Introduction

Global shear velocity tomographic models show two large-scale low velocity structures, so-called superplumes, in the lower mantle, under Africa and under the mid-Pacific. Sharp lateral velocity changes have been documented for some parts at the borders of the superplumes (e.g.Wen, 2001, To et al., 2005, He et al., 2006). We evaluate the distribution and amplitude of the anomalies given by a tomographic model for the D'' layer around the Pacific region by measuring travel times of Sdiff, SKS and SKKS phases. We search for sharp lateral velocity changes, which have not been found yet, at the borders and also inside, according to tomographic models, the superplume.

Figure 2.61: (a) Top panel: Distribution of diffracting portion of all the Sdiff paths whose travel time anomalies are measured. The background model is SAW24B16 (Mégnin and Romanowicz, 2000) at the depth of 2850km. Bottom panel: Sdiff travel time anomalies with respect to PREM is plotted at the midpoint of the paths. (b) (c) (d) Top panel: Sdiff travel time anomalies are plotted at the midpoint of the path. Diffracting portion of Sdiff waves are shown in thick gray lines. The back ground model is SAW24B16. The event locations are shown by a stars. The station locations are shown by triangles. Bottom panel: Travel time anomalies of Sdiff (solid red circle), SKS (blue triangle) and SKKS (green square) phases with respect to azimuth or back azimuth. Synthetic travel time anomalies obtained by 1D ray theory for SAW24B16 are shown by open red circles.