Global Structure of the D" Region Using Diffracted Waves and Finite Frequency Kernels

The base of the mantle serves as a thermal, chemical, and mechanical boundary layer, separating the liquid iron core and the solid silicate mantle. Studies of the region reveal a large number of unusual features, such as the D" discontinuity, anisotropy, ultra-low velocity zones, high amplitude anomalies, and anti-correlation of bulk sound speed and shear velocity. However, the structure at the base of the mantle is not well understood, due to poor coverage by standard seismic phases, particularly in the southern hemisphere. Using a variant of cluster analysis, we have created a new data set of long period diffracted S and P times, consisting of 20,000 diffracted S and 31,000 diffracted P measurements, which significantly improves coverage at the CMB. Finite frequency kernels, made using adjoint methods, for diffracted phases are quite different from ray theory kernels and tend to have increased sensitivity to structure at depths above the CMB. Differences in travel time anomalies predicted for a model using the two different theories can often exceed one second, which is a significant fraction of the observed signal. However, preliminary modeling of Pdiff using finite-frequency kernels, in conjunction with P, PP-P, PKPab, and pP-P data sets, gives models that are very similar to ray theory models, although the finite frequency models tend to have somewhat larger amplitudes. Shear and compressional models derived from our data show large, slow regions at the base of the mantle underneath the Pacific and Africa, circumscribed by fast regions, although the patterns of the shear and compressional models are not well-correlated. To model bulk sound speed, we create bulk sound speed rays by combining our S and P data for common source-receiver pairs and invert directly for bulk sound speed. The now well resolved bulk sound speed pattern is anti-correlated with shear velocity over most of the base of the mantle, confirming the results of previous studies. This anti-correlation suggests the presence of chemical or phase heterogeneity at the base of the mantle.