In the simplest of terms, land subsidence is the sinking of the earth’s surface due to subsurface movement. Aquifer compaction from groundwater withdrawal, oil and gas extraction, underground mining, sinkholes, drainage of organic soils, and natural depositional compaction are the most common causes of subsidence in the United States. More specifically for the greater Houston-Galveston region, land subsidence is the decrease in land-surface elevation caused by aquifer compaction due to abundant, long-term groundwater withdrawals.
In the greater Houston-Galveston region, groundwater is pumped primarily from the Chicot and Evangeline aquifers as a source of freshwater. The geology of these aquifers includes discontinuous layers of gravel, sand, silt, and clay. The extraction of large volumes of groundwater causes clays to depressurize and compact. Aquifer compaction affects overlying stratigraphic units all the way to the land surface and is observed as subsidence.
Illustration of subsidence in a Gulf Coast aquifer as a result of groundwater withdrawals producing a decrease in the potentiometric surface (the groundwater level). Source: Kasmarek, M.C., Ramage, J.K., and Johnson, M.R., 2016, Water-level altitudes 2016 and water-level changes in the Chicot, Evangeline, and Jasper aquifers and compaction 1973–2015 in the Chicot and Evangeline aquifers, Houston-Galveston region, Texas: U.S. Geological Survey Scientific Investigations Map 3365, pamphlet, 16 sheets, scale 1:100,000, http://dx.doi.org/10.3133/sim3365.