Groundwater-related subsidence subsidence Groundwater-related subsidence is the sinking of land resulting from groundwater extraction. It is a growing problem in the developing world as cities increase in population and water use, without adequate pumping regulation and enforcement. One estimate has 80% of serious land subsidence problems associated with the excessive extraction of groundwater, making it a growing problem throughout the world. In this way, land subsidence has the potential of becoming self-perpetuating, having rates up to 5 cm/yr.
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.
Dissolution of limestone Subsidence 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. This type of subsidence can cause
sinkholes which can be many hundreds of meters deep.
Mining Several types of
sub-surface mining, and specifically methods which intentionally cause the extracted void to collapse (such as pillar extraction,
longwall mining and any
metalliferous mining method which uses "caving" such as "block caving" or "sub-level caving") will result in surface subsidence. 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). Mining-induced subsidence is nearly always very localized to the surface above the mined area, plus a margin around the outside. The vertical magnitude of the subsidence itself typically does not cause problems, except in the case of drainage (including natural drainage)–rather, it is the associated surface compressive and tensile strains, curvature, tilts and horizontal displacement that are the cause of the worst damage to the natural environment, buildings and infrastructure. Where mining activity is planned, mining-induced subsidence can be successfully managed if there is co-operation from all of the stakeholders. This is accomplished through a combination of careful mine planning, the taking of preventive measures, and the carrying out of repairs post-mining.
Extraction of petroleum and natural gas If
natural gas is extracted from a
natural gas field the initial pressure (up to 60
MPa (600
bar)) in the field will drop over the years. The pressure helps support the soil layers above the field. 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. Extraction of
petroleum likewise can cause significant subsidence. The city of
Long Beach, California, has experienced over the course of 34 years of petroleum extraction, resulting in damage of over $100 million to infrastructure in the area. The subsidence was brought to a halt when
secondary recovery wells pumped enough water into the oil reservoir to stabilize it. The
Geospatial Information Authority of Japan reported immediate subsidence caused by the
2011 Tōhoku earthquake. In Northern Japan, subsidence of 0.50 m (1.64 ft) was observed on the coast of the
Pacific Ocean in
Miyako,
Tōhoku, while
Rikuzentakata, Iwate measured 0.84 m (2.75 ft). In the south at
Sōma, Fukushima, 0.29 m (0.95 ft) was observed. The maximum amount of subsidence was 1.2 m (3.93 ft), coupled with horizontal
diastrophism of up to 5.3 m (17.3 ft) on the
Oshika Peninsula in
Miyagi Prefecture.
Faulting induced 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. 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.
Isostatic subsidence The crust floats buoyantly in the
asthenosphere, with a ratio of mass below the "surface" in proportion to its own density and the density of the asthenosphere. If mass is added to a local area of the crust (e.g., through
deposition), the crust subsides to compensate and maintain
isostatic balance.
Seasonal effects Many soils contain significant proportions of clay. Because of the very small particle size, they are affected by changes in soil moisture content. Seasonal drying of the soil results in a lowering of both the volume and the surface of the soil. If building foundations are above the level reached by seasonal drying, they move, possibly resulting in damage to the building in the form of tapering cracks. Trees and other vegetation can have a significant local effect on seasonal drying of soils. Over a number of years, a cumulative drying occurs as the tree grows. That can lead to the opposite of subsidence, known as heave or swelling of the soil, when the tree declines or is felled. As the cumulative moisture deficit is reversed, which can last up to 25 years, the surface level around the tree will rise and expand laterally. That often damages buildings unless the foundations have been strengthened or designed to cope with the effect.
Weight of buildings High buildings can create land subsidence by pressing the soil beneath with their weight. The problem is already felt in
New York City, the
San Francisco Bay Area, and
Lagos. == Impacts ==