日本における最近70年間の総括的上下変動

抄録

The precise levelling in Japan commenced in about 1884. Past surveys were irregular as regards both repeating interval and area. However, it is possible to arrange them into 4 epochs of reduction. In this report, synthetic vertical movements in the whole land of Japan during the period from about 1895 to about 1965 is studied. Principles of the present reduction are as follows; (1) As 4 epochs of reduction, 1895, 1930, 1950 and 1965 are adopted. (2) Many studies of movements in particular regions and of particular periods have been made by various authors. These results are used for compilation with the synthetic movements. New data in the recent 10-20 years are com-puted this time. (3) Change in mean sea level is used for given conditions in reduction of the recent survey. Number of the adopted tidal stations amounts to 8. (4) In order to obtain a map which is significant for neotectonics motion, abrupt and large movements influ-enced by earthquakes are eliminated as far as possible. (5) By the similar reason as (4), influence of artificial ground subsidence is eliminated. (6) In order to secure nearly equal accuracy all over the whole land, 4-5 bench marks in each mesh of longitude 1° by latitude 1°, are selected to draw a contour map. Fig. 4 shows the final result. The contour is drawn in unit of mm/year. From this figure, the following distinctive movements can be found. (1) The amount of curstal movements in almost all the coastal areas of Japan Sea of Hokkaido and Honshu Islands is small, and it is safely said that these areas are stable. On the contrary, considerably large subsidence is seen in the area of Pacific Ocean side of Hokkaido and the northern part of Honshu Islands. These two facts seem very favorable to an explanation by the plate tectonics; that is, the subsidence of the continental crust is explained by drag down of the sinking lithosphere. (2) In the central part of the Honshu Island, considerable large zone of upheaval can be clearly seen. The northern part of this zone coincides with active volcanic chains, so it can be considered that the volcanic activity still accompanies the land rise. On the other hand, the southern part of the concerned zone is not volcanic area but old moun-taneous region. So it should be considered that this zone continues to the upheaval zones which are seen in Kui Peninsula and Shikoku Island. (3) In the western half of Honshu, Shikoku and Kyushu Islands, it is very remark-able that upheaval and subsidence zones are seen alteratively in parallel to the island arc. This phenomenon may be understood as the folding motion caused by horizontal pressure which acts in perpendicular to the island arc. The half wave length of this motion is about 100 kms. Here only 3 main distinctive movements of Japan Islands in the recent 70 years are cited. The author is apt to consider that almost all these movements are understood on the basis of the plate tectonics. The conclusion, however, is not definitive up to the time when reliable horizontal movements are obtained, because horizontal movements are probably more primary than vertical movements in the field of the plate tectonics.