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BRIEF DESCRIPTION: Shallow- focus earthquakes (those with sources within a few tens of kms of the Earth's surface) are caused by crustal movements along strike- slip, normal and thrust faults, though they can also be induced anthropogenically. They can result in marked temporary or permanent changes in the landscape, depending on the magnitude of the earthquake, the location of its epicenter, and local soil and rock conditions [see surface displacement]. Deep- focus earthquakes (below about 70 km), unless of the highest magnitude, are unlikely to have serious surface manifestations.
To avoid, reduce or warn of environmental impacts, it is necessary to know the size, location, and frequency of seismic events. These parameters can identify active faults and the sense of motion along them. Also of great importance is the spatial pattern of seismicity, including the presence of seismic gaps, and the relationship to known faults and active volcanoes. At least three, and generally many more, monitoring sites are required to determine the necessary parameters.
Seismic observations constitute one of the oldest forms of systematic earth monitoring (geoindicators). There are now in operation many national, regional and international seismic networks, which provide information about the location, size and motion of earthquakes anywhere in the world. However, shallow- focus tremors of lower magnitude, may not be detected by these means, and must be monitored more closely, on a local basis. Seismic hazard maps can be constructed to identify areas at varying risk from earthquake damage.
SIGNIFICANCE: Earthquakes constitute one of the greatest natural hazards to human society. Between 1960 and 1990 earthquakes killed about 439,000 people worldwide and caused an overall economic loss of some $US 65 billion. The 1994 Northridge earthquake in California alone resulted in over US$30 billion in property damage, and the 1995 Kobe earthquake over $100 billion. Surface effects include uplift or subsidence, surface faulting, landslides and debris flows, liquefaction, ground shaking, and tsunamis (`tidal' waves caused by undersea tremors). Damage to buildings, roads, sewers, gas and water lines, power and telephone systems, and other built structures commonly occurs.
HUMAN OR NATURAL CAUSE: Earthquakes are predominantly natural events. However, shallow-focus seismic tremors can be induced by human actions that change near- surface rock stresses or fluid pressures. These actions include: extracting (or pumping back into the ground for storage or for secondary hydrocarbon recovery) water, gas, petroleum, waste fluids; mining or quarrying activities; and loading the surface with large water bodies (reservoirs). Underground explosions, particularly for nuclear testing, can also generate seismic events.
ENVIRONMENT WHERE APPLICABLE: Any area of active tectonics or weakness in old cratons, or where human activities change subsurface rock pressures.
TYPES OF MONITORING SITES: Remote, away from obvious sources of ground shaking, such as traffic, mines, quarries, and heavy industry. For heavily populated areas in seismically- active areas, a dense array of seismographs is recommended.
SPATIAL SCALE: mesoscale to regional / global
METHOD OF MEASUREMENT: Standard seismographs. These should be able to record three components of ground acceleration with a dynamic range of 10-5 to 1 g (acceleration due to gravity) in the frequency band 0.1 to 20.0 Hz, maintaining absolute time to a precision of 5 ms. Monitoring seismicity induced by mining or fluid extraction activities generally requires networks of closely-spaced (5 km) instruments that can record considerably higher frequencies (20-1500 Hz) than for natural seismicity. Seismic data should be transmitted quickly (preferably in real time) to central analysis units. The effects of increases in crustal stress, which can be released through earthquakes, is becoming increasingly important as a tool for estimating seismic hazard. Stress increase may be detected indirectly in many ways, for example by monitoring actual earth stress in mines and boreholes, magnetic, gravity and electric fields, water levels in wells, surface deformation (creep, tilt, extension or shortening). However, these are not a substitute for direct observations of seismicity using seismographs.
FREQUENCY OF MEASUREMENT: continuous
LIMITATIONS OF DATA AND MONITORING: Monitoring seismicity will identify where earthquakes are likely to occur and their potential magnitude, but not when they might be expected.
APPLICATIONS TO PAST AND FUTURE: Seismic records for the past century are available for many parts of the Earth. Extending this record through historical and paleoenvironmental studies may be important in establishing spatial and temporal patterns of significant seismicity. Despite many efforts, there is still no assured method for predicting when earthquakes will occur. Increases and impending release of crustal stresses may be indicated by certain geological and geophysical precursors, including fluctuations in water table levels in wells, changes in geomagnetic fields, piezoelectric effects, and ground surface tilting, shortening and displacement.
POSSIBLE THRESHOLDS: A threshold is reached when natural or induced stresses overcome the strength (resistance to failure) of a rock mass and rupture occurs, expressed as an earthquake. Several scales of earthquake magnitude are in common use, based on their surface effects. Near- surface tremors with magnitudes <5 may be felt, but are rarely damaging. Those >M5 can induce significant damage. Earthquakes above M7 can be expected to have severe environmental and human impacts.
Bolt, B.A. 1993. Earthquakes, New York: W.H. Freeman.
McGuire, R.K. (ed.) 1993. The practice of earthquake hazard assessment. International Association of Seismology and Physics of the Earth's Interior.
National Research Council 1991. Real- time earthquake monitoring: early warning and rapid response. US National Academy Press, Washington.
OTHER SOURCES OF INFORMATION: Some national building codes contain valuable information about seismic zones and risks. Geological surveys, emergency preparedness and disaster relief agencies, US Geological Survey (Office of Earthquakes, Volcanoes and Engineering, USGS, 905 National Center, Reston VA 22092, USA), IAEG, IDNDR, IASPEI (International Association of Seismology and Physics of the Earth's Interior: Secretary- General E.R.Engdahl, USGS, MS 967, Box 25046, Denver Federal Center, Denver, Colorado 80225, USA), UNDRO, World Data Center-A for Natural Hazards.
RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: Near- surface earthquakes can induce a wide range of important and generally irreversible changes in landscape morphology: including faults and surface fissure [see surface displacement], sand-soil liquefaction, rockfalls, debris flows and other forms of slope failure [see slope failure (landslides)]. Human activities such as constructing dams and reservoirs, pumping out or in waste fluids, hydrocarbons and water can trigger seismic activity in normally aseismic ('quiet') areas. The social and economic impacts of major earthquakes can be devastating, particularly in urban areas. A proper building code that sets standards for construction and maintenance should be based on knowledge of both seismicity and local ground conditions.
OVERALL ASSESSMENT: It is essential to monitor the seismicity of any tectonically active area so as to avoid or minimize injury to life and damage to property.
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