Land subsidence is sinking of the land surface. The elevation of the land surface is lowered due to the compaction of fine-grained sediments, mainly clay and silt, in the subsurface. In the greater Houston area, land subsidence is caused by the compaction of fine-grained sediments in the aquifer as a result of groundwater withdrawal.
In other parts of the world, subsidence occurs due to oil and gas withdrawal, underground mining, and natural depositional compaction. Natural land subsidence occurs over long periods of time due to the deposition sediments over thousands to millions of years; but it doesn’t compare to the rates of subsidence caused by humans.