The modeling approach utilizes increasingly advanced theoretical frameworks and numerical methods in order to obtain improved models of regional seismic structure. Specifically, a large-scale regional Eurasian model will be developed from a large dataset of seismic waveforms using the path-average approximation (PAVA) and NACT (Non-linear Asymptotic Coupling Theory; *Li and Romanowicz*, 1995), which are well-developed normal-mode based approaches which consider 1D (PAVA) and 2D (NACT) waveform sensitivity in the vertical plane along the great-circle path between source and receiver. This model will then be refined in a smaller region using an implementation of Born single-scattering theory (*Capdeville*, 2005), which more accurately represents the 3D sensitivity of the seismic wavefield. Finally, we will utilize the Spectral Element Method (SEM), a numerical approach that accurately models both 3D and non-linear effects (e.g. *Faccioli et al.*, 1996; *Komatitsch and Vilotte*, 1998). To conserve computational resources we will restrict the use of SEM to the upper mantle by coupling to a normal mode solution (CSEM; *Capdeville et al.*, 2003) and applying appropriate boundary conditions.

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