Global Seismic Image of Mantle Transition Zone based on Finite-Frequency Theory.

Analysis of the differential travel times between SS and its precursors (i.e., S410S and S660S) are widely used to estimate the mantle transition zone (MTZ) thickness, which helps to constrain temperature and mineralogical variations in the mantle. Most analyses have been based on ray-theoretical travel time calculations. However, several studies have demonstrated that ray theory is inaccurate to compute corrections for upper mantle velocity structure since the sensitivity of SS to seismic structure extends several thousand kilometers beyond the ray-theoretical reflection point. Thus, the SS travel time must be calculated by using 3D Fréchet kernels. In this study, we will analyze a new global dataset of SS-waves and its precursors using finite-frequency theory. 3D travel time sensitivity kernels will be calculated from pre-computed databases of strain Green's tensors by direct solution method so that finite-frequency effects may be taken into account. We will make travel time corrections for each trace of SS and its precursors of our dataset and then convert the travel time anomalies to obtain the MTZ thickness. Comparison between the MTZ thickness map obtained by ray and finite-frequency theories will allow us to understand how data analysis are compromised by theoretical simplifications.