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 differen tial 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)) d iscontinuity at the base of the mantle. Two different localized D'' re gions 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 thicknesse s found in other regions, which average near 280 km. The data presente d 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. Ou r 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 f act 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.