Resolving the influence of mantle heterogeneity on estimates of inner core anisotropy

地球的核心来自液态铁外核的凝固，而液态铁外核提供了产生磁场所需的能量。30年前，人们发现，路径与地球自转轴平行的地震波在内核中传播的速度比路径沿赤道方向的地震波传播得更快。这种对波传播方向的依赖被称为各向异性，并归因于内核中铁晶体在旋转轴方向上的排列。造成地震各向异性的机制尚未完全阐明，可能包括内核中的大范围流动，或在地球磁场影响下的优先生长方向。由于内核地震各向异性的起源和大小没有得到很好的约束，而且这些波总是穿过不均匀的地幔，这影响了它们的旅行时，因此迄今为止对每种影响的分离都是有限的。另一个复杂因素是地震和地震台站在全球的分布不均匀，导致对核心地震路径的采样不足，特别是那些平行于地球自转轴的路径(即极地路径)。在这个项目中，研究人员将分析最近在阿拉斯加和南极洲部署的数据，以找出阿拉斯加观测到的南桑威奇群岛地震走时异常大规模传播的神秘来源，这很可能不是由内核结构引起的。这些新数据使填补极地路径覆盖范围的空白成为可能，同时阿拉斯加地幔结构的改进图像也变得可用。我们将量化地幔结构对内核敏感地震波传播的影响，并探索内核以外的其他可能来源，以解释在这些极地路径中观察到的旅行时模式。特别是，将寻求对南桑德威岛-阿拉斯加异常起源问题的更明确答案，从而能够建立更准确的内核地震各向异性模型。该项目的更广泛的影响包括指导博士后研究员和开发一个旨在向公众开放的网站。准确地解决内核(IC)地震各向异性的大小及其空间和深度相关性，对于我们理解内核的形成和演化至关重要。在对内核结构敏感的数据中，PKP数据最适合构建这种详细的模型，但它们受到地幔非均质性的污染，需要首先对其进行校正。特别是，推断出的大的各向异性，在西半球高达6-8%，很难与矿物物理模型相一致。以前许多基于PKP波的IC研究几乎没有极地路径的数据，而且西半球IC各向异性的大小取决于阿拉斯加台站对南桑威奇群岛事件的观测。已知该地区的板片结构很强，因此IC各向异性与地幔结构之间的权衡一直不清楚。找出PKP数据的污染源，并进行适当的校正，对于调和对内核各向异性强度的不同估计，以及建立更稳健的内核各向异性结构图像至关重要。将分析最近在阿拉斯加和南极洲部署的新数据。这些数据现在可以填补极地路径覆盖范围的空白，同时改进的阿拉斯加板块的断层图像现在也可以使用。通过最新的层析成像模型和合成板片模型的一维和三维射线追踪和三维波形模拟，将更好地量化上地幔结构对PKP差异走时的影响，并探索其他污染源。对于SSI-阿拉斯加异常的起源问题，将寻求更明确的答案，从而建立更准确的IC各向异性模型。更广泛的影响包括提供一套新的PKP旅行时间集合，供未来的研究人员使用，以及指导一名博士后研究员。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The inner core of the Earth grows from the solidification of the liquid iron outer core, which provides the energy that powers the generation of its magnetic field. Thirty years ago, it was discovered that seismic waves that propagate through the inner core with paths parallel to the earth's rotation axis travel faster than those with paths oriented in the equatorial direction. This dependence on the direction in which the waves propagate is known as anisotropy, and is attributed to the alignment of iron crystals in the inner core in the direction of the rotation axis. The mechanisms responsible for the observed seismic anisotropy have not been completely elucidated, and may include large scale flow in the inner core, or preferential growth directions under the influence of the earth's magnetic field. Since the origin and magnitude of the seismic anisotropy in the inner core is not well constrained and these waves always traverse the heterogeneous mantle, that influences their travel time, separating each effect has been limited to date. Another complication comes from the uneven distribution of earthquakes and seismic stations around the globe, resulting in poor sampling of the inner core seismic paths, especially those parallel to the earth's rotation axis (i.e. polar paths). In this project, the researchers will analyze data from recent deployments in Alaska and Antarctica to find out the mysterious source of the large spread of travel time anomalies for earthquakes in South Sandwich Islands observed in Alaska, which is likely not caused by inner core structure. These new data make it possible to fill gaps in coverage for polar paths, while improved images of the structure in the mantle beneath Alaska are becoming available. We will quantify the effect of mantle structure on the propagation of inner core sensitive seismic waves and explore other possible sources outside of the inner core to explain the travel time patterns observed in these polar paths. In particular, a more definitive answer to the question of the origin of the South Sandwih Island-Alaska anomalies will be sought, allowing the construction of more accurate models of inner core seismic anisotropy. Broader impacts of this project include mentoring of a postdoctoral fellow and development of a website aimed towards public outreach.Resolving the magnitude of inner core (IC) seismic anisotropy accurately, as well as its spatial and depth dependence, is critical for our understanding of inner core formation and evolution. Among data sensitive to inner core structure, PKP data are best suited to construct such detailed models, but they suffer from contamination by mantle heterogeneity, which they need to first be corrected for. In particular, the large magnitude of the anisotropy inferred, up to 6-8% in the western hemisphere, is hard to reconcile with mineral physics modeling. Many of the previous studies of the IC based on PKP waves included few data for polar paths and the magnitude of the IC anisotropy in the western hemisphere hinged on observations at stations in Alaska from events in the South Sandwich Islands. Slab structure in this region is known to be strong and thus the trade off between IC anisotropy and mantle structure has been unclear. Identifying sources of contamination of PKP data and making appropriate corrections is crucial for reconciling different estimates of inner core anisotropy strength and building a more robust image of inner core anisotropic structure. New data from recent deployments in Alaska and Antarctica will be analyzed. These data now make it possible to fill gaps in coverage for polar paths, while improved tomographic images of the Alaska slab are now also becoming available. By 1D and 3D ray tracing and 3D waveform modelling through recent tomographic models and synthetic slab models, the effect that upper mantle structure can have on PKP differential travel times will be better quantified, and other sources of contamination will be explored. A more definitive answer to the question of the origin of the SSI-Alaska anomalies will be sought, allowing the construction of more accurate models of IC anisotropy. Broader impacts include making available the new set of PKP travel time collection to be used by future researchers, as well as the mentoring of a postdoctoral fellow.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.