Seismic study of upper mantle and transition zone beneath hotspots

We compute receiver functions for 18 permanent seismic stations on oceanic islands after selecting earthquake records with high signal-to-noise ratio. P-to-S conversions from the 660 km discontinuity are well observed at all except one station. The 410km discontinuity is in some cases not equally well observed. All measured 410 conversion times are close to or later than predicted by IASP91, some are as late as to 6s. This extreme case corresponds to averaged velocities in the upper mantle down to 410km depth that are 15% lower than in the IASP91 reference model. The transition zone thickness, derived from differential times of the P-S conversions from the 410 and 660km discontinuities, is similar or up to 15% (4 s in time) thinner than predicted by the IASP91 reference model. We found no case of an oceanic station with 4 10 arrival times earlier and only one station with transition zone thickness larger than predicted by IASP91. We have also identified measurable topography of the transition zone underneath a few stations. (C) 2003 Elsevier Science B.V. All rights reserved.