Global variations of temperature and water content in the mantle transition zone from higher mode surface waves

We performed a fully nonlinear joint inversion of surface wave overtone phase velocities for velocity structure down to a depth of 1500 km and topography of transition zone discontinuities using neural networks. The resulting models are compared with existing mantle models from linearized inversions techniques and the agreement between vertically averaged S-wave velocity models and various 660-km topography models is very good, whereas the agreement between all the 400-km topography models is very poor. The latter might be due to a broader than previously reported 400-km discontinuity where the minerals co-exist with water induced melt. From our models of transition zone thickness and S-wave velocity we estimated lateral variations of temperature and water content in Earthʹs mantle transition zone, linearly relating variations in S-wave velocity and transition zone thickness to variations in temperature and water content. Within the transition zone we observe relatively cold subduction zones as well as relatively warm regions beneath continents and hot spots. We find that the transition zone is wettest away from subduction zones.