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

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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.