Land subsidence

Land subsidence is the gradual or sudden lowering of the ground surface. It commonly results from groundwater withdrawal, sediment compaction, mining, or dissolution of underground rock.

Groundwater extraction: The removal of water from wells or other groundwater sources for human use. Pumping lowers pore-water pressure, allowing compressible aquifer layers to compact.
Interferometric synthetic-aperture radar: A radar technique that measures surface displacement by comparing the phase of satellite images. It maps subsidence over broad areas without requiring instruments at every site.
Relative sea-level rise: The rise of sea level relative to the land at a particular location. Sinking land increases local relative sea-level rise even when ocean levels are unchanged.
Mexico City: The capital of Mexico, built on a former lakebed within the Valley of Mexico. Its compressible lake sediments and intensive groundwater pumping have caused severe, uneven sinking.
Aquifer-system compaction: The compression of sediments and rock layers that store or transmit groundwater. Compaction of these layers is a direct cause of subsidence in pumped basins.
GNSS: A satellite-based positioning system used to determine precise locations on Earth. Continuous GNSS stations track vertical ground motion at specific points.
Coastal flooding: The inundation of coastal land by seawater, often during storms or unusually high tides. Subsidence lowers coastal elevations and can worsen flood frequency and reach.
San Joaquin Valley: An agricultural valley in California’s Central Valley. Groundwater pumping for irrigation has caused extensive aquifer compaction and subsidence.
Effective stress: The stress carried by the solid framework of a porous material after fluid pressure is accounted for. Falling groundwater pressure raises effective stress and compresses sediment grains.
Leveling: A surveying method that measures elevation differences between points using a level instrument. Repeated surveys can establish how quickly benchmarks are sinking.

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