Travel-time residuals of P and S waves from local deep earthquakes are used to apply geophysical constraints to the laterally heterogeneous structure associated with the Japan subduction zone. Theoretical residuals for two-dimensional upper mantle models that have high-velocity dipping slabs are calculated by applying Jacob's (1970) ray-tracing method. The forms of the spatial distribution of the observed and the calculated residuals are matched to determine the velocity contrasts across the asthenosphere and the subducting lithosphere boundary and a possible velocity structure inside the slab.
We find 5 ± 1 per cent for Vp contrast and 7 ± 2 per cent for Vs contrast. These contrasts cannot simply be explained by a temperature effect alone and it is likely that other mechanisms such as partial melting in the asthenosphere are required for the observed velocity change.
The data indicate that the subducting lithosphere can be modeled by a two-layer structure similar to that found for oceanic lithosphere (Shimamura and Asada, 1976) but with a smaller velocity contrast. Two-layer model calculations show that the subducting lithosphere is about 120 km thick. The upper layer of the slab is about 40 to 50 km thick and has about 1 per cent lower velocity than the underlying layer.