偏离了一个非旋转的、均匀分层的行星，这就消除了正常模式的简并性，就像原子遇到磁场时电子能级被分裂一样。这种分裂的测量允许拉斯克成像对称的横向异质性。不对称的地球结构会导致模式之间的耦合，因此耦合系数的分析使她能够充分了解地球的三维非均匀性。在苏门答腊-安达曼地震之前，地球的衰减结构特别难以评估，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，如1994年的大玻利维亚地震，激发模式是敏感的内核结构。由于其非常大的破裂面积，苏门答腊-安达曼地震也激发了这些模式的水平，这是自玻利维亚地震以来没有观察到的。虽然最近，较小的地震被用来限制内核旋转速率，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。拉斯克现在可以在近30年的时间跨度内测试内核旋转速率。 该项目的更广泛影响之一是分析地球的自由振荡，为地球科学的邻近学科提供了地球结构的关键约束。特别是密度的限制是非常难以获得使用其他地震方法，如果不是不可能的。 此外，该项目将有助于培训一名研究生和一名本科生。