Factor analysis on land subsidence in the Nobi Plain, southwest Japan

The land subsidence of the Nobi Plain largely ceased with the commencement of pumping regulations beginning in 1975. However, a small amount of land subsidence, less than one centimeter, is still observed at the Delta zone in the southwest part of the Plain. The authors attempted to investigate the cause of the small amount of existing subsidence. The alluvial clay layer deposits are more than 15-20 m thick and the withdrawal is small. The decrease of the yearly average groundwater level has not been confirmed. On the other hand, the seasonal change in the groundwater level is clearly observed. This investigation focuses on the seasonal change of groundwater level each year and its relation to the thickness of alluvial clay layer. The Delta zone was divided into several cells and a multiple regression analysis was applied to the seasonal change of the groundwater level and the thickness of alluvial clay layer of the cell. The study concluded that the small amount of land subsidence was caused of drawdown of piezometric head of groundwater every year during the summer.