3. Influence of 3-D seismic heterogeneity on the CRP images
The map of the depth of the 410-km discontinuity shown in Figure 4b is reminiscent of the estimated shear-wave velocity variations in the upper mantle beneath the US from the modeling of regional S-waves (e.g., Grand & Helmberger, 1984), surface-waves (e.g., Van der Lee & Nolet, 1997), and, more recently, ambient noise (e.g., Bensen et al. 2008), P-wave polarization (Park et al. 2019) and surface-wave amplification (Eddy & Ekström, 2014; Bowden & Tsai, 2017). This indicates strongly that shear-velocity variations in the mantle affect Ss410s-S and Ss660s-S difference times and that a mapping method based on a 1-D velocity structure would overestimate undulations of the 410-km and 660-km discontinuities.