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

解决地幔非均质性对内核各向异性估计的影响

• 批准号: 1829283
• 负责人: Barbara Romanowicz
• 金额: $ 15.21万
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-08-01 至 2020-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1829283&HistoricalAwards=false
• 关键词: Resolving; influence; mantle; heterogeneity; estimates

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.

地球内核是由液态铁外核凝固而成的，它提供了产生磁场的能量。三十年前，人们发现，通过内核传播且路径平行于地球自转轴的地震波比路径沿赤道方向传播的地震波传播得更快。这种对波传播方向的依赖性被称为各向异性，归因于内核中铁晶体在旋转轴方向上的排列。造成观测到的地震各向异性的机制尚未完全阐明，可能包括内核中的大规模流动，或在地球磁场影响下的优先生长方向。由于内核中地震各向异性的起源和震级没有得到很好的约束，并且这些波总是穿过异质地幔，这会影响它们的传播时间，迄今为止，每种效应的分离受到限制。另一个复杂问题来自于全球地震和地震台分布不均匀，导致内核地震路径采样不良，特别是那些平行于地球自转轴（即极地路径）的地震路径。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范围传播的神秘根源，这很可能不是由内核结构引起的。这些新数据使得填补极地路径覆盖范围的空白成为可能，同时阿拉斯加下方地幔结构的改进图像也正在变得可用。我们将量化地幔结构对内核敏感地震波传播的影响，并探索内核之外的其他可能来源，以解释在这些极路径中观察到的走时模式。特别是，将寻求对南桑威岛-阿拉斯加异常起源问题的更明确的答案，从而能够构建更准确的内核地震各向异性模型。 该项目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宣传的网站。准确解决内核 (IC) 地震各向异性的大小及其空间和深度依赖性，对于我们理解内核地震各向异性至关重要。核心的形成和演化。在对内核结构敏感的数据中，PKP数据最适合构建如此详细的模型，但它们受到地幔异质性的污染，需要首先对其进行校正。特别是，推断出的各向异性幅度很大，在西半球高达 6-8%，很难与矿物物理模型相一致。之前许多基于 PKP 波的 IC 研究几乎没有包含极路径数据，而且西半球 IC 各向异性的大小取决于阿拉斯加站对南桑威奇群岛事件的观测。已知该区域的板片结构很强，因此 IC 各向异性和地幔结构之间的权衡尚不清楚。识别 PKP 数据的污染源并进行适当的校正对于协调内核各向异性强度的不同估计和构建更可靠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断层扫描图像现在也已可用。通过最近的断层扫描模型和合成板片模型的 1D 和 3D 射线追踪以及 3D 波形建模，上地幔结构对 PKP 差分走时的影响将得到更好的量化，并探索其他污染源。我们将寻求对 SSI-阿拉斯加异常起源问题的更明确的答案，从而能够构建更准确的 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，以及对博士后研究员的指导。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值进行评估，被认为值得支持以及更广泛的影响审查标准。

项目成果

Upper mantle slab under Alaska: contribution to anomalous core-phase observations on south-Sandwich to Alaska paths

阿拉斯加下的上地幔板片：对南桑威奇至阿拉斯加路径异常核心相观测的贡献

• DOI: 10.1016/j.pepi.2020.106427
• 发表时间: 2020-02-01
• 期刊: Physics of the Earth and Planetary Interiors
• 影响因子: 2.3
• 作者: D. Frost;B. Romanowicz;S. Roecker
• 通讯作者: S. Roecker