Subsidence—sinking of the ground because of underground material movement—is most often caused by the removal of water, oil, natural gas, or mineral resources out of the ground by pumping, fracking, or mining activities.
Subsidence can also be caused by natural events such as earthquakes, soil compaction, glacial isostatic adjustment, erosion, sinkhole formation, and adding water to fine soils deposited by wind (a natural process known as loess deposits). Subsidence can happen over very large areas like whole states or provinces, or very small areas like the corner of your yard. In the Chesapeake Bay area, for example, land subsidence may be caused by a combination of sediment loading (when rivers deposit sediment in an area that then sinks under the additional weight) and sediment compaction after groundwater is removed.
One way to measure these changes is through the Global Positioning System (GPS). NOAA’s National Geodetic Survey maintains a national network of high-precision GPS receivers, called a Continuously Operating Reference Station (CORS), that can be used to track height changes at the station. Areas without a CORS can measure subsidence from satellite InSAR (Interferometric Synthetic Aperature Radar), temporary GPS receivers, repeated surveys of geodetic leveling, or installations of ground and water sensors.