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

地震产生的地震波在整个地球内部传播。这些波被用来成像行星内部。地震层析成像工具经常被用来产生地幔(大致占据地球外半部分的壳体)内地震波速度变化的图像。我们拟议的工作旨在通过使用更大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的和新颖的方式进行测量，以提高精度。其中一些波包括多次从地核反弹并返回地表的地震S波。这使我们能够增加对地球的地震波采样？？S南半球，在层析成像研究中，南半球的采样比北半球更差。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的性质的理解是重要的，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的斑点状结构内层析图像的清晰度，这两个结构大小是大陆的，向上延伸至少1000公里进入地幔(科学上称为大的低剪切速度省，或LLSVP，强调通过这些结构的剪切波速度的大幅降低)。用地震层析成像的方法描述LLSVP的大尺度方面得到了不同研究小组的类似结果。然而，较小的规模结构在不同的模型中有所不同。这个为期两年的项目旨在为对小尺度结构敏感的过程增加更多信息，即仔细测量多反射S波和SCS波以及绕射S和P波的旅行时。我们将在正反向建模方法中使用这些方法。将对P和S构造进行全层析反演。正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。对于较大的地震，可以清楚地观察到5次反弹的波，因此可以清楚地观察到旅程的6段，例如S6和ScS6(即6条S和SCS路径)。多回波数据非常适合于改进地幔成像，因为它们允许小弧线和大弧线的传播路径(即地震和台站之间的大圆周路径，以及沿相反方向的大圆周路径，即绕地球很长的路)。我们将为这些长路径数据开发有限频率核。正演和逆演的最终3D模型将被用来计算3D合成地震记录，以便与实际数据进行比较。这将评估模型的稳健性，并比较正向方法和反向方法的解。地幔的不均一性取决于温度、矿物学、相和状态，虽然断层成像只为我们提供了今天的时间快照，但它可能与地球所走的演化路径有关。最近，人们越来越关注地表观测数据之间的关系，例如热点火山活动的位置与具有深部结构(例如LLSVP)的大型火成岩省的起源位置之间的关系。因此，提高地幔不均匀地震图像的分辨率使我们更接近了解地球的结构、动力学和演化。

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.