Three-dimensional velocity structure and resolution of the core-mantle boundary region from whole-mantle inversions of body waves

Abstract

The nature of the core-mantle boundary (CMB) has important implications for deep-Earth processes, particularly those which have their origin in the lower mantle or outer core. The possibility of the existence of lateral P-wave velocity variations in the outermost core is explored here by obtaining models of three-dimensional, CMB region velocity structure using an inversion of waveform data. The numerical inversions involve determination of the three-dimensional structure of the Earth's outermost core and lower mantle, using over 5000 long-period seismic waveforms collected from global digital seismic networks. Spheroidal modes with periods between 33 and 100 s were selected to model the body-wave portion of seismograms recorded from global earthquakes. First-order perturbations in S-wave velocities in 11 mantle layers 200–350 km thick and P-wave velocities in one outermost core layer comprise the least-squares solutions to the inverse problem. Although the statistical F test supports the additional layer in the inversions, this layer is not required by the F ratio to occur in the outermost core. Pattern-retrieval resolution tests, in which synthetic data constructed from an initial synthetic Earth model are inverted for model parameters, are used to compare parameters for one region to those of another in which some coefficient power smearing is suspected. Depth resolution tests indicate that the best radial resolution at the CMB is 500 km. Thus, this method does not produce the resolution required for outermost core lateral velocity variation analysis. An examination of the limitations of this approach demonstrates the need for waveform inversion techniques which more accurately predict raypaths in the deep mantle. Improvements in radial resolution are needed before whole-mantle velocity models can begin to distinguish whether significant heterogeneity related to core-mantle interactions occurs in the CMB region.