We have assembled a large dataset of Sdiff waveforms and travel time throughout the Pacific region.
Also in other regions bordering the South Pacific superplume, observed Sdiff travel times vary rapidly over small ranges of azimuth and/or distance (Figure 13.51).
The observed travel times can be better fit by increasing the amplitude and lateral gradients of the large scale velocity anomalies in a tomographic S velocity model.
Modeling the pulses and the travel time jump due to lateral heterogeneity can help constrain the shape and velocity contrast at the superplume boundaries, at the base of the mantle.
Synthetic waveforms calculated by CSEM down to 8 seconds. [Left] The waveforms from the original tomographic model (Figure 13.50 left panel); [Middle] the waveforms from the modified model (Figure 13.50 right panel). Gray lines follow the first trough, black lines follow the secondary arrivals.
[Right] Observed velocity waveforms for event 19970904 in Fiji-Tonga (6.8) recorded in South Africa. Bandpass filtered with corner frequencies at 0.01 and 0.125 Hz. Y-axis shows the back azimuth. The broken line is the expected Sdiff arrival for the PREM model. Gray lines follow the trough of the first pulse. Black solid lines follow the secondary pulse, which is only observed in the vicinity of the structural boundary.
Left panel: the original SAW24B16 (Mégnin and Romanowicz, 2000) model at three depths in the bottom 300km of the mantle. The source in Fiji Tonga region is located at the apex. The stations in Africa are shown by triangles.
Right panel: a model which is modified from SAW24B16. The boundary of the fast and slow anomalies is the contour line of 0% anomaly of SAW24B16. The anomaly jump is from -2.75 to 1.75%.
Both models have 1D PREM structure from surface down to 370 km above the CMB. The 3D velocity anomalies linearly increase from 370 to 300 km above the CMB.
[Left] Sdiff ray path distributions where the rapid travel time shifts with respect to azimuth or back azimuth are observed. The background color map shows SAW24B16 model at CMB. Yellow circles and green triangles show the observed Sdiff travel time residuals with respect to PREM. Symbols are plotted at the middle point of the diffracting portion at CMB. White star and white triangles show the epicenter (Event1) and the stations (YAK and BDFB) which are discussed in the right figure. [Right]The Sdiff travel time residuals as a function of azimuth (for Event1) and back azimuth (for YAK and BDFB).
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