Investigation of the anisotropy of the mantle transition zone using PP and SS precursors

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The aim of this extension proposal is the investigation and characterization of deformation that leads to seismic anisotropy in the transition zone between upper and lower Earth's mantle. During the previous three years of the project, a detailed mapping of the Atlantic with underside reflections was carried out and provides a large dataset that showed strong temperature dependence of the mantle discontinuities in the area of hotspots in the Atlantic. This dataset also indicated polarity variations, not predicted by wave propagation modelling. Through theoretical calculations of a simple model of anisotropic, deformed olivine over an isotropic wadsleyite layer we found that there are visible effects for the SH-wave underside reflections off the discontinuity at 410 km depth for different travel directions, albeit at short epicentral distances. In this extension proposal we aim to build on the successful first part of the project and want to continue and extend the modelling of reflection coefficients for underside reflections, including an anisotropic lower layer for wadsleyite but also predict polarities and amplitudes for receiver functions as well as shear-wave splitting for the deformed materials above and below the discontinuity. Furthermore we aim to extend these predictions of reflection coefficients, transmission coefficients (receiver functions) and shear wave splitting to the 660 km discontinuity and possibly deeper discontinuities. To test the predictions with real data, we use and extend our current dataset of underside reflections and add receiver function analysis as well as shear wave splitting for the mantle transition zone. We have to find suitable seismic waves that can probe the deformation as the predictions from our starting model at the moment point to very short epicentral distance waves. This however, may change for two anisotropic layers and will be determined better during the course of the project. The final goal is to develop a new diagnostic tool to detect the style of deformation by combining several seismic observables (with the least number of independent observables) and in different directions, and through this detection of deformation constrain flow in the depths around the mantle transition zone.