Preliminary evidence for a lower mantle shear wave velocity discontinuity beneath the central Pacific

In a dataset consisting of long-period waveforms (5-20 s) and differential travel times of S, ScS, and the arrival from the D″ layer, Scd, evidence is found for a laterally varying shear wave velocity (V_S) discontinuity at the base of the mantle. Two different localized D″ regions beneath the central Pacific have been investigated. Predictions from a model having a V_S discontinuity 180 km above the core-mantle boundary (CMB) agree well with observations for an eastern mid-Pacific CMB region. This thickness differs from V_S discontinuity thicknesses found in other regions, which average near 280 km. The data presented here from a western mid-Pacific CMB region are more complicated, and are difficult to fit with a one-dimensional (1-D) structure. Some of the data from the western region show evidence for a thicker D″ layer (approximately 280 km), though this finding is poorly constrained. Our data do not resolve the 'sharpness' of the V_S jump at the top of D″, i.e. the depth range over which the V_S increase occurs, and in fact may be modeled equally well by a lower mantle with the increase in V_S at the top of D″ occurring over a 100 km depth range. The Scd-S and ScS-S differential travel times were corrected for a published 3-D mantle model, to study upper and middle mantle heterogeneity effects on these times.