What is land subsidence?
Land subsidence is a gradual settling or sudden sinking of the Earth's surface owing to subsurface movement of earth materials. The principal causes are aquifer-system compaction, drainage of organic soils, underground mining, hydrocompaction, natural compaction, sinkholes, and thawing permafrost (National Research Council, 1991). Three distinct processes account for most of the water-related subsidence--compaction of aquifer systems, drainage and subsequent oxidation of organic soils, and dissolution and collapse of susceptible rocks.
How is land subsidence measured?
Measurements of elevations, aquifer-system compaction, and water levels are presented, interpreted, and integrated to improve understanding of the processes responsible for land-surface elevation changes. Elevations, and elevation changes, have been measured using Interferometric Synthetic Aperture Radar (InSAR), Continuous GPS (CGPS) measurements, campaign Global Positioning System (GPS) surveying, and spirit-leveling surveying.
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