The Philippine Sea plate subducting beneath the Japanese Islands is colliding with the Pacific plate at the depth of 50 to 80km beneath southwestern Ibaraki and central Chiba prefectures, the eastern Kanto district. Relocating earthquakes caused by this collision, we investigated focal mechanisms and a fine structure in the source region in detail.
Earthquakes observed by a network of National Research Institute for Earth Science and Disaster Prevention are relocated by the Joint Hypocenter Determination (JHD) method. The JHD method was extended by using a following constraint: A station correction is independent of both a distance and an azimuth from a center of a studied region to a station. This prior information makes results by the JHD method stable.
Almost all earthquakes in this region are thrust events of which nodal planes dip westward at a low angle. They occurred inside a boundary layer with a thickness of about 4km dipping westward at an angle of about 30°, that is a boundary of the subducted two plates. Stress state and a seismic velocity in the boundary layer imply an origin of the layer is a subducted oceanic crust in the Pacific plate. Its size is about 90km in the NS direction and 7-16km in the EW direction. It inclines westward at an angle of about 25° in the northern part, and inclines west-southwest at an angle of about 30° in the southern part. Although all events inside the boundary layer are thrust type of which nodal planes are parallel with this layer, many of them may have occurred along planes which are parallel to nodal planes dipping eastward at a high angle.
However, a magnitude 6.1 earthquake with a vertical fault plane of which east side subsided occurred inside the Pacific plate in Oct. 1985. This event implies coupling of the two plates above it is stronger than that of the surrounding region. Furthermore, a few earthquakes occurring at the western edge of the boundary layer show normal faulting with the NE-SW or ENE-WSW tension axes, which are supposed to be caused by stress concentration at the ends of the boundary layer mentioned above. Therefore these two kinds of exceptional events define the boundary between strong- and weak-coupling areas of the two plates very well.