Nobuaki Fuji

Abstract: We develop a quantitative and objective methodology for inversion of body-wave waveform data for elastic and anelastic structure and apply it to the upper mantle and mantle transition zone (MTZ) beneath the Northwestern Pacific, using PREM as the starting model. We derive the Jacobian for elastic and anelastic perturbations to the initial model in order to efficiently calculate the partial derivatives. This study is the first attempt to invert body-wave waveforms to estimate the radial dependence of regional anelastic structure. As waveform inversion has often been thought to be a �black-box-like� procedure, we explicitly show that the model obtained by the inversion is just the projection of the data onto the model space using orthogonal vectors. We use the singular value decomposition (SVD) as the basis for our inversion and use Akaike�s Information Criterion (AIC) to evaluate the statistical significance of the models and determine the appropriate size of the basis. Models are examined using several statistical tests and also confirmed by comparison to models obtained using the conjugate gradient (CG) method. We obtain an average positive S-wave velocity anomaly in the MTZ beneath the Philippine Sea Plate and the Japan Sea. Our inversions obtained a lower Qμ, as compared to PREM, for all of the subregions. A slight sub-regional dependence of Qμ structure is found. Beneath the Philippines Sea region, we find Qμ 50 throughout the MTZ down to 660 km, while we find an increasing Qμ at depths of 500-600 km beneath the Pacific. This may reflect the water rich content beneath the mantle wedge driven down to the lowermost part of the MTZ in the Philippines Sea.