UC San Diego

The base of the mantle is characterized by many unusual features, including the anti-correlation of shear velocity and bulk sound speed. The spatial extent of the anti- correlation, however, is not well determined, due to poor coverage by standard seismic phases. To improve coverage at the base of the mantle, we created a new data set of 27, 000 Sdiff and 35,000 Pdiff arrival times from long period seismograms, using a new picking technique. The inclusion of diffracted data in our inversions greatly improves coverage and resolution at the base of the mantle. Earthquake mislocation is a potentially large source of error in tomographic inversions. We compared three methods of dealing with location error. We find that models made using iterative relocation do not fit the data well, whereas models made using projection and a joint inversion for perturbation in velocity and earthquake location do. An examination of the effects of different relocation techniques on the correlation of shear velocity with bulk sound speed shows that iterative relocations results in lower and possibly negative correlations between shear velocity and bulk sound speed over the entire mantle. We present the results of inversions using ray theory and finite frequency kernels for our new data set of long period Sdiff and Pdiff travel time measurements. Finite frequency kernels for diffracted phases are quite different from ray theory kernels and have increased sensitivity to structure at depths well above the CMB. Shear velocity models derived from our data are very similar, with the finite frequency model showing larger amplitudes at the base of the mantle. Compressional velocity models display greater differences in the lower mantle, particularly at shorter wavelengths. To model bulk sound speed, we jointly inverted for shear velocity and bulk sound speed. Bulk sound speed is anti-correlated with shear velocity over most of the base of the mantle, starting at a depth of 2000 km, confirming the results of previous studies. This anti-correlation suggests the presence of chemical or phase heterogeneity at the base of the mantle