(Front) Map showing S-wave velocity distribution along a curved surface located 10km above the Pacific plate (Nakajima and Hasegawa, 2010). The color scale shows S-wave velocities. The contact zone between the Pacific and Philippine Sea plates is enclosed by two thick broken lines. Red stars denote five M 7-class earthquakes that have occurred in southern Kanto since 1885. Beach balls show the focal mechanisms of the 1921 M 7.0 Ryugasaki earthquake (two solutions shown) and the 1987 M 6.7 east off Chiba earthquake. Black circles represent aftershocks of the 1987 earthquake.
(Back) Serpentinized mantle of the Philippine Sea plate at its easternmost portion (shaded in brown). The dashed pink line represents the approximate location of the western boundary of the serpentinized mantle. Grey broken contour lines show depths to the upper surface of the Philippine Sea plate. Green squares and circles plot interplate earthquakes on the upper surface of the Pacific plate, and gray dots plot earthquakes in the mantle of the Philippine Sea plate. Beach balls also show the focal mechanisms of four earthquakes in the mantle of the Philippine Sea plate. The purple circle denotes an aftershock (M 7.1) of the 1923 Kanto earthquake that occurred ∼30h after the main shock. The Philippine Sea plate is being torn into two parts along the dashed pink line: the easternmost portion of the serpentinized mantle subducts at a slower rate, having been left behind the western part, the trunk of the Philippine Sea plate. The pink ellipse denotes a seismic gap pointed out by Okada and Kasahara (1990).
(Akira HASEGAWA)
