東北日本弧における二重深発地震面と発震機構

抄録

A precise relocation of earthquakes using data listed in the Seismological Bulletin of the Japan Meteorological Agency reveals a double-planed structure of the deep seismic zone in the whole region of the northeastern Japan arc. The two planes are nearly parallel to each other and the distance between them is from 30km to 40km. The lower seismic plane appears at depths greater than about 60km in the Tohoku District, northeastern Japan, while that appears at depths greater than about 90km in the Kanto District.
Some interesting characteristics are found from focal mechanism study for large earthquakes and microearthquakes in the upper seismic plane. In the Tohoku District, the predominant fault-plane solution for earthquakes shallower than about 60km is low angle thrust faulting and that for deeper earthquakes is down dip compression. In the Kanto District, similar characteristics are found for earthquakes in the upper seismic plane, except that the depth at which the predominant focal mechanism changes is about 90km. This difference may be closely associated with the subduction of the Philippine Sea plate overriding the Pacific plate in the Kanto District.
In the whole region of the northeastern Japan arc, the earthquakes in the lower seismic plane is characterized by down dip extensional fault. Down dip compressional and down dip extensional earthquakes form a pair in the double-planed deep seismic zone, since the depth at which the lower seismic plane appears is consistent with the depth at which the predominant focal mechanism changes in the upper seismic plane.
The depth to the upper seismic plane just beneath the volcanic front is about 100km in the Tohoku District, whereas the depth is much deeper in the Kanto District, where the triple junction among the Philippine Sea plate, the Pacific plate and the Eurasian plate is situated.