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NAME: Surface displacement
BRIEF DESCRIPTION: In many regions the Earth's surface is subject to small but important displacements (uplift/heave, subsidence, lateral movement, rotation, distortion, dilation) that affect elevation, horizontal position or both.. Such movements may result from - active tectonic processes within the Earth; collapse into underground cavities; compaction of surficial materials; seismically induced mass movement (e.g. liquefaction); volcanic activity; and swell/shrink behaviour of clays. Downslope movements of rock, soil, debris and snow are covered in a separate checklist entry [avalanches and landslides].
Sudden movements may be caused by faulting associated with earthquakes [see seismicity] and from the collapse of rock or sediment into natural holes in soluble rocks (e.g. salt, gypsum, limestone [see karst activity]), or into cavities produced by extraction of near-surface mineral deposits by conventional (especially coal) or solution mining (especially salt). Slower local subsidence may also be induced by: fluid withdrawal (gas, oil, groundwater, geothermal fluids); densification or loss of mass in peat being developed for agriculture; drainage of waters from wetlands, which can cause oxidation, erosion and compaction of unconsolidated soils and sediments [see wetlands extent, structure and hydrology]; and seepage of surface water through porous metastable sediments such as loess, causing hydrocompaction. Uplift may also be caused by stress-relief during valley downcutting (valley bulging).
In tectonically active mountains, uplift may be as much as 20 mm/year. Although vertical crustal movements of continental platforms may range be less than 1 mm/1000 years, the ground surface in northern Manitoba, near the centre of the former Laurentide ice sheet, is estimated to be rising about 1 cm/year. In southern California, groundwater pumping in the San Joaquin Valley between 1925 and 1967 led to land subsidence of up to 9 m, and oil withdrawal at Long Beach caused part of the city to subside 9.5 m. Pumping and recharging of ground water in the Los Angeles basin leads to ground rise and subsidence as much as 10 cm/year. The outflow of geothermal fluids has caused up to 4.5 m subsidence at Wairaki, New Zealand. Surface subsidence due to sediment compaction in the Nile Delta ranges up to 50 mm/year, and sections of the Dead Sea coast subsided up to 6.5 cms between 1992 and 1999 when falling groundwater levels led to compaction of soils, sands and gravels. Parts of central California near the San Andreas fault have moved laterally as much as 3.2 cm/yr over the past two decades. Large-scale lateral movements of tectonic plates may average as much as 7 cm/year and more: the Pacific plate is now converging on the Tonga Ridge near Samoa at rates of up to 24 cm/year. During a major earthquake in Taiwan in 1999, the lateral surface displacement was up to 9.8m in places.
Fissures and faults can develop suddenly during earthquakes and as a result of volcanic processes, or more slowly as a result of differential compaction during subsidence. In arid and semi-arid terrains, fissures up to several km long and a few cm wide may be rapidly eroded by surface run-off to gullies, some as much as 1-2 m wide and 2-3 m deep. In China, surface cracks due to fault growth have been observed to extend laterally at rates well over 100 m per year. In the USA, surface fault scarps have been noted up to 16 km long and 1 m or more high, growing vertically by non-seismic creep at rates up to 60 mm/yr. Regional shortening of 15 cm over a distance of 50 km was measured in Japan prior to an earthquake in April, 1995, following which there was a return to the former condition.
SIGNIFICANCE: Many surface displacements have but minor effects on landscapes and ecosystems, but fault displacements may disrupt surface drainage, rapidly create new relief features, rupture or destroy utilities, lines of communication, and other structures, or cause emergence or permanent inundation of inter-tidal ecosystems. Extraction of fluids beneath urban areas can induce land subsidence (as in Bangkok, Mexico City, Shanghai, and Venice) and may cause flooding, especially of coastal communities near sea-level. Subsidence can damage buildings, foundations and other structures: in the Houston-Galveston area of Texas, movements on more than 80 surface faults due to regional subsidence have caused millions of dollars of property damage. Extraction of gas along the north Italian coast has also lead to subsidence. Coastal subsidence can give rise to shoreline changes with consequent effects on coastal communities [shoreline position]. Uplift and subsidence can warn of impending earthquakes or eruptions or may be associated with a build up of crustal stress.
HUMAN OR NATURAL CAUSE: Surface displacements are natural phenomena associated with: plate movements (e.g. warping of Vancouver Island due to locking onto the Juan de Fuca subduction zone); post-glacial rebound, seismic faulting;. However, human activities such as extraction of groundwater, oil and gas can also induce surface displacements.
ENVIRONMENT WHERE APPLICABLE: Tectonically active areas (active fault zones, areas of high seismicity), areas formerly covered by ice sheets and now subject to isostatic rebound, karst regions, areas of near-surface mining or where subsurface fluids are being withdrawn.
