A slow subsidence of the earth level has been long recognized in the vicinity of Koto delta districts in Tokyo as a result of a series of level survey carried out by the Japan Geographical Survey Institute. For more than twenty years, the survey of subsidence of the earth level has been conducted in Ko-to; delta districts (surrounded with the Sumida river and Arakawa diversion channel) and in Johoku low districts (alongside of the Arakawa river) by Tokyo Metropolitan Government
The survey methods of subsidence of the earth level are by levelling and observation wells. At present, extent of the area affected by subsiding land is approximately 190km2 and the annual maximum subsiding land of 1961, i. e. 189. 8 mm, took place in Gobeicho, Adachiku.
Subsidence of the earth level is remarkable in the area where the alluvial deposits is thick. Consequently, it may be closely related to the thickness of the alluvial deposits.
Further subjects to be studied on the survey of subsiding land may be as follows:
1) new layout of bench marks for levelling.
2) mechanism of observation wells.
3) shrinkages of sand and gravel beds.
