Subducted lithosphere or 530 km discontinuity?

Tomographic images of the mantle west of Japan and south Kuril islands suggest that in this region the subducted lithospheric slabs instead of penetrating the lower mantle, lie flat on top of it [van der Hilst el al., 1991; Fukao el al., 1992]. To test these models with independent data and techniques, we analyse the slowness of the P arrivals of Kuril-Japan seismic events at a seismic network in Baikal region. Relative to IASP91 standard earth model, the flat lying slab should generate an additional refraction line at epicentral distances less than 30 degrees. The presence of this line is very clear in our data. The slope of the line and its position relative to the other lines are qualitatively consistent with the estimates of the slab velocity and thickness in the tomographic models. However, a similar wavefield can be generated by 530 km discontinuity, the presence of which is established by long-range refraction profiling in the neighbouring regions of northern Eurasia.