Causes of Land Subsidence

Mining of Groundwater

•Communities use aquifers as source of freshwater by pumping out the groundwater.
•If the area pumped out exceed the recharge from precipitation it is considered as groundwater mining.
•Excess of extraction resulting ground subsidence.
•Mostly aquifers consists of thick accumulation of sand & gravels with water filling between grains.•Discharge of water from pore spaces cause them to pack more tightly together & take up less space.•Large water loss can cause permanent reduction of aquifer capacity & subsidence of ground.
 

Extraction of Natural Gas & Petroleum

•If natural gas were extracted from the natural gas field the initial pressure (up to 60MPa) in the field will drop over the years.
•Gas pressure support the soils layer above field.
•Pressure drops, soil pressure increase and lead to ground subsidence.

Dissolution of Limestone

•Subsidence frequently causes major problems in karst terrains - dissolution of limestone by fluid  flow in the subsurface causes the creation of voids (caves).•If the roof of these voids becomes too weak, it can collapse and the overlying rock and earth will  fall into the space, causing subsidence at the surface.•This type of subsidence can result in sinkholes.

Earthquake Induced

•Movement that occurs along faults can be horizontal or vertical or have a component of both.
•As a result, a large area of land can subside drastically during an earthquake.
•Land subsidence can also be caused during liquefaction.
•Liquefaction can result in the settling and compacting of unconsolidated sediment in an event of a major earthquake. This can result in the lowering of the land surface.

Faulting Induced

•When differential stresses exist in the Earth, these can be accommodated either by geological faulting.
•Where faults occur, absolute subsidence may occur in the hanging wall of normal faults.
In reverse, or thrust, faults, relative subsidence may be measured in the footwall