State of Stress in the Kanto-Tokai Area

Abstract

A map of the maximum compressive stress direction has been made for the Kanto-Tokai area from the data of in-situ stress measurements, focal mechanisms of earthquakes, and geological evidences. Data compiled, which amounts to 77 points, includes the following: 10 points of hydraulic fracturing stress measurments, 3 points of overcoring stress measurments, 7 points of Quaternary dike trends, 9 points of Quaternary monogenetic volcano alignments (including flank volcanoes), 41 points of active fault movements and 7 points of focal mechanism solutions of shallow earthquakes (depth <20km, M>6.5). The data points which lie close to one another give a similar stress condition: the stress orientation and stress state classified by fault type such as normal, reverse, and strike-slip faults. We have divided the Kanto-Tokai area into seven provinces on the basis of the data compiled after consideration of the plate interation among the Eurasian, the Philippine Sea and the Pacific plates. Stress condition within each province appears almost uniform. The seven stress provinces and their averaged azimuths for the maximum horizontal compressive stresses S_Hmax are as follows: southern part of Nagano Prefecture (NA), N85°W; northern part of the Kanto district (KA), N80°W; Shizuoka district (SZ), N40°W; Nishiizu district (NI), N10°E; Higashiizu district (HI), N40°W; Tanzawa district (TN), N-S; and the Miura-central Boso district (MB), N35°W. The azimuths of S_Hmax in stress provinces NA and KA are both nearly E-W, which is attributed to the westward relative movement of the Pacific plate against the Eurasian plate. The azimuths of N35°-40°W in provinces SZ, HI and MB are considered to be due to the northwestward movement of the Philippine Sea plate relative to the Eurasian plate. The cause for the individual orientation of S_Hmax as N10°E in province NI is interpreted as being a result of the downward bending of the Philippine Sea plate at the Suruga trough. That is, the extension perpendicular to the bending axis is effected on the surface by the downward bending, which result in the parallel orientation of the S_Hmax to the trough. In the case of province TN, the stress orientation in the province is not interpreted in terms of downward bending because the subduction has not been expected in the province. This is one of the subjects for future study. In stress province MB, the direction of S_Hmax agrees well with the horizontal maximum compressive strain accumulated during the period from 1882/1902 to 1973/80. However, the direction does not agree well with that of the strain accumulated during the period from 1924/25 to 1973/80. The difference is one reason for low seismicity in the upper crust in province MB, since current crustal movement does not work efficiently to increase the in-situ differential stress. The difference may be attributed to the long-term after-effect of the 1923 Kanto earthquake.