Regional variations of amplitudes of ScSp phases observed in the Japanese Islands

ScSp phases are examined with special reference to their amplitude variations over seismograph networks in the Japanese Islands. ScSp of relatively large amplitude are observed along the Pacific side of the Japanese Islands. Amplitude ratios of the ScSp with respect to ScS are explained in terms of ScS-to-P conversion at the upper boundary of a descending Pacific plate which dips at about 30° and whose seismic wave velocity is between 5 and 10 per cent higher than that of the surrounding mantle. Large-amplitude ScSp are observed at stations nearest the Kurile or Japan trench. The amplitude ratios of ScSp/ScS are found to decrease rapidly with distance inland from the trench. An estimate is made of the effects of sharpness of the conversion interface, the incident angle of ScS upon it, and lateral variations of anelasticity within the asthenosphere overlying the descending Pacific plate, upon the ScSp/ScS ratios. We show that the sharpness of the conversion interface is the dominant factor. This result implies that the nature of the ScS-to-P conversion interface varies with distance from the trench or equivalently with depth. The presence of a low-velocity zone in the vicinity of the conversion interface is inferred from the relative polarity between ScSp and ScS observed at the stations on the outermost side of north-eastern Japan. ScSp arrivals from shallower conversions at depths less than about 80 km, observed in central and south-western Japan, are also examined to elucidate the structure of the Philippine Sea plate descending beneath this region. A discussion is made on the correlations between dipping subcrustal seismicity and the ScS-to-P conversion interfaces. Except for the Chugoku region, the ScS-to-P conversion interfaces correlate well with the dipping subcrustal seismicity. Our ScSp observations indicate the presence of a dipping discontinuity in the uppermost mantle beneath the Chugoku region, where no subcrustal earthquakes have been detected.