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

对地球自由振荡的观测和分析有助于地震学家对地球内部结构进行成像。特别是，自由振荡分析提供了研究密度变化的独特机会，这一参数对于其他地震学应用来说仍然有些难以捉摸。另一方面，密度限制在理解地球上邻近学科提出的过程中发挥着关键作用。例如，地球坚固的外核向内核的转变与密度的突然增加有关。在讨论地球发电机的维护问题时，这种密度跃升是一个重要的制约因素。另一个令人感兴趣的领域是几年前提出的地幔最低100公里处的“热深海层”。它带有低地震速度的“热”物质的地震特征，但它仍然不浮力。这一层被认为异常致密，其厚度肯定在横向上变化很大，因为地震学一直无法检测到相关的全球不连续。这样的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，地球的自由振荡还不足以确凿地证明或反驳“热的深渊层”是否真的致密。自由振荡分析的另一个重要领域是地核的结构和动力学。20世纪90年代，有人提出内核独立于地球其他部分自转，最初发表的一篇文章称，每年6度的超级自转与掠过南美下方地球内核的体波一致。这种超自转对地球的地球发电机以及地幔和地核的引力耦合有着深远的影响。后来对地球自由振荡的分析已经证明了这种高自转速度是错误的，但自由振荡观测的保真度到目前为止还不能使地震学家将不确定度降低到0.15度/年以下。2004年12月6日苏门答腊岛-安达曼大地震将地球的自由振荡激发到了自1964年阿拉斯加耶稣受难日地震以来未曾见过的水平。事实上，它几乎可以与1960年5月22日智利大地震相媲美，当时首次观察到了自由振荡。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这次地震。皮拉斯克和她的团队测量了地球平均内部结构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，在这种技术中，地震过程的细节不必知道。这使得可以分析震源机制相对复杂的事件，例如苏门答腊岛-安达曼地震，其震动持续了近10分钟。Laske主要测量全球变化的模式频率和衰减率。地球偏离非旋转的、均匀分层的行星，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Laske成像对称的横向非均质性。不对称的地球结构导致模式之间的耦合，因此耦合系数的分析使她能够完全成像地球的3D异质性。在苏门答腊岛-安达曼地震之前，地球的衰减结构一直特别难以评估，因为它引起的地震信号相对较小。通常需要非常深的地震，如1994年玻利维亚大地震，才能激发出对内核结构敏感的振型。由于其非常大的破裂面积，苏门答腊岛-安达曼地震也将这些模式激发到自玻利维亚地震以来未曾观察到的水平。虽然最近较小的地震被用来限制内核自转速度，但苏门答腊岛-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的自转速度。在这个项目的更广泛的影响中，对地球自由振荡的分析为地球科学的邻近学科提供了对地球结构的关键限制。特别是对密度的限制，如果不是不可能的话，也很难用其他地震方法来获得。此外，该项目将有助于培养一名研究生和一名本科生。

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.