31. 東京都南東部の地盤沈下測量

概要

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The ground of the south-eastern part of Tokyo was found to be subsiding moderately as reported in many cases. The boundary of high land and low land of Tokyo is regarded as the western limit of the subsiding area. But the eastern boundary was not yet made clear, so to find it precise levellings were carried out in the area in the southeastern part of Tokyo and north-western part of Tiba prefecture. As the result of the surveying, the subsiding area was found to be limited by the Edogawa (river). Then the settling of the sediments of the Arakawa (river) must be the main cause of the subsiding. The levelling of the bench marks Nos. 9837 and 9838 was carried out twice, in Dec. 1950 and in Feb. 1951. On Jan. 9. 1951 a remarkable earthquake occurred near the area. But difference of heights obtained by the two levellings was only 1.2 mm, which is too small to be considered as the effect of the earthquake on the land deformation.