Seismological evidences for the existence of soft thin layers in the upper mantle under Japan

From the seismological data on the crust-mantle structure under Japan, it is impossible to explain the phase velocities of Love and Rayleigh waves by any single model with a weakly heterogeneous, isotropic upper mantle. The significance of the problem is confirmed by the data on S waves from local mantle earthquakes. To explain these observations, a laminated mantle model is proposed, in which soft horizontal layers are interleaved in hard material. The data support a model having 2% soft material with shear velocity as low as 1.1 km/sec.