Global shear velocity tomographic models show two large-scale low velocity structures in the lower mantle, under southern Africa and under the mid-Pacific. While tomographic models show the shape of the structures, the gradient and amplitude of the anomalies are yet to be constrained. By forward modeling of Sdiffracted phases using the Coupled Spectral Element Method (C-SEM,Capdeville et al., 2003), we have previously shown that observed secondary phases following the Sdiff can be explained by interaction of the wavefield with sharp boundaries of the superplumes in the south Indian and south Pacific oceans (To et al., 2005). We search for further constraints on velocity gradients at the border of the Pacific superplume all around the Pacific using a multi-step approach.
The data were downloaded from IRIS DMC and CNSN.
Li, X.D. and B. Romanowicz, Global mantle shear-velocity model developed using nonlinear asymptotic coupling theory, Geophys. J. R. Astr. Soc., 101, 22,245-22,272, 1996.
Mégnin C. and B. Romanowicz, The three-dimensional shear velocity structure of the mantle from the inversion of body, surface and higher-mode waveforms. Geophys. J. Int., 143, pp. 709-728, 2000
Capdeville, Y., A. To and B. Romanowicz, Coupling spectral elements and modes in a spherical earth: an extension to the "sandwich" case, Geophys. J. Int., 154, 44-57, 2003
To, A., B. Romanowicz, Y. Capdeville and N. Takeuchi, 3D effects of sharp boundaries at the borders of the African and Pacific Superplumes: observation and modeling,Earth and Planet. Sci. Lett., 233, 137-153, 2005
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