DI002-0013
Global observations of mantle discontinuities from ScS reverberations
Monday, 7 December 2020
Poster
Rashni Rashni Anandawansha, New Mexico State University Main Campus, Physics, Las Cruces, NM, United States, Lauren Waszek, University of Cambridge, Department of Earth Sciences, Cambridge, United Kingdom, Benoit Tauzin, Australian National University, Research School of Earth Sciences, Canberra, Australia and Jorge Garcia, New Mexico State University, Las Cruces, NM, United States
Abstract:
Seismic tomography models show that both downgoing slabs and upwelling plumes are deflected and become stagnant in the mantle transition zone (MTZ) and mid-mantle (800 – 1300 km depth). While the geodynamic effects of the major MTZ discontinuities at 410 and 660-km depth are relatively well-understood, the cause of deflection in the mid-mantle is unclear. Furthermore, there is no candidate mineral physics phase change to explain the numerous reflectors observed at these depths. The relationship between the seismic properties of the MTZ discontinuities and mid-mantle reflectors is poorly understood, but is an important constraint in resolving the geodynamics and composition of the upper mantle. In order to do so, we require high quality global observations of the MTZ and mid-mantle features, combined with forward modelling for mantle mineral assemblages.
In this study, we introduce a new large global dataset of ScS reverberations, compiled with an automatic waveform identification code which uses a Convolutional Neural Network (CNN). The CNN model was trained on a handpicked global dataset of SS phases, and here we show its application to the identification for other shear wave phases with similar frequencies. We build new topography maps for the MTZ and mid-mantle discontinuities using an adaptive stacking technique based on Voronoi tessellation, which automatically adjusts its parameterisation to account for topography of the discontinuities, noise, and data coverage. We consider our observations in the context of mineral physics predictions for various thermochemical models. The new geometries provided by this dataset supplement our existing maps of the MTZ and mid-mantle, built with SS and PP precursors, providing improved resolution in regions with previously poor data coverage. Our results offer new insight into the relationship between the MTZ discontinuities and mid-mantle reflectors, informing regarding the regionally diverse styles of mantle mixing.
