Figure 11 . Maps of the 410-km (a and b) and 660-km (c and d) discontinuities, and the thickness of the MTZ (e and f) inferred for a model with the S40RTS model velocity structure for the mantle and 5x5 harmonic undulations on the 410. In the maps along the right column (b, d, f) the effects of the velocity structure have been removed by using ray-theoretical traveltime corrections.