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We estimate Qs values (quality factor for S-wave) of the slab near the southern Kurile trench by using seismic data observed at ocean bottom seismometer (OBS) stations from selected local earthquakes. The seismic rays pass mainly through the slab and this enable us to directly estimate the slab Qs values. The spectral inversion and coda normalization method are applied to these data. The estimated Qs values from the two methods are nearly the same and increase with frequencies. However, these Qs values are not high enough to explain the abnormal distribution of ground motion that has been recognized as abnormal distribution of seismic intensities at the Japanese arc. Thus we propose a two-layer Qs structure of the slab to explain the both facts, our slab Qs values and the abnormal distribution of ground motion. The two-layer Qs structure consists of the upper layer with not so high Qs values (39f1.0) and lower layer with very high Qs values (500f1.0); the upper layer has a thickness of about 50 km. Seismic activity is restricted only within the upper layer of the two-layer Qs structure. This may mean that the two-layer Qs structure reflects the different material property between the upper and lower layer of the slab.

Two-layer Q s structure of the slab near the southern Kurile trench