Fine scale imaging of structures at and near the mantle transition zone using a generalized Radon transform

Abstract

We apply a generalized Radon transform (GRT) to the study of the transition zone discontinuities. The transition zone discontinuities, e.g. the '410' and '660', result from mineral phase changes that occur at depths constrained by temperature and chemistry, and can provide information about the conditions of the mantle within the transition zone. Previous global studies of topography on the transition zone discontinuities use SS data and are limited by the low lateral resolution provided by SS waves, on the order of about 1000 kilometers. The GRT employs inverse scattering theory to image perturbations in mass density and elastic parameters of a medium, and can resolve structure on the order of 100 kilometers; the limit of this resolution has yet to be tested. This study maps discontinuity depths in the northwest Pacific Ocean with a lateral spatial sampling of 1° and a vertical sampling of five kilometers. We observe striking variations in the depth, strength, and continuity of the '410', '520', and '660' seismic discontinuities, as well as the presence of structure beyond the bounds of what is traditionally considered to be the transition zone. Topographies on the '410' and '660' are alternately positively and negatively correlated, suggesting that both composition and temperature contribute to the observed depths. Preliminary analysis of '410' depths, which assumes no variations in chemistry, yields an upper bound for temperature variations of ±280K. Future tests and data preprocessing should further improve the GRT results.