Figure 6 . Depth of the 410-km discontinuity (in a) and the thickness of the transition zone (in b) estimated after ray-theoretical traveltime corrections have been applied. The color scale in a is the same as Figure 4a. Panels c and d show histograms of the resolved depth of the 410-km discontinuity and the thickness of the MTZ beneath the USArray with (black line) and without (purple line) traveltime corrections. Compare with Figure 4b.