Seismic hazard

Seismic hazard

Seismic hazard refers to the study of expected earthquake ground motions at the earth's surface, and its likely effects on existing natural conditions and man-made structures for public safety considerations; the results of such studies are published as seismic hazard maps, which identify the relative motion of different areas on a local, regional or national basis.[1] With hazards thus determined, their risks are assessed and included in such areas as building codes for standard buildings, designing larger buildings and infrastructure projects, land use planning and determining insurance rates. The seismic hazard studies also may generate two standard measures of anticipated ground motion, both confusingly abbreviated MCE; the simpler probabilistic Maximum Considered Earthquake, used in standard building codes, and the more detailed and deterministic Maximum Credible Earthquake incorporated in the design of larger buildings and civil infrastructure like dams or bridges. It is important to clarify which MCE is being discussed.[2]

Surface motion map for a hypothetical earthquake on the northern portion of the Hayward Fault Zone and its presumed northern extension, the Rodgers Creek Fault Zone

Calculations for determining seismic hazard were first formulated by C. Allin Cornell in 1968[3] and, depending on their level of importance and use, can be quite complex.[4] The regional geology and seismology setting is first examined for sources and patterns of earthquake occurrence, both in depth and at the at the surface from seismometer records; secondly, the impacts from these sources are assessed relative to local geologic rock and soil types, slope angle and groundwater conditions. Zones of similar potential earthquake shaking are thus determined and drawn on maps. The well known San Andreas Fault is illustrated as a long narrow elliptical zone of greater potential motion, like many areas along continental margins associated with the Pacific ring of fire. Zones of higher seismicity in the continental interior may be the site for intraplate earthquakes) and tend to be be drawn as broad areas, based on historic records, like the 1812 New Madrid earthquake, since specific causative faults are generally not identified as earthquake sources.

Each zone is given properties associated with source potential: how many earthquakes per year, the maximum size of earthquakes (maximum magnitude), etc. Finally, the calculations require formulae that give the required hazard indicators for a given earthquake size and distance. For example, some districts prefer to use peak acceleration, others use peak velocity, and more sophisticated uses require response spectral ordinates.

The computer program then integrates over all the zones and produces probability curves for the key ground motion parameter. The final result gives you a 'chance' of exceeding a given value over a specified amount of time. Standard building codes for homeowners might be concerned with a 1 in 500 years chance, while nuclear plants look at the 10,000 year time frame. A longer-term seismic history can be obtained through paleoseismology. The results may be in the form of a ground response spectrum for use in seismic analysis.

More elaborate variations on the theme also look at the soil conditions.[5] If you build on a soft swamp, you are likely to experience many times the ground motions than your neighbour on solid rock. The standard seismic hazard calculations become adjusted upwards if you are postulating characteristic earthquakes.

Areas with high ground motion due to soil conditions are also often subject to soil failure due to liquefaction. Soil failure can also occur due to earthquake-induced landslides in steep terrain. Large area landsliding can also occur on rather gentle slopes as was seen in the "Good Friday Earthquake" in Anchorage, Alaska, March 28, 1964.



In a normal seismic hazard analyses intended for the public, that of a "maximum considered earthquake", or "maximum considered event" (MCE) for a specific area, is an earthquake that is expected to occur once in approximately 2,500 years; that is, it has a 2-percent probability of being exceeded in 50 years. The term is used specifically for general building codes, which people commonly occupy; building codes in many localities will require non-essential buildings to be designed for "collapse prevention" in an MCE, so that the building remains standing - allowing for safety and escape of occupants - rather than full structural survival of the building.

A far more detailed and stringent MCE stands for "maximum credible earthquake",[6][7] which is used in designing for skyscrapers and larger civil infrastructure, like dams, where structural failure could lead to other catastrophic consequences. These MCEs might require determining more than one specific earthquake event, depending on the variety of structures included.[8]

US seismic hazard maps

Some maps released by the USGS are shown with peak ground acceleration with a 10% probability of exceedance in 50 years, measured in Metre per second squared. For parts of the US, the National Seismic Hazard Mapping Project in 2008 resulted in seismic hazard maps showing peak acceleration (as a percentage of gravity) with a 2% probability of exceedance in 50 years.

See also


External links

U.S. Geological Survey National Seismic Hazard Maps:

The Global Seismic Hazard map:

Wikimedia Foundation. 2010.

Игры ⚽ Нужно сделать НИР?

Look at other dictionaries:

  • Seismic microzonation — map of Greater Bangkok prepared based on predominant period of site obtained from microtremor observations [1] Main article: Earthquake engineering Seismic microzonation is defined as the process …   Wikipedia

  • Seismic risk — uses the results of a seismic hazard analysis, and includes both consequence and probability. A building located in a region of high seismic hazard is at lower risk if it is built to sound seismic engineering principles. On the other hand, a… …   Wikipedia

  • Seismic retrofit — Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. Other retrofit techniques are applicable to areas subject to tropical cyclones,… …   Wikipedia

  • Seismic Hazards Mapping Act — The Seismic Hazard Mapping Act was in acted by the California legislature in April of 1997, primarily as a result of the Northridge earthquake of 1994. The act requires the creation and publication of maps showing areas where earthquake induced… …   Wikipedia

  • Mitigation of seismic motion — Main article: Earthquake engineering Mitigation of seismic motion is an important factor in earthquake engineering and construction in earthquake prone areas. The destabilizing action of an earthquake on constructions may be direct (seismic… …   Wikipedia

  • Natural hazard — This article is about the natural hazards that may lead to disasters. For natural disasters, see natural disaster. A natural hazard[1] is a threat of a naturally occurring event that will have a negative effect on people or the environment. Many… …   Wikipedia

  • Household seismic safety — Contributions of photographs related to details of preparation are needed. Household seismic safety is an issue for areas of the world with a high risk of seismic activity, and is concerned with the following: *Appropriate securing of household… …   Wikipedia

  • New Madrid Seismic Zone — Coordinates: 36°34′57″N 89°35′17″W / 36.58245°N 89.58801°W / 36.58245; 89.58801 …   Wikipedia

  • Urban seismic risk — is a subset of the general term seismic risk which describes the problems specific to centers of population when they are subjected to earthquakes. Many risks can be minimised with good earthquake construction, and seismic analysis. One of the… …   Wikipedia

  • Pacific Northwest Seismic Network — The Pacific Northwest Seismic Network, or PNSN collects and studies ground motions from about 400 seismometers in Oregon and Washington to monitor volcanic and tectonic activity, and gives advice and information, and works to mitigate earthquake… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”