Land subsidence and earth fissures in southern Jiangyin

The southern Jiangyin region is tectonically divided into two upheavals and one depression.The southern and northern margins of the region are buildup zones,and the central part is a plain underlain by Quaternary sediments.Difference is found in the thickness and lithology of the layered sediments,and the structure and hydrogeology of the region,which are controlled by the basement relief and the ancient river course.Due to long-term groundwater over-exploitation from the second confined aquifer,regional depression cones of groundwater piezometric heads have occurred with maximum groundwater piezometric drawdown of 75 m below the land surface,and the land subsidence center was formed to reach the maximum land subsidence value of 1 300mm.The Hetang earth fissures,Changjing earth fissures and Zhutang earth fissures were originally small local ground cracks in concrete pavements and started in 1995.Their continuous active time has been over 12 years and now the earth fissures are slow down.There are some earth fissures zones consisting of main fissures and their northwestern associated depressions are 20 to 60m wide.Their linear extension length ranges from 300 m to 500m,and the strike direction is 20 to 45 degree in the northeast.Due to differential subsidence,the main fissure shaped as a 10～30cm high scarp,and there are a lot of small fissures in the depression zone.It is verified by shallow seismic prospecting and drilling that a bedrock ridge underlies the Quaternary sediments directly below the main earth fissure.It is the dominant factor for the formation of earth fissures that long-term groundwater over-exploitation results in compaction of the deep confined sandy aquifer and consolidation of its underlying and overlying clayey aquitards.It is the controlling/background factor that the underlying bedrock ridge and the depth and structure of the sediments control the spatial and temporal characteristics of the earth fissure zone because of its leading to differential land subsidence.The depth of the earth fissure may reach the bedrock ridge,and a small fault is actually developed from the main fissure.