Figure 9 . Depths of the 410-km discontinuity (top row) and the thicknesses of the transition zone (bottom row). Panels (a) and (b) are estimated from spectral-element-method seismograms calculated for model S40RTS. Panels (c) and (d) show the same estimates after ray-theoretical corrections have been applied to the waveforms.
If ray-theoretical traveltime corrections are precise, we must expect that the CRP images of the ray-theoretical corrected S40RTS, SEMUCB-WM1, and TX2015 waveforms are similar to the CRP image for the PREM model because the 410-km and 660-km discontinuities are horizontal boundaries in all models. However, we find this not to be the case. While the elevation of the 410-km and 660-km discontinuities beneath the western US (by 10 and 11 km, respectively) and their depressions beneath the central-eastern US (by 11 and 12 km, respectively) have the expected trends, the corrections are larger than expected ray-theoretically. In the corrected image, the 410-km discontinuity is shallower in the western US than in the eastern US (Figure 9c) opposite to the uncorrected CRP image (Figure 9a). In the western US, the inferred and predicted depth correction differ by a factor of 1.7. The MTZ thickness has a smaller variation than the depth of the 410-km discontinuity (Figure 9c and 9d). After corrections, a larger area has a thickness within 265-275 km. The corrected Pacific coast changes from thicker to slightly thinner than the surrounding area, and the central US has the largest thickening by about 15 km.