Anomalous distribution of seismic intensities due to the descending Philippine Sea plate beneath the southern Kanto district, Japan.

Abstract

An examination has been made of the distribution of seismic intensities by the earthquakes which occurred in the uppermost mantle beneath the Kanto district, Japan, during the period between 1949 and 1978. Anomalously high seismic intensities were observed on the eastern side of the Izu Peninsula (Ajiro (AJI) and Oshima (OSH)) for the earthquakes at depths between 60 and 70km beneath northern and central Tokyo Bay. High seismic intensities were also observed along the western shore of Tokyo Bay (Tokyo (TOK) and Yokohama (YOK)) and on the eastern side of the Izu Peninsula (Ajiro and Oshima) for the earthquakes in the depth range from 60 to 80km beneath northern and central Chiba. The high seismic intensities observed at Tokyo and Yokohama must have been due to an amplification of ground motion by the thick alluvial deposits along the western shore of Tokyo Bay. It is in our opinion that these anomalously high seismic intensities observed at Ajiro and Oshima are caused by the practically unattenuated transmission of seismic waves through the descending Philippine Sea plate that dips northeastwards from the Sagami trough beneath the southern Kanto district. The existence of this plate had been suggested from seismicity and earthquake mechanism studies, but not confirmed from seismic wave transmission. High seismic intensities on the eastern side of the Izu Peninsula were also observed from the earthquakes at depths between 70 and 80km beneath southern Ibaraki. The earthquakes in the depth range from 30 to 60km beneath southwestern Ibaraki show normal concentric distribution of seismic intensities. These characteristic distributions of seismic intensities can be explained by the locations of earthquakes and the geometry of the high-Q plates.A comment is made on a possible systematic error of epicenter locations of the historically earlier earthquakes along the western shore of Tokyo Bay (Tokyo-Yokohama area).