The basement rocks distributed in Central Honshu are divided from the Sea of Japan to the Pacific side into the Hida, Hida Marginal-Joetsu, Mino-Ashio, RyOke, Sambagawa, Chichibu, and Shimanto Belts. The Median Tectonic Line forms the boundary between the Ryoke and the Sambagawa Belts. The Hida and the Mino-Ashio Belts are characterized by some Helium-rich natural gases. Some geologists has postulated the existence of the Kashiwazaki-Choshi Tectonic Line trending NW-SE from Kashiwazaki in the Sea of Japan side to the southern part of the Kashima-nada in the Pacific side. The line is confirmed on the subsurface geology known from many deep wells and some geophysical data. Although there are three tectonic belts, that is the Sambagawa, Chichibu, and Shimanto Belts on the southern side of the Kashiwazaki-Choshi Tectonic Line, the Mino-Ashio and Ryoke Belts are recognized on the northern side of the line. At present Northeast Japan belongs to the north American plate. The front of the plate was situated at the Itoigawa-Shizuoka Tectonic Line by the middle Miocene age. Afterward the front has migrated from the line to the Kashiwazaki-Chashi Tectonic Line, on which the nest of earthquakes on the southwestern part of Ibaraki Prefecture rests.
Bouguer anomaly map of the Kanto district was compiled. On the whole, gravity anomalies are high in Tsukubasan areas, and low in the central parts of the Kanto plain. Negative anomalies in the Boso peninsula and the Tama hills suggest the basement structures of the sedimentary basins. The Kanto mountains district is bounded on the east by steep gradient of gravity anomaly. The above gravity features can be recognized in the map of the upward continuation. The gravity trend along Narashino, Sakura and Omigawa is more clearly indicated in the map of the first derivative, and the tectonic line covered by sediments can be supposed. In the Izu peninsula and the Tanzawa mountains district, gravity anomalies are rather high and low anomaly zone is recognized between these two areas. The northern border part of this zone correspond to Kannawa fault and Kozu-Matuda fault. Aeromagnetic map was compiled with the data obtained by New Energy Development Organization and Geological Survey of Japan. Generally, the Kanto district is surrounded by intense magnetic anomalies with short wavelength which are distributed along Nasu volcanic zone and Fuji volcanic zone. In the central part of the Kanto plain, weak magnetic anomalies are scattered zonally in the E-W direction. According to the geological and well data, this zone seems to be corresponded to Sambagawa metamorphic belt and these anomalies are presumably caused by the basic or ultrabasic rocks. The Hakone mountains district has sharp anomalies with short wavelengths, and the maximum amplitude of the anomalies reachs 900nT. Low anomaly zones are recognized arround this high anomalies. The source of this low anomalies is not clear, but it is of great interest geologically and geophysically.
Studies of the crust and upper mantle structure in and around the Kanto Plain was reviewed. It has been revealed that the crust under the Kanto Plain has no distinct granitic layer and that the crust and the upper mantle under the southern half of the Kanto Plain is the Philippine Sea Plate which is subducting under the North American Plate. It is supposed that the anomalous crustal structure, i.e., lack of the granitic layer, is a cause of low activity of shallow earthquake under the Kanto Plain. The tearing of the Philippine Sea Plate under the Tokyo Bay is considered to be the cause of the occurrence of destructive earthquakes in the metropolitan Tokyo.
The occurrence of large earthquakes in the Kanto-Tokai (K-T) district has been interpreted by the interaction among the Philippine Sea (PHS), the Eurasian (EUR) and the Pacific (PAC) Plates the PHS and PAC plates underthrust beneath the EUR plate in this region. In order to understand the tectonic process taking place beneath the K-T region, the seismic activity, velocity structure and source mechanisms were studied by using the data from recently developed seismological observation networks. The double-planed structure of the intermediate-depth seismic zone was observed clearly beneath the Kanto districts, and the PHS plate underthrusting from the Sagami and Suruga troughs were clearly delineated by the high velocity and high seismic region. The distribution of pressure axes corroborated the results derived from the hypocentral distribution.
Quaternary crustal movements in the Kanto district, the area where Tokyo is situated in that central part, are overviewed with newly compiled maps (Figs. 1-3), profiles (Fig. 4), and sequence diagrams of vertical crustal movements in twelve parts of the Kanto (Fig. 5). Relations between types of Quaternary crustal movement and types of plate convergence in the area concerned (Fig. 6) are briefly mentioned. Figure 1 shows the depth distribution of the base of Neogene and Quaternary deposits in the Kanto plain by contours (drawn by halftone lines and figures in km), and the distribution of pre-Neogene rocks in the mountains around the plain and in the continental shelf. Profiles in Fig. 4 show that the plain consists of thick Plio-Pleistocene strata (the Kazusa Group), whose base is marked with K, and also fairly thick middle to late Pleistocene strata (the Sagami-Shimousa Group), whose base is marked with S. These strata were deposited in tectonic basins, called the Kazusa trough and the Kanto tectonic basin. The outline of the Kazusa trough is shown by an oval line with teeth in Fig. 1. This trough-making down-warping seems to be formed as a forearc basin above the subducted Philippine Sea plate (PHS) beneath the Kanto plain, denoted by c in Fig. 6. The Kazusa trough changed its shape to conform with the Kanto tectonic basin, shown in Figs. 2 and 3, by the uplift of its eastern part during 1.0-0.5 Ma BP. Figure 2 shows vertical crustal movements during the past about 1 Ma by contours (with figures in meters) with late Quaternary faults lines, and Fig. 3 shows approximate vertical crustal movements since the last interglacial period, using the contour lines of the last interglacial depositional surface formed nearly at the interglacial sea level. In Fig. 3, distribution of late Quaternary faultings and foldings are shown : anticlinal and synclinal axes are shown by rows of solid and open circles, respectively. Distribution of Holocene uplift during the past 6 ka is also shown in Fig. 3 by thin dotted lines (height in meters) for the southern coastal area. The sequence diagrams of vertical movements, deposition, and erosion are illustrated for twelve parts of the Kanto in Fig. 5. Diagrams (1) and (4) show the uplift and erosion of western mountains of the Kanto during the Quaternary, while diagrams (7) and (11) show rapid subsidence and later uplift of southeastern parts of the Kanto during the same period. Diagrams from central parts of the plain, (2) and (6) indicate continuous subsidence through the Quaternary. Rapid uplift of the western mountains, especially of the Tanzawa Mountains shown by diagram (4) suggests that a collision is occurring at b in Fig. 6 between the northern tip of the Philippine Sea plate (PHS) and a collisional zone (f) between the Northeast Japan arc (NEJ) and Southwest Japan arc (SWJ). The subsidence and later uplift at the southeastern part of the Kanto suggest some relation between the subduction of PHS and that of the Pacific plate (PAC) and their changing features during the Quaternary under the Kanto district.