Evidence for a sharp lateral variation of velocity at the core-mantle boundary from multipathed PKPab

Rapid changes in differential travel times between the various PKP branches have been attributed to strong lateral velocity variations at the base of the mantle, Differential time PKPab-df residuals showing jumps of up to 2 s for paths only 100 km apart have been reported. Such extreme changes in tr avel times for signals with comparable wavelengths suggest ray bifurcation with multipaths containing slow and fast contributions to PKPab. Here we re port on some well-documented normal and multipathed PKPab phases along with rapidly varying differential time residuals observed for Fiji events with paths sampling the northern edge of the mid-Pacific large slow structure be neath Hawaii. Systematic analyses of these data with two-dimensional models suggest the presence of a ridge-shaped structure at the core-mantle bounda ry containing abrupt reductions of P-wave velocity of up to 10%. This narro w ultralow velocity zone (ULVZ) is located at the boundary between a large scale low velocity and high velocity structure. corroborating recent dynami c models which propose that ULVZs preferentially occur at the boundary betw een hot and cold mantle domains and could be related to plume activity at H awaii. (C) 2001 Elsevier Science B.V. All rights reserved.