Lateral variations at the base of the mantle from profiles of digital S-diff data

Average long-wavelength shear velocity structure in several regions at the base of the mantle is determined using the profiles of core-diffracted SH w aves (SHdiff) There is significant lateral variation in the S-diff apparent ray parameters, suggesting considerable lateral shear velocity anomalies i n D ", the region just above the core-mantle boundary (CMB). Apparent ray p arameters are determined by least squares fits through the pulse maxima of the instrument-deconvolved ground displacements, and heterogeneities are qu antified through comparisons with reflectivity synthetic ray parameters. Co rrections are applied to the data to account for the effects of the Earth's ellipticity and of mantle heterogeneities along the SHdiff upswing paths. A total of 161 SHdiff profiles were obtained, greatly expanding the number of ray parameter measurements reported in previous studies. Most of the obs erved slownesses fall in the range between 8.2 and 8.8 s/deg. These corresp ond to slowness anomalies of +/- 3.5%. The most robust and extensive featur e resolved is under the northern and northeastern Pacific Ocean. The slowes t values (Delta p = + 4%) occur toward the southeast, and there is a trend toward fast velocities moving to the northern and eastern rims of the Pacif ic. A model of D " shear velocities is obtained by converting the slownesse s to velocity anomalies, superposing the D " path profiles onto the CMB, an d applying a weighted moving cap spatial average. This model agrees well wi th many current tomographic models, both at large (similar to 5000 km) and intermediate (similar to 1000 km) scales. The fact that this occurs with a different type of data and technique of analysis suggests that we are now a ble to recognize some of the smaller-scale lateral variations at the base o f the mantle. Locations of fast and slow velocity anomalies at the CMB are consistent with the model of cold paleoslabs pending at the CMB and forcing D " rock laterally to form hot aggregates that give rise to plumes in the mantle and hot spots at the surface.