Deep reflections: Imaging reflectors in the lowermost mantle using one- and three-dimensional stacking techniques

The Earth's core-mantle boundary (CMB) is the site of the largest density contrast and most profound chemical contrast within the planet. The CMB separates the vigorously convecting molten iron-alloy outer core and the more slowly convecting crystalline silicate/oxide mantle. The lowermost few hundred kilometers of the mantle, known as the D" region, has a thermal boundary layer caused by heat flowing from the core into the mantle. There is seismic velocity complexity in the lowermost mantle that cannot be accounted for by thermal structure alone. Lay and Helmberger (1983) first observed triplicated arrivals from a rapid increase in shear wave velocity near the top of the D" region, about 250 km above the CMB. It is the goal of this thesis to detect, characterize and document detailed observations of both P and S reflectors within D. P-wave reflectors in D" are detected and characterized using double-array stacking, a one-dimensional imaging technique. Results are found to be consistent with the perovskie to post-perovksite phase transition. Using a three-dimensional imaging technique, steep topography of the D" is mapped with steep topography suggesting the presence of subducted slab material.