We observed that the SHdiff phases generated by earthquakes in the Fiji-Tonga, recorded by Indian seismic array, are accompanied by secondary pulses (Figure 1A-B, E). The secondary pulse is sharper on the northern part compared to the southern part of array, with azimuthal variation. We choose to interpret the second phase as the direct SH wave that samples the top edge of the large low-shear velocity province (LLSVP) and then gets refracted (or diffracted) before reaching the receiver. We attempt to model the relative timing of the first and secondary pules as well as the absolute arrival time of the first pulse by incorporating varying thickness and shear wave velocity on either side of the boundary (Figure 1C-D). Our preferred model shows that the western edge of the Pacific LLSVP extends in the NE-SW direction and there is a lateral variation in topography; the southern part reveals a thicker (300 km) low-velocity region compared to the northern part (200 km) (Figure 1E). However, the velocity perturbations of the LLSVP appear to be compatible (-1.5%). The results are consistent with a scenario that LLSVP is a chemically distinct pile with significant surface topography.
Figure 1. Displacement waveforms recorded by the Indian network for the event D from the Fiji-Tonga slab. The waveforms are plotted in terms of (A) azimuth and (B) distance. The observed differential traveltimes (black pluses) between the second and first pulses as a function of distance for the (C) northern and (D) southern seismic array, respectively. (E) Source-receiver geometry of our data set for the five deep earthquakes from the Fiji-Tonga slab (filled star) and the Indian seismic array (inverse triangle). The magenta and dark green lines demarcate a ray segment of SHdiff phase on the core-mantle boundary (CMB) for the northern and southern seismic array, respectively. Traveltime residuals (aqua circles) projected at the entry point of the SHdiff ray path to the CMB. The background color represents the shear wave velocity variation in the lowermost mantle in the model SH18CE (Takeuchi, 2012).
Roy, S. K., Takeuchi, T., Srinagesh, D., Kumar, M. R., Kawakatsu, H., 2019. Topography of the western Pacific LLSVP constrained by S-wave multipathing, Geophys. J. Int. 218, 190-199.