Seismic evidence for a thinner mantle transition zone beneath the South Pacific Superswell

Broadband seismograms recorded by the seismic stations deployed on oceanic islands in the South Pacific for two deep earthquakes in 1998 are used to i nvestigate the mantle transition zone structure beneath the South Pacific, where a large-scale hot plume might ascend from the core-mantle boundary (C MB). By stacking S waves reflected at the mantle transition zone discontinu ities, we find clear signals associated with the 410-km and 660-km disconti nuities. The transition zone thickness, which is constrained from the trave l time difference between the reflected waves from the 410-km and 660-km di scontinuities, is observed to be approximately 15 km thinner beneath the So uth Pacific Superswell than that beneath other regions. The result suggests that the transition zone temperature beneath French Polynesia is approxima tely 100 similar to 200 K higher than the surrounding mantle.