A Study Of Transition Zone Discontinuities Beneath The Northeastern Pacific Ocean Using The Radon Transform

Transition zone discontinuities highlight density and velocity variations in the mantle and provide key information on the Earth's thermal and chemical evolution. Since SS precursors are highly sensitive to upper mantle discontinuities, subtle structural perturbations can be detected using differential travel times of SS and its precursors. In this study, we determine the depth of transition zone discontinuities beneath the northeastern Pacific Ocean by applying the Radon transform to a selected SS precursor data set. While stacking is commonly used to increase the signal-to-noise ratio in SS precursor travel time analysis, ray parameters are typically less well constrained. Furthermore, undesired seismic arrivals with vastly different slownesses (for example, scattering waves) could potentially contaminate SS precursor stacks. To overcome these pitfalls we have adopted a processing scheme based on Radon transform. Through well-constrained inversion, the Radon transform is proved to be particularly effective in suppressing background noise, isolating the desired slowness window, and determining high-resolution tau-p phase attributes. We obtained depth estimates of the transition zone discontinuities from the optimal tau-p value of each 5-degree cap. The depth ranges of the 410-km and 660-km discontinuities are determined to be 391-420 km and 640-667 km, respectively, beneath the northeastern Pacific. A low-velocity anomaly inside the transition zone is revealed by seismic tomography, which is consistent with a locally depressed 410-km discontinuity and an elevated 660-km discontinuity. This anomaly suggests higher-than-average temperatures or the presence of partial melt within the transition zone.