Land Subsidence in the Urbanizing Rural Area and Geological Issues

Case Study in the North of the Kanto Plain

Abstract

The northern part of the Kanto Plain is the area where the most severe land subsidence have been taking a place in recent years. This paper presents the relation between the change of circumstances of groundwater recharge which is caused by the change of the land use and the mechanism of the land subsidence.
The land subsidence in this area is classified into two types by the depth of the compacted beds. The one is the compaction of alluvium strata that is about 30 meters in thickness and developed only the lowland. The rest is the compaction in the Pleistocene beds. This compaction is mainly occurring more than 200 meters beneath and spreading widely over the whole area.
The groundwater head become lower according to the depth of the aquifers. In the aquifer from 200 to 300 meters under the surface, groundwater head is lowest and it ranks about 25 meters below sea level. Because of the wide lowering of the groundwater head of the deep aquifers, the groundwater is recharged vertically from the adjacent aquifers by leakage. The original source of the groundwater is rainfall and irrigation water. Especially, the recharge from paddy fields takes an important role for raising groundwater table of the unconfined aquifer.
For the effective prevention of the land subsidence, it is necessary not only to reduce the amount of the groundwater use from the deep aquifers with low groundwater head, but also to manage the proper land use that is maintain the function of the groundwater recharge.