TYPES OF MONITORING SITES: Active fault zones, reservoirs, coastal communities, deltas, and zones of groundwater, oil, gas, salt, and coal extraction, especially beneath urban areas.
SPATIAL SCALE: patch to regional
METHOD OF MEASUREMENT: Instrumented techniques include repeated precise levelling and ground surveys; geodetic techniques, including GPS and laser range finders; gravity determinations; and tide-gauge records in coastal zones. Methods employing Light Detection & Ranging, particularly where aircraft-mounted, have enabled local subsidence to be rapidly monitored. The development of satellite-borne techniques, such as Synthetic Aperture Radar Interferometry and Permanent-Scatterer Interferometry, has further increased capability for subsidence monitoring, and for detection over relatively large areas. Such techniques have the capability to resolve centimetre-scale movements over very large areas and it is now possible to track plate-tectonic movements using satellite based measurements. Measurements derived from repeated air photographs may reveal vertical and horizontal displacements. Past vertical movements may be inferred from archaeological studies of former coastal settlements, now below or above sea level. Stratigraphic methods involve detailed studies of deposits which, together with radiocarbon dating, may establish the approximate dates of large prehistoric displacements.
FREQUENCY OF MEASUREMENT: This depends on the type of movement taking place and the risk involved in such movements. Features such as active faults, and subsidence features, which directly threaten settlements or installations, are now routinely subjected to continuous monitoring with strain-sensitive instruments. Tele-metering of data via either land lines or wireless devices is routinely employed. Areas of lesser hazard tend to be monitored on an annual to decadal basis, using either ground-based surveying or photogrammetry.
LIMITATIONS OF DATA AND MONITORING: The timing of sudden collapse of the ground surface in karst terrain or above mined cavities, and surface movements due to earthquake faulting are not generally predictable, although the locations of many potentially hazardous sites have been identified. A major focus in both seismic and landslide studies is the recognition of premonitory events, or movements, which may signal that a catastrophic displacement or collapse is imminent. This requires continuous or periodic monitoring of displacements, is very expensive, and usually cannot be justified unless the risk from hazards is high. In areas of complex mountain terrain, the problem is especially acute, since multiple hazards from both seismic and gravitational deformations commonly occur in the same area. A concern with seismic hazards is that many earthquakes occur along faults that have little or no surface expression as scarps. This is common where active faults are obscured by tens to hundreds of metres of unconsolidated recent sediments. Long term monitoring of neotectonic or isostatic changes does not provide a means of predicting sudden displacements along faults and fissures due to earthquakes.
APPLICATIONS TO PAST AND FUTURE: Measured trends of slow regional subsidence, uplift, or lateral displacement may be used, with caution, as a basis for predicting likely long-term consequences, but only where long-term records are available. However, movement rates may suddenly accelerate beyond predicted values.
POSSIBLE THRESHOLDS: Most ground displacements involve a rupture of soil or rock material along a well-defined plane. This indicates that applied stresses have exceeded the local strength of the materials. In the case of seismic faults, the fault plane of rupture already exists, which predisposes it to future displacements as tectonic stress accumulates in the Earth's crust.. Premonitory movements typically occur as creep, a slow, progressive type of displacement: accelerating rates usually indicate that rupture is imminent.
Cooper, A.H. (1998) Subsidence hazards caused by dissolution of Permian gypsum in England: geology, investigation, and remediation. In: Geohazards in Engineering Geology. Special Publication 15 of the Geological Society, London, pp265-276.
Culshaw, M.G., McCann, D.M., Bell, F.G. (2004). Modern reconnaissance methods for geohazard detection and monitoring in site investigation. In: Advances in Geotechnical Engineering. The Skempton Conference. Thomas Telford, London.
Johnson, A.I. (ed) 1991. Land subsidence. Proceedings of 4th International Symposium on Land Subsidence . International Association of Hydrological Sciences Publication 200.
Land Subsidence in the US. 1999. USGS Circular 1182,
National Research Council 1986. Active Tectonics . National Academy Press, Washington.
OTHER SOURCES OF INFORMATION: National geological surveys, IAEG, IGA, Crustal Dynamics Data Information System of the Goddard Space Center, International Global Navigation Satellite System (IGS)
RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: Surface flooding in subsiding areas, damage to built structures, changes to hydrological systems. Rapid, warping of the ground surface may be a sign of impending sudden stress release, a precursor of an earthquake or, in an area of volcanic unrest, an eruption.
OVERALL ASSESSMENT: Displacements of the ground surface can be used to assess and warn of environmental problems, especially in coastal areas and in areas liable to subsidence from bedrock solution, mining and fluid extraction.
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