The Sdiff waveforms which sample South Pacific superplume or the southeastern edge of the African superplume both show a rapid shift of the arrival time with respect to azimuth and they are followed by postcursors. In order to explain these features, we modified a tomographic SH model by increasing the gradient between the fast and slow anomalies, but keeping the shape of the boundary fixed (Figure 13.50 (Right)). We used the coupled mode/spectral element method (CSEM, Capdeville et al., 2003), which can handle strong lateral variations of velocity in the D", to construct synthetic waveforms. The synthetics from the original tomographic model (Figure 13.49 (Left)) do not generate the secondary arrival or the rapid shift of the first arrival. On the other hand, the synthetics from the modified model with the sharp boundaries (Figure 13.49 (Middle)) capture the features of the observed waveforms. The move out of the secondary arrival, which actually appears in multiple branches, shows a slope which is consistent with observations, although it appears at a slightly different azimuth. Moreover, the jump of the first arrival occurs around the azimuth of 215 degrees, which is also consistent with the observations. Particle motion analysis of observed and synthetic waveforms shows that the first pulse arrives from the southern side and the second pulse arrives from the northern side. Both the first and second arrivals are estimated to be refracted waves and their paths are described schematically in Figure 13.50 by yellow and green lines, respectively. The result suggests that it is important to take into account the heterogeneity outside of the great circle path.
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