1991 Volume 100 Issue 4 Pages 503-513
A review of the seismicity in Izu-Ogasawara region is presented. In this region, earthquakes occur associated with the interaction of the subducting Pacific Plate and the overriding Philippine Sea Plate. The Wadati-Benioff zone, the deep seismic zone of the descending salb, is steep in this region. The Japan Trench subduction zone, which is the northern extension of Izu-Ogasawara arc has a less steep dip angle. The Mariana slab, the southern extension of the Izu-Ogasawara slab dips almost vertically. Within the Izu-Ogasawara slab, the dip angle becomes steeper towards the Mariana arc.
The seismic activity differs at three depth ranges. The top zone, shallower than 100 km depth, is active probably due to plate interaction at subduction. The middle zone between the depths of 100 km and 300 km is low in activity. The bottom zone is an active seismic zone, outstanding in the western Pacific region. This bottom active zone deepens toward the south. At the southern deepest end of the slab, however, the slab is suggested to be bent horizontally. No earthquakes occur deeper than 600 km.
Earthquake mechanisms in this region can be characterized by locations in the plate. There are several groups of different mechanisms indicating the regional stress field: normal type caused by plate bending seaward of trench; thrust type were two plates contact; high angle normal fault type within the descending Pacific Plate; down dip compression type near bottom of the top active zone; and tensile type in the back arc region suggesting an east to west extensional field. Finally, a peculiar tsunami earthquake in 1984 that suggests a magma intrusion in the back arc region is described.