STUDY OF THE STRUCTURE OF THE SLAB MANTLE INTERFACE USING REFLECTED AND CONVERTED SEISMIC-WAVES

Seismic waves reflected and converted at the interface between a downg oing slab and the mantle above it can provide important constraints on the velocity contrast at the interface and hence on the properties of the downgoing lithosphere. Modelling studies suggest that different p hysical mechanisms may contribute to the velocity contrast at differen t depths along the interface. To explore the possible mechanisms, we e xamine the amplitudes of seismic waves reflected and converted at the slab-mantle interface in Japanese subduction zones. Slab-face P-reflec tion amplitudes suggest a velocity contrast ranging upwards to approxi mately 10 per cent at depths of between 300 and 400 km. P/S amplitude ratios from ScS-to-P conversions at the slab-mantle interface depend o n the depth of conversion and are large, 0.14 and 0.12, at stations SH K and MAT. The size of the velocity contrasts and amplitude ratios sug gests that a model of the interface more complicated than one involvin g merely temperature and/or compositional effects is required. We use these observations to constrain layered models of the slab-mantle inte rface using Thomson-Haskell matrix modelling, comparing observations w ith calculated amplitude ratios. The results suggest that the crustal layer of basaltic composition, persisting to depth during subduction, plays a role in the generation of the shallow converted waves. The dee per conversions appear to require layering as well, perhaps involving eclogite at the slab surface in addition to oriented olivine in the ov erlying mantle and the alpha-beta (and alpha-gamma) phase transformati ons in olivine within the slab.