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

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成像。特别是，自由振荡分析为研究密度的变化提供了独特的机会，密度是其他地震学应用中仍然难以捉摸的一个参数。另一方面，密度约束在理解地球上邻近学科提出的过程中起着关键作用。例如，地球的固体外核向内核的转变与密度的突然增加有关。在讨论维持地球的地球发电机时，这种密度的跳跃是一个重要的限制。另一个令人感兴趣的领域是几年前提出的地幔最下面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，但它仍然没有浮力。这一层被认为是异常致密的，它的厚度一定在横向上有很大的变化，因为地震学一直无法探测到相关的全球不连续。这一层被认为是在海洋岛屿上发现的熔岩的最终起源，这些岛屿的化学成分与海洋中脊上发现的熔岩非常不同。不幸的是，我们对地球的自由振荡还没有足够精确的了解，无法非常有把握地证明或反驳“热深海层”是否真的很致密。自由振荡分析的另一个重要贡献领域是地核的结构和动力学。早在20世纪90年代就有人提出，地球内核的自转独立于地球的其他部分，每年6度的超级自转最初被发表为与南美洲地下地球内核的体波相一致。这种超级自转对地球的地球动力学和地幔与地核的引力耦合具有深远的影响。随后对地球自由振荡的分析否定了如此高的自转速率，但迄今为止，自由振荡观测的保真度还不能使地震学家将不确定性降低到0.15度/年以下。2004年12月6日的苏门答腊-安达曼大地震激发了地球的自由振荡，达到了自1964年阿拉斯加耶稣受难日地震以来从未见过的水平。事实上，它几乎可以与1960年5月22日的智利大地震相媲美，当时首次观测到自由振荡。这一次，许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和她的团队测量了地球平均和横向变化的内部结构的自由振荡参数。该小组开发了一种分析技术，无需了解地震过程的细节。这样就可以对震源机制相对复杂的事件进行分析，比如持续了近10分钟的苏门答腊-安达曼地震。Laske本质上是测量全球变化的模式频率和衰减率。地球偏离非旋转、均匀分层的行星，消除了正常模式的简并，就像电子能级在原子遇到磁场时分裂一样。这种分裂的测量使Laske能够对对称的横向非均匀性进行成像。不对称的地球结构导致了模态之间的耦合，因此对耦合系数的分析使她能够充分成像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。通常需要非常深的地震，如1994年的玻利维亚大地震，才能达到对内核结构敏感的激发模式。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从未观察到的水平。虽然最近，较小的地震被用来限制内核的旋转速度，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的旋转速率。该项目的广泛影响包括地球自由振荡的分析，为地球科学的邻近学科提供了地球结构的关键约束。特别是对密度的限制，用其他地震方法很难获得，如果不是不可能的话。此外，该项目将有助于培训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.