Toward Constraints on Lateral S Wave Velocity Gradients around the Pacific Superplume

Akiko To, Barbara Romanowicz

Introduction

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.

Preliminary Results

We have assembled a large dataset of Sdiff waveforms and travel time throughout the Pacific region. As shown in Fig.31.1, the new dataset has more sampling than the original one especially in the south Pacific. The original dataset was used to construct the original model, SAW24b16 (Mégnin and Romanowicz, 2000). We first apply a finite frequency tomographic inversion methodology (NACT, Li and Romanowicz, 1996), which provides a good starting 3D model. In particular, the inversion method allows us to position the fast and slow anomalies and their boundaries quite well, as has been shown previously, but underestimates the gradients and velocity contrasts. The result of the inversion using the newly obtained dataset shows shrinking of the Pacific superplume area in its northern and southern edge (Fig.31.2). Based on this starting model, we will perform forward modeling to search for further constraints on the gradients at the boundary of the Pacific superplume.

Figure 31.1: Sampling density of the collected Sdiff data. Top: The Sdiff phase raypath distribution of the previously collected dataset. The data is used in the construction of the original model, SAW24B16. Bottom: The raypath distribution of the newly obtained dataset. Compared to the previous dataset, there are more samplings in the south Pacific.

Figure 31.2: Contours of the Pacific superplume. Gray line: Original model, SAW24B16. Black line: New model, obtained from the recently collected dataset. The lines are the contour lines of -0.4% S velocity anomaly at a depth of 2820km. The east sides of the contours overlap each other, whereas the contours are shifted on the north and south side in the new model.

Acknowledgements

The data were downloaded from IRIS DMC and CNSN.

References

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