Upper mantle transition zone: Structure and topography of discontinuities

We investigate global depth variations in the topography of the 410- and 660-km discontinuities by analyzing the underside reflection ( SS precursors) from more than 21,000 SH-component records. We find that the undulations on the 410- and 660-km discontinuities are decorrelated. The long wavelength pattern in the topography of the 410-km discontinuity is dominated by the degree 1 spherical harmonic, and is weakly correlated with seismic velocity perturbations in the mantle above it. The global topography of the 660-km discontinuity, on the other hand, is dominated by the degree 2 spherical harmonic, and is strongly correlated with shear velocity variations in the transition zone. A global mapping of transition zone thickness shows peak-to-peak variations exceeding ±25 km.; We find that the 220-km discontinuity is only a regional feature that is most frequently observed under the continents. This implies a potential difference in rheology or composition between the continents and oceans at depths below 200 km. We also observe a weak reflector near 520 km in the oceanic mantle, but there is little indication of such a reflector under the shield regions.; We obtain the three-dimensional shear velocity structure of the mantle from inversions of travel time and waveform of surface and body waves. Our best fit models (parameterized in B-splines) show a significant difference in the large-scale structure between the upper and lower mantle as a function of wave number, which could imply a possible reorganization of the convective flow between these two regions. No other flow discontinuity, e.g., close to 400 km or 1000 km, is indicated by our data sets.; Finally, we perform a simultaneous inversion for the shear velocity and depth variations of the transition zone discontinuities. The best fit model satisfies the travel times of both reflected and transmitted waves at the transition zone discontinuities. This approach effectively minimizes problems with tradeoff between the shear velocity and boundary topography.