東北地方南東部の第四紀地殻変動とブロックモデル

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This study attempts to explain the Quaternary crustal movements in the southeastern region of the Northeast Japan, using a kind of block model. In this region, many active faults are observed (Fig. 1), and some characteristics of them (Table 1) can be summarized as follows. i) Almost all of the faults which are developed along the eastern margin of the Backbone Range have the NNE-SSW strike and east side downthrow. Some of them are comfirmed as reverse faults. ii) Almost all of the faults striking NE-SW are thrust faults. iii) Almost all of the faults striking NNW-SSE have vertical fault plane, and among them, at least the Futaba Sheared Zone is a left-lateral fault. iv) Faults striking NW-SE are considered as reactivized preexisting faults or faults which were developed from extention joints. v) Almost all of the active faults observed in this region are reactivized preexisting faults. vi) The displacement rate along the active fault is usually less than 1 mm/year. The Quaternary tectonic stress field in this region were restored based on the major and minor fault analysis (Fig. 7), the focal mechanism of shallow earthquakes (HASEGAWA and UMINO, 1976) and geodesy (SuzuKi et al., 1975). The results suggest that the maximum principal stress axis (compression is positive) was horizontal, its direction being N55°W, and the minimum principal stress axis was vertical. As showing in Fig. 1, this region can be geomorphologically divided into Backbone Range, Kitakami Mountainlands, Abukuma Mountainlands, Abukuma Lowlands, Ohsaki Plain, Rikuzen Hilly Lands and Coastal Lowlands. Almost all of the boundaries between geomorphological provinces have an abrupt change of geomorphological height. Furthermore, the distributions of active faults and epicenters of shallow earthquakes are restricted along these boundaries. This indicates that each of geomorphological province might geodynamically act as a block, although Abukuma Mountainlands and Abukuma Lowlands might be one block. Therefore we set up a kind of qualitative block model which can be summarized as follows. "The crust in this region is composed of the polygonal blocks, and each block is bounded by preexisting faults. When the tectonic stress acts in such a crust, preexisting fault series which are most suitable to release the elastic strain energy become active. If the energy was not released sufficiently, new active faults may be originated." As shown in Fig. 8, there are four kinds of fault series and their associated tectonic movement patterns which play some important roles to release the elastic strain energy in this region. Some important phenomena of the Quaternary crustal movements in this region can be expounded using such a block model. i) Allotment of total long-period displacement among three active faults which form, a triple junction. Suppose N_p, E_p, F_p be components parallel tc the maximum pricipal stress axis of net shift along the Nagamachi-Rifu Line, Enda Fault and the Futaba Sheared Zone respectively. N_p≒E_p+F_p, from Table 1. This relationship suggests that the displacement of the Nagamachi-Rifu Line is a cumulative result of the tectonic movement patterns (1) and (2) of Fig. 8. ii) Contribution of crustal movements to the origin of geomorphological provinces. Origin of geomorphological provinces can be explained by the vertical movements of blocks accompanied by the activity of faults. The westward tilt of Abukuma Mountainlands (KoiKE, 1969) and the origin of Abukuma Lowlands are considered to be related to the activity of the Western Marginal Fault of Fukushima Basin (the tectonic movement pattern (2)〜(4) of Fig. 8) and the northward tilt of Abukuma Mountainlands (KoiKE, 1969) and Coastal Lowlands (NAKAGAWA, 1961 a) to the crustal movement patterns (1) and (2) of Fig. 8. The crustal movement pattern (1) of Fig. 8 seems to contribute to the origin of Coastal Lowlands. Rikuzen Hilly Lands and its two side areas, Ohsaki Plain and Sendai Plain seem to be formed by movement patterns (1)〜(3) of Fig. 8. iii) Seismic activity and block model In this region two pairs of large earthquakes have been recorded up to now: the 1731 earthquke of M≒6.2 occurred on the northern part of the Western Marginal Fault of Fukushima Basin and the 1736 earthquake of M≒5.9 occured on the Nagamachi-Rifu Line; the 1956 earthquake of M=6.2 occured again on the northern part of the Western Marginal Fault of Fukushima Basin and the 1962 earthquake of M=6.5 occured in tne neighbourhood of the Sanbongi Fault. In each pair of seismic activity, the first and second ones correspond to the cases (2) and (3) of Fig. 8 respectively, and the migration of epicenters seems to be related to the shift of dilated and contracted areas caused by formerly occured earthquakes as shown in Fig. 9.
1977-01-25

東北地方南東部の第四紀地殻変動とブロックモデル

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