Imaging mantle discontinuities using least squares Radon transform

Differential traveltimes of SS precursors have been widely used to map large-scale mantle structure and depths of discontinuities. Measurements are commonly made by stacking individual records to increase the signal-to-noise ratio pertaining to these mild reflections. However, ray parameters of the SS precursors are typically less well constrained and undesired seismic arrivals with vastly different slownesses ( for example, scattered waves) could potentially contaminate the time domain stacks. To overcome these pitfalls, we introduce a processing scheme based on the Radon transform and well-constrained inversions. Our method is particularly effective in suppressing background noise, constraining differential ray parameter and traveltime, and detecting weak reflected or converted phases. We apply the Radon-based method to delineate the discontinuity structure beneath the northeastern Pacific Ocean and the northwestern Canada. An elevated 410-km discontinuity and a thickened transition zone ( 252 km) are observed beneath the northern British Columbia, which may be caused by remnant oceanic lithosphere from the subduction of Kula-Farallon plate under North America. A thin transition zone is identified beneath the northern Pacific Ocean and its presence is supported by a low shear-velocity anomaly from recent tomographic models. The improved accuracy and resolution using the least squares Radon transform also offer clear evidence for a regional 520-km discontinuity and several mild reflectors in the depth ranges of 250-330 km and 900-1200 km. We do not observe a 220-km discontinuity beneath the study region.