Precise focal mechanism study of earthquakes occurring in Hokkaido and northern Honshu is made by using 35 stations of microearthquake observation networks of Tohoku University and of Central Research Institute of Electric Power Industry.
In the northern Honshu and at the junction between the Northeastern Japan Arc and the Kurile Arc, the upper seismic plane of the double-planed deep seismic zone is characterized by down dip compression and the lower seismic plane by down dip extension. The events occurring in the shallower part of the upper seismic plane (shallower than 60km in the northern Honshu and shallower than 70km at the junction) have low-angle thrust fault type.
The upper seismic plane is not down dip compression but has various types of focal mechanisms beneath Hokkaido, the southwestern end of the Kurile Arc, although the shallower part of it has low-angle thrust fault type as the case of northern Honshu. This upper seismic plane disappears at the depths greater than about 120km. The lower seismic plane is characterized by down dip exension, which is the same as northern Honshu. In Hokkaido, the lower seismic plane with down dip extension type extends to the deeper part down to about 300km depth. On the contrary, in nortern Honshu, it is the upper seismic plane with down dip compression type that extends to the deeper part down to at least 200km depth.
The characteristics of the double-planed deep seismic zone in the southwestern end of the Kurile Arc may suggest relatively strong slab pull force superimposed on the stress system generating the double-planed deep seismic zone.
