Focal mechanism solutions of 60 deep and intermediate earthquakes (depths greater than 200km) which occurred during the period from 1931 to 1974 in the Izu-Bonin and the Northeastern Japan arcs except in Central Japan, where ITO and ANNAKA (1977) have investigated, have been determined from the first motions of P-waves. Fourteen well determined mechanism solutions published by the previous investigators were also adopted.
Regional variations of the focal mechanism have been studied in detail, with reference to the spatial distribution of hypocenters. The Izu-Bonin arc is divided into eight regions, and the Northeastern Japan arc is divided into two regions, based on the discontinuity of the spatial distribution of hypocenters or the change of the configuration of the deep seismic zone. The predominant types of the focal mechanism in each region are determined.
The axes of maximum compression for the deep earthquakes in most of the regions are dipping nearly westward in the dip angle about 35°. The local dip direction of the deep seismic zone changes remarkably in those regions, but it hardly influences the direction of the axis of maximum compression. The directions of the axes of minimum compression in those regions change from region to region, and they seem to be related to the configuration of the deep seismic zone. The axes of minimum compression are dipping nearly eastward, or nearly horizontal in the north to the south, for the earthquakes in the regions where the local dip of the deep seismic zone is nearly westward. On the other hand, they are dipping nearly northeastward for most of the earthquakes in the regions where the local dip of the deep seismic zone is nearly southwestward, or the configuration of the deep seismic zone changes abruptly.
For the intermediate earthquakes in two arcs, for the deep earthquakes in the southern end region of the Izu-Bonin arc, where the deep seismic zone is near vertical, and for some of the deep earthquakes in the Northeastern Japan arc, the axes of maximum compression aren't dipping nearly westward in the dip angle about 35°, but are dipping nearly northwest-ward. And the axes of minimum compression are approximately perpendicular to the deep seismic zone.
The hypocenters seem to be distributed on one plane for three regions where the earthquakes occur in small and isolated area. And the plane corresponds to one of the two nodal planes of the focal mechanism for the earthquakes in the region. It may suggest that the earthquakes are caused by the relative motion between two sides of the plane.