1989 年 42 巻 1 号 p. 13-19
The characteristic structure of the Wadati-Benioff zone (WBZ) beneath Kyushu is presented from detailed examinations of the space distribution of intermediate-depth earthquakes. The data for this study are from the Japan Meteorological Agency Seismological Bulletins from January 1983 to April 1987.
The WBZ beneath Kyushu, north of the Tokara Channel which lies above the significant transverse discontinuity of the zone, is divided into three segments by two transverse discontinuities which are located in central Kyushu and southern Kyushu (the central part of Kagoshima prefecture).
In the northern segment of about 130km in strike length, the WBZ of 10 to 20km in thickness dips at an angle of about 60° at depths of around 80km, and about 65° in the lower portion extending to a depth of about 140km.
The central segment of about 125km length changes strike from that of the northern segment, N15°E, to N27°E. The WBZ of 20 to 30km thick dips at an angle of about 60° at depths of 80 to 100km, and 60° to 70° at depths of about 100km or more. The deepest portion of the zone is around 170km.
The southern segment of about 250km length strikes to N24°E. The zone of 50 to 60km in thickness dips at angles of 55° to 60° at depthes of about 80km and greater, and extends to about 200km in depth.
The major structure defined by the WBZ seems to be mirrored in the structure of the oceanic region ranging from the zone to the trench axis. The transverse discontinuity separating the northern and central segments may be connected to the tectonic line separating source regions of the 1968 and 1984 large Hyuga-nada earthquakes from those of the other large earthquakes of 1931, 1941 and 1961. The discontinuity separating the central and southern segments connects probably with the tectonic line separating Hyuga-nada region and the southeast of Kyushu region.
The active volcanoes in southern Kyushu (the central and southern segments), which is characterized by a fairly good volcanic lineament, are situated on the WBZ at depths of about 80 to 100km. This is a general characteristic for the active volcanoes in the island arcs. In the northern segment, however, the volcanoes seem to be hardly related to the WBZ. For instance, Asosan volcano lies to the west of the zone, though this volcano is petrologically referred to the Ryukyu type as those in southern Kyushu.