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.

地震产生的地震波会穿过地球的整个内部。 这些波被用来拍摄行星内部的图像。 地震层析成像的工具经常被用来产生地幔（外壳大致占据地球的外半部）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大的地震波类型套件来改进过去的层析成像研究，这些地震波将以一种新的和新颖的方式进行测量，以提高精度。 其中一些波包括地震S波，它从地核反弹回地表，多次。 这使我们能够添加地震波采样地球？在断层扫描研究中，南半球的采样比北方半球的采样更差。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至关重要，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球地幔底部两个巨大的斑点状结构内的层析图像的清晰度，这两个结构的大小为大陆状，延伸到地幔中至少1000公里（科学上称为大低剪切速度省，或LLSVP，其强调剪切波通过这些结构的速度的大幅降低）。通过地震层析成像的方法，LLSVP的尺度方面从不同的研究小组得到类似的结果。 然而，较小的尺度结构在模型之间是不同的。 这个为期2年的项目旨在为对较小尺度结构敏感的过程添加更多信息，即仔细测量多反射S波和ScS波以及衍射S波和P波的旅行时间。我们将在正向和反向建模方法中使用这些方法。将对P和S结构进行全层析反演。前向方法使用新数据迭代更新现有的层析成像模型，并更好地保留更清晰的结构。波浪反弹多达5次，因此行程为6段，例如，S6和ScS6（即，六个S和ScS路径），清楚地观察到较大的地震。多次反弹数据非常适合于改进地幔成像，因为它们允许小弧和大弧行进路径（即，地震和台站之间的大圆路径，以及分别沿相反方向的大圆路径、绕行星的长距离沿着）。我们将为这些长路径数据开发有限频率内核。正演和反演方法的最终三维模型将用于计算三维合成地震图，以与实际数据进行比较。这将评估模型的鲁棒性以及比较正向和反向方法的解决方案。地幔的不均匀性取决于温度、矿物学、相位和状态，虽然断层成像只能为我们提供一个当前的快照，但它可能与地球的进化路径有关。 最近，人们越来越关注地表可观测量之间的关系，例如热点火山活动的位置和大火成岩省的起源位置，以及深部结构（例如，LLSVP）。因此，提高地幔不均匀性地震图像的分辨率使我们更接近于了解我们星球的结构，动力学和演化。