Subsidence

Geological subsidence involves the settling or sinking of a body of rock or sediment. Subsidence is a type of mass wasting, or mass movement-transport of large volumes of earth material primarily by gravity. Subsidence may occur as the result of either natural or human-caused events.

Earthquakes are commonly associated with subsidence. When two blocks of the earth's crust slide against each other, causing an earthquake, ground movement may occur, raising or lowering the ground surface. During Alaska's 1964 Good Friday earthquake, an area of at least 70,000 sq mi (180,000 sq km), much of it coastline, subsided 3 ft (1 m) or more—some areas now flood at high tide.

Another way that earthquakes can cause subsidence is by rapidly decreasing the load-bearing capacity, or strength, of loose earth materials, or sediments, due to liquefaction. Liquefaction occurs when vibrations from an earthquake, or other disturbance, cause water-saturated sediments to temporarily lose their grain-to-grain contact, which is what gives them their load-bearing capacity. For just an instant, the weight of the overlying materials is supported only by the water between the grains. An instant later, when the grains begin to settle, the weight of the overlying sediment (or buildings) causes the grains to be forced closer together and the land to subside.

During the 1989 earthquake along the San Andreas Fault in California, some of the most serious damage occurred in San Francisco's Marina District. Buildings were constructed on old bay deposits, unconsolidated water-saturated sediments, which when shaken by the earthquake temporarily lost their strength due to liquefaction. In the Marina district, sediment that had previously supported large buildings quickly turned into a mud-like material that could no longer support a building's weight.

Another example of natural subsidence can be found in regions where caves are common. Caves form when underground water dissolves limestone and carries it away. The resulting void spaces grow larger and larger over time until they become the features we call caves.

If limestone dissolves for a long enough time, the hole (cave) that forms becomes too large to support the weight of its walls. In such a case, the ceiling of the cave will subside, either slowly or in a catastrophic collapse, forming a large depression at the surface, known as a sinkhole. If conditions are right, the sinkhole may eventually fill with water, forming a lake. A landscape containing many dry or water-filled sinkholes formed by limestone dissolution is called karst topography.

Yet another cause of subsidence is volcanic eruption. Whether molten rock, or magma, is lost suddenly and dramatically, as in the Mt. St. Helens eruption of May 18, 1980, or the slow flow of lava (magma flowing on the surface), as happens in the Hawaiian Islands, land subsidence is likely to follow. The material ejected from the earth's interior leaves an empty space that must be filled in one way or another. In many cases, a large section of the overlying crust—along with the geologic and human features attached to it—collapses into the earth's interior, forming what is called a caldera.

Subsidence may also result from the accumulation of large volumes of sediment at the earth's surface in what is known as a sediment basin. An obvious setting in which this occurs is at river deltas. Each day, the Mississippi River deposits up to 1.8 million metric tons of sediment at its mouth near New Orleans. The weight of this sediment contributes to a gradual subsidence of the land on which New Orleans resides. Basins between mountains also can subside due to the weight of accumulating sediments.