Keywords:core-diffracted S waves, Core-Mantle boundary, mantle plumes, multipathing
The waveforms of five deep earthquakes from the Fiji-Tonga subduction zone recorded by a seismic array in India reveal a secondary pulse just after the Sdiff phase. We obtained 51 observations for this pulse, in the distance range of 102.0° to 115.1°. The pulse is sharper on the northern part compared to the southern part of the array, with a azimuthal variation. The relative arrival time of the second pulse varies from 3.0 to 9.9 s with respect to the first pulse, with its arrival getting delayed from north to the south along the seismic array, albeit the south array being closer to the earthquake sources. At present, we choose to interpret the first pulse in terms of the Sdiff phase whose ray path reaches the CMB inside the Pacific Large Low-Shear-Velocity Province (LLSVP) and then passes along the vertical side of the LLSVP. The second pulse is probably the direct S and its diffracted at the top of the LLSVP. The ray path goes into the LLSVP but bottoms above the CMB. It appears that the second wave seems to have stronger amplitudes when the ray bottoms inside the LLSVP and the sharp vertical boundary lies after the bottoming point. We attempt to model the relative timing of the first and secondary pulses as well as the absolute arrival times of the first pulse by incorporating varying thickness and shear wave velocity on either sides of the boundary. At the time of the presentation, we plan to show our velocity model including azimuthal variability, to explain the observed data.