Abstract
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.
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