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What is Land Subsidence?
Subsidence (land surface sinking) occurs in many parts of the world, particularly in densely populated deltaic regions, causing extremely expensive damage. Subsidence has resulted from natural causes, such as tectonic motion and sea level rise, from man-induced causes, such as the heavy withdrawal of groundwater, geothermal fluids, oil, and gas, or the extraction of coal, sulphur, gold, and other solids through mining, or underground construction (tunnelling), or from other mixed causes, such as the hydro-compaction of loosely deposited sediments, oxidation and shrinkage of organic deposits, or the development of sinkholes in karstic terrain.
Over 150 areas of contemporary subsidence are known, some with as much subsidence as 10 m in countries such as Mexico, Japan, and the United States. Many more areas of subsidence are likely to develop in the next few decades as a result of accelerated exploitation of natural resources in order to meet the demands of increasing population and industrial development in many developed countries of the world. As developing countries expand their industry, subsidence is likely to occur in many more areas.
Most of the major subsidence areas around the world have developed in the past half century, probably starting to a great extent during World War II and continuing since then, at accelerated rates due to the rapidly increasing use of groundwater and of oil and gas. Most areas of known subsidence are along coasts where the phenomenon becomes quite obvious when the ocean or lake waters start coming further up on the shore. In some such areas, the usually dense population and intensive industrial development are protected from being flooded by several metres of water only by construction of extensive and expensive systems of dikes, flood walls, and pumping stations. The most common subsidence, that due to underground fluid withdrawal, is such a subtle phenomenon — often of large areal extent and at a slow rate — that the problem is not evident in inland areas until new precise levelling takes place or underground pipelines crack, well casings fail or stand above ground level, surface drainage patterns change, or canals no longer carry original design flows.
Developers, as well as the engineers and scientists making the studies and plans for industrial complexes, urban developments, water supply systems, and natural resource extractions need to know about the potential hazards, costs, and socioenvironmental impacts that can result from land subsidence.