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.