Interaction of subducted slabs with either a phase change or a chemical boundary, or both, at 650 km may determine important aspects of mantle convection and mixing in the mantle. Observations of S-P conversion from the discontinuity provide a means to map deformation, or disturbance of the discontinuity beneath seismically active zones. The research proposed here is primarily an extensive and detailed study of the phase S650P beneath Tonga and other western-Pacific subduction zones. The phase S65oP has been tentatively observed by other researchers, using the short period Warramunga Seismic Array (WRA) in central Australia, from deep events beneath Tonga and the Izu- Bonin arc. A much larger data set from WRA, including more than 300 deep events from Tonga, has been collected. With this 8-fold increase in high quality data, the following exciting results have been obtained from preliminary analysis: (1) much clearer observation of S650P than before, (2) geographical variation in the apparent depth of the discontinuity (deformation), between 650 and 720 km, and (3) observation of other possible converted phases.
