Investigations of Changjing earth fissures, Jiangyin, Jiangsu, China

Fissuring of the earth began at Changjing, Jiangyin City, Jiangsu Province, in 1995. The earth fissure zone consists of a main fissure with a number of secondary fissures on both sides, which range from 20 to 60 m wide. The main fissure is about 500 m long, trends 15° to 30°, and has a scarp 100–200 mm high. Extensive field investigations, including shallow seismic surveys and geological drilling, have been conducted over 20 years. Deposits of a Quaternary paleoriver at 150–160 m depth curve around a bedrock spur which is situated beneath, and lies parallel to, the fissure at about 90 m. A regional trough has formed due to land subsidence over the paleoriver channel, with Changjing at its eastern edge. The fissures resulted from differential subsidence of the Quaternary deposits as a result of extended over-exploitation of groundwater. Groundwater extraction facilitates compaction of the Quaternary aquifers and aquitards from the increase of the effective in situ stresses, which leads to subsidence. Positive relief of the subsurface bedrock defines the location and formation of the earth fissures. On one hand, differential subsidence causes the Quaternary sediments to rotate over the ridge, leading to tensional cracking. On the other hand, temporal differential in subsiding aggravates the vertical shearing or faulting of the fissure, forming a scarp.