DEFINITION OF SUBSIDENCE

Sinking of the land relative to sea level or some

other uniform surface. It can be a gradual barely perceptible

process, or it may occur as a catastrophic collapse of the surface.

Subsidence naturally occurs along some coastlines and

in areas where groundwater has dissolved cave systems in

rocks such as limestone. It may occur on a regional scale,

affecting an entire coastline, or it may be local in scale, such

as when a sinkhole suddenly opens and collapses in the middle

of a neighborhood. Other subsidence events reflect the

interaction of humans with the environment and include

ground surface subsidence as a result of mining excavations,

groundwater and petroleum extraction, and several other

processes. Compaction is a related phenomenon, where the

pore spaces of a material are gradually reduced, condensing

the material and causing the surface to subside. Subsidence

and compaction do not typically result in death or even

injury, but they do cost Americans alone tens of millions of

dollars per year. The main hazard of subsidence and compaction

is damage to property.

Subsidence and compaction directly affect millions of

people. Coastal subsidence can affect entire cities, regions,

and countries. Residents of New Orleans, Louisiana, live

below sea level and are constantly struggling with the consequences

of living on a slowly subsiding delta. Coastal residents

in the Netherlands have constructed massive dike

systems to try to keep the North Sea out of their slowly sub-

siding land. The city of Venice, Italy, has dealt with subsidence

in a uniquely charming way, drawing tourists from

around the world to their flooded streets. Millions of people

live below the high-tide level in Tokyo. The coastline of Texas

along the Gulf of Mexico is slowly subsiding, placing residents

of Baytown and other Houston suburbs close to sea

level and in danger of hurricane-induced storm surges and

other more frequent flooding events. In Florida sinkholes

have episodically opened up swallowing homes and businesses,

particularly during times of drought.

The driving force of subsidence is gravity, with the style

and amount of subsidence controlled by the physical properties

of the soil, regolith, and bedrock underlying the area that

is subsiding. Subsidence does not require a transporting medium,

but it is aided by other processes, such as groundwater

dissolution, which can remove mineral material and carry it

away in solution, creating underground caverns that are

prone to collapse.

Natural subsidence has many causes. Dissolution of

limestone by underground streams and water systems is one

of the most common, creating large open spaces that collapse

under the influence of gravity. Groundwater dissolution

results in the formation of sinkholes, large, generally circular

depressions caused by collapse of the surface into underground

open spaces.

Earthquakes may raise or lower the land suddenly, as in

the case of the 1964 Alaskan earthquake where tens of thousands

of square miles suddenly sank or rose 3–5 feet, causing

massive disruption to coastal communities and ecosystems.

Earthquake-induced ground shaking can also cause liquefaction

and compaction of unconsolidated surface sediments,

leading to subsidence. Regional lowering of the land

surface by liquefaction and compaction is known from the

massive 1811 and 1812 earthquakes in New Madrid, Missouri,

and from many other examples.

Volcanic activity can cause subsidence, as when underground

magma chambers empty out during an eruption. In

this case, subsidence is often the lesser of many hazards that

local residents need to fear. Subsidence may also occur on

lava flows, when lava empties out of tubes or underground

chambers.

Some natural subsidence on the regional scale is associated

with continental scale tectonic processes. The weight of

sediments deposited along continental shelves can cause the

entire continental margin to sink, causing coastal subsidence

and a landward migration of the shoreline. Tectonic processes

associated with extension, continental rifting, strikeslip

faulting, and even collision can cause local or regional

subsidence, sometimes at rates of several inches per year.

See also GROUNDWATER; PLATE TECTONICS.

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