Waveform inversion for localized seismic structure and an application to D structure beneath the Pacific

In order to extract more information on localized seismic structure from observed seismic data, we have developed and applied a new method of waveform inversion. The calculation of synthetic seismograms and their partial derivatives is the key steps in such an inversion. We formulate the inverse problem of waveform inversion for localized structure, computing partial derivatives for the 1-D and 3-D anisotropic elastic moduli at particular depth or at points in space. Our method does not use any great circle approximations in computing the synthetics and their partial derivatives. In this study we invert for the vertical dependence of the shear velocity in the lowermost mantle beneath the Pacific using the transverse component of broadband waveforms for the period range 8-200 s. We find 1%-1.5% velocity decreases and increases in the zones from 400 to 500 km and from 300 to 400 km above the core-mantle boundary (CMB), respectively. In addition, we find 0.5%-1% velocity increases and decreases in the zones from 100 to 200 km and from 0 to 100 km above the CMB, respectively. This is interpreted as evidence for phase transitions between perovskite and postperovskite (although a chemical stratification model cannot be excluded).