GROUND SUBSIDENCE OF THE MODERN YELLOW RIVER DELTA AND ITS CAUSES

Three digital elevation models(DEM) were generated from 1:50 000 topographical maps of the Yellow River delta,which was surveyed in 1956,1967 and 1980,respectively.Knowledge of ground subsidence in the delta from 1956 to 1980 was obtained through spatial analyses of these DEMs,and three major impact factors of ground subsidence were addressed. The results show that ground surface of the Yellow River delta experienced a widespread subsidence during the past 24 years,with an average rate of several centimeters per year in the subsidence areas. Neotectonic sinking had a faint but persistent influence on ground subsidence in the Yellow River delta. Consolidation of deposits,a major cause of ground subsidence in the delta before the Yellow River switched to the Qingshuigou course,induced sheets of subsidence.Human activity played an increasing role in ground subsidence in the delta with the rapid development of oil industry and the over-exploitation of underground water, which usually induced small points of subsidence at the beginning and then gradually expanded to the whole region.