Study of the structure of the slab-mantle interface using reflected and converted seismic waves

Abstract

Seismic waves reflected and converted at the interface between a downgoing 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 physical mechanisms may contribute to the velocity contrast at different depths along the interface. To explore the possible mechanisms, we examine the amplitudes of seismic waves reflected and converted at the slab-mantle interface in Japanese subduction zones. Slab-face P-reflection amplitudes suggest a velocity contrast ranging upwards to ~ 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 on the depth of conversion and are large, 0.14 and 0.12, at stations SHK and MAT. The size of the velocity contrasts and amplitude ratios suggests that a model of the interface more complicated than one involving merely temperature and/or compositional effects is required. We use these observations to constrain layered models of the slab-mantle interface using Thomson-Haskell matrix modelling, comparing observations with 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 deeper conversions appear to require layering as well, perhaps involving eclogite at the slab surface in addition to oriented olivine in the overlying mantle and the α−β (and α−γ) phase transformations in olivine within the slab.