Fine Scale Imaging of Structure at and Near the Mantle Transition Zone Using a Generalized Randon Transform

The transition zone discontinuities, e.g. the '410' and '660', result from mineral phase changes that occur at depths constrained by temperature, pressure, and mineralogy, and detailed images of them can provide information about thermal and chemical variations in the upper mantle. We apply a generalized Radon transform (GRT), modified from underside core-mantle boundary imaging with SKKS (Wang et al., GJI, 2007), to image the transition zone discontinuities with the broadband wavefield (5-75 sec) containing precursors to SS. Previous studies of topography on the transition zone discontinuities mostly use stacks of SS data with lateral resolution of order of about 1000 kilometers. The GRT employs inverse scattering theory to detect and characterize perturbations in mass density and elastic parameters of a medium and can resolve lateral variations in structure at lateral scale lengths of 100 km. We map upper mantle discontinuities beneath the northwest Pacific Ocean with a lateral spatial sampling of one degree and a vertical sampling of five kilometers. We clearly detect interfaces near 410, 520, 660, and, more tentatively, near 800 km depth. In cross section, the '410', '520', and '660' reveal substantial variations in strengths, depth, and width. Moreover, the pulse shapes in the reflectivity profiles are frequency dependent. The '520' has larger topography than '410' and '660' and, locally, appears stronger than the '410'. Split pulses occur locally for '520' and '660'. We also detect (broader) scatterers outside what is traditionally considered to be the transition zone (e.g., near 800 km depth), but this signal shows significant lateral variation and may not represent globally continuous structures.