Abstract
SUMMARY The lowermost mantle is investigated using a data set of high-quality waveforms and a phase stripping technique, which removes the S and ScS phases in order to isolate more subtle arrivals. Two migration methods were applied to locate scattering bodies in the lower mantle. The first, a simple backprojection, is useful for identifying regions of strong scattering. The second migration scheme uses weights based on the generalized Radon transform. We present results for a region of lower mantle beneath Alaska and western Canada, and test the influence of experimental geometry on the migration results by migrating synthetic data sets generated for various distributions of point scatterers. We show that, although the exact geometry of scattering bodies is poorly constrained, the majority of the features in the data can be recreated with a few simple structures. The results show the presence of a D �� discontinuity, which can be simulated using a flat scattering body ∼250 km above the core‐mantle boundary near the Alaska‐Canada border. We also present evidence for positive velocity anomalies in the bottom 100 km of the mantle, beneath the D �� reflector and near the Aleutians. The association of these scattering structures with a region of high velocity is consistent with a cause related to lower mantle heterogeneity introduced by a subducting slab.
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