Collaborative Research: Improving lower mantle seismic sampling and model resolution using multi-bounce and diffracted waves

合作研究：利用多次反射波和衍射波提高下地幔地震采样和模型分辨率

• 批准号: 1648770
• 负责人: Stephen Grand
• 金额: $ 10.47万
• 依托单位: University of Texas at Austin
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-12-15 至 2019-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1648770&HistoricalAwards=false
• 关键词: Collaborative; Research; Improving; lower; mantle

Earthquakes generate seismic waves that travel through the entire interior of the planet. These waves are used to image the planetary interior. The tool of seismic tomography is frequently used to produce images of the variation of seismic wave speeds within the Earth's mantle (the shell roughly occupying the outer half of the planet). Our proposed work aims to improve upon past tomographic studies by using a larger suite of types of seismic waves that will be measured in a new and novel way in order to improve accuracy. Some of the waves include seismic S waves that bounce off of the Earth's core and back to the surface, multiple times. This enables us to add seismic wave sampling of Earth?s southern hemisphere, which is more poorly sampled than the northern hemisphere in tomography studies. Improving the resolution of Earth's mantle structure is important for improving our understanding of the nature of global internal processes, including the convective dynamics of the mantle and evolution of the planet. Also of interest is improving the clarity of tomographic images within two massive blob-like structures at the base of Earth's mantle, which are continental in size and extend at least 1000 km up into the mantle (scientifically referred to as Large Low Shear Velocity Provinces, or LLSVPs, which emphasizes the large reduction in the speeds of shear waves through these structures).The characterization of the large-scale aspects of LLSVPs by the method of seismic tomography yields similar results from different research groups. However, the smaller scale structure differs between models. This 2-year project aims to add more information to the process that is sensitive to the smaller scale structure, namely careful travel time measurements of multi-bounce S and ScS waves, and also diffracted S and P waves. We will use these in both forward and inverse modeling approaches. The full tomographic inversion will be for both P and S structure. The forward approach iteratively updates existing tomography models using new data, and better preserves sharper structures. Waves bouncing up to 5 times, thus 6 legs of the journey, e.g., S6 and ScS6 (i.e., six S and ScS paths), are clearly observed for larger earthquakes. The multi-bounce data are well suited for improving mantle imaging, since they allow for both minor and major arc travel paths (i.e., the great circle path between earthquake and station, as well as along the great circle path in the opposite direction, the long way around the planet, respectively). We will develop finite frequency kernels for these long path data. Final 3D models from both the forward and inverse approaches will be used to compute 3D synthetic seismograms to compare to actual data. This will both assess model robustness as well as compare solutions for the forward and inverse methods. Mantle heterogeneity depends upon temperature, mineralogy, phase, and state, and while tomographic imaging only provides us with a present-day snapshot in time, it can be related to the evolutionary pathway Earth has taken. Recently, there has been increased attention to the relationship between surface observables, such as the locations of hot spot volcanism and the origination locations of large igneous provinces, with deep structures (e.g., LLSVPs). Thus improving resolution in seismic images of mantle heterogeneity brings us closer to understanding the structure, dynamics, and evolution of our planet.

地震产生的地震波穿过地球的整个内部。 这些波用于拍摄行星内部的图像。 地震层析成像工具经常用于生成地幔（大致占据地球外半部分的外壳）内地震波速度变化的图像。 我们提出的工作旨在通过使用更多类型的地震波来改进过去的层析成像研究，这些地震波将以一种新颖的方式进行测量，以提高准确性。 其中一些波包括地震 S 波，它会多次从地核反弹回地表。 这使我们能够添加地球南半球的地震波采样，在断层扫描研究中，南半球的采样质量比北半球要差。提高地球地幔结构的分辨率对于提高我们对全球内部过程性质的理解非常重要，包括地幔的对流动力学和地球的演化。 同样令人感兴趣的是提高地幔底部两个巨大斑点状结构内断层扫描图像的清晰度，这些结构的大小为大陆性，延​​伸至地幔至少 1000 公里（科学上称为大低剪切速度省，或 LLSVP，它强调通过这些结构的剪切波速度的大幅降低）。通过地震层析成像方法对 LLSVP 的大尺度特征进行表征产生了类似的结果来自不同的研究小组。 然而，不同型号的较小规模结构有所不同。 这个为期两年的项目旨在向对较小尺度结构敏感的过程添加更多信息，即多次反射 S 波和 ScS 波以及衍射 S 波和 P 波的仔细旅行时间测量。我们将在正向和逆向建模方法中使用它们。完整的层析成像反演将针对 P 结构和 S 结构。前向方法使用新数据迭代更新现有的断层扫描模型，并更好地保留更清晰的结构。对于较大的地震，可以清楚地观察到波浪最多弹跳 5 次，因此可以清楚地观察到 6 个旅程段，例如 S6 和 ScS6（即 6 个 S 和 ScS 路径）。多次反射数据非常适合改善地幔成像，因为它们考虑了小弧和大弧行进路径（即地震和台站之间的大圆路径，以及沿相反方向的大圆路径，分别绕地球很远）。我们将为这些长路径数据开发有限频率内核。正向和逆向方法的最终 3D 模型将用于计算 3D 合成地震图，以与实际数据进行比较。这将评估模型的稳健性以及比较正向和逆向方法的解决方案。地幔异质性取决于温度、矿物学、相和状态，虽然断层扫描成像只能为我们提供当前的快照，但它可能与地球所采取的进化路径有关。 最近，人们越来越关注地表观测数据（例如热点火山活动的位置和大型火成岩省的起源位置）与深层结构（例如 LLSVP）之间的关系。因此，提高地幔非均质性地震图像的分辨率使我们更了解地球的结构、动力学和演化。