Thickness estimates of the upper-mantle transition zone from bootstrapped velocity spectrum stacks of receiver functions

We modify the receiver-functions stacking technique known as velocity spectrum stacking (VSS) so as to estimate combinations of velocity model (V_P and V_s) and depth that stack the Ps conversion from upper-mantle discontinuities most coherently. We find that by estimating the differences in the depths to the 660 and 410 km discontinuities using velocities that maximize the stacked amplitudes of P410s and P660s phases we can estimate the thickness of the transition zone more accurately than the depths to either of these discontinuities. We present two examples indicating that the transition zone beneath Obninsk, Russia, is 252 ± 6 km thick and that beneath Pasadena, California, is only 220 ± 6 km thick.