Subsidence
Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities.
[3] Processes that lead to subsidence include dissolution of underlying carbonate rock by groundwater; gradual compaction of sediments; withdrawal of fluid lava from beneath a solidified crust of rock; mining; pumping of subsurface fluids, such as groundwater or petroleum; or warping of the Earth's crust by tectonic forces.
[4] Pumping of groundwater or petroleum has led to subsidence of as much as 9 meters (30 ft) in many locations around the world and incurring costs measured in hundreds of millions of US dollars.
[6] Subsidence frequently causes major problems in karst terrains, where dissolution of limestone by fluid flow in the subsurface creates voids (i.e., caves).
If the roof of a void becomes too weak, it can collapse and the overlying rock and earth will fall into the space, causing subsidence at the surface.
Mining-induced subsidence is relatively predictable in its magnitude, manifestation and extent, except where a sudden pillar or near-surface tunnel collapse occurs (usually very old workings[8]).
This is accomplished through a combination of careful mine planning, the taking of preventive measures, and the carrying out of repairs post-mining.
If the gas is extracted, the overburden pressure sediment compacts and may lead to earthquakes and subsidence at the ground level.
Since exploitation of the Slochteren (Netherlands) gas field started in the late 1960s the ground level over a 250 km2 area has dropped by a current maximum of 30 cm.
It is a growing problem in the developing world as cities increase in population and water use, without adequate pumping regulation and enforcement.
The resulting aeration of the soil leads to the oxidation of its organic components, such as peat, and this decomposition process may cause significant land subsidence.
This applies especially when groundwater levels are periodically adapted to subsidence, in order to maintain desired unsaturated zone depths, exposing more and more peat to oxygen.
Water management used to be tuned primarily to factors such as crop optimization but, to varying extents, avoiding subsidence has come to be taken into account as well.
When differential stresses exist in the Earth, these can be accommodated either by geological faulting in the brittle crust, or by ductile flow in the hotter and more fluid mantle.
Due to the weight of the water once held in the lake, the earth's crust subsided nearly 200 feet (61 m) to maintain equilibrium.
Additionally, they found that, aside from continuous acquisition systems typically installed in areas with rapid subsidence, InSAR had the highest measurement frequencies.
[48][49] Quasi-three-dimensional seepage models apply Terzaghi's one-dimensional consolidation equation to estimate subsidence, integrating some aspects of three-dimensional effects.
