Seismic hazard

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Seismic hazard map
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Rodgers Creek Fault Zone

A seismic hazard is the probability that an

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 (or Event[3] ), 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.[4]

Calculations for determining seismic hazard were first formulated by C. Allin Cornell in 1968[5] and, depending on their level of importance and use, can be quite complex.[6] The regional geology and seismology setting is first examined for sources and patterns of earthquake occurrence, both

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 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.

Good Friday earthquake in Anchorage, Alaska
, March 28, 1964.

MCEs

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",[8][9] 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.[10]

probability of exceedance
in 50 years

US seismic hazard maps

Some maps released by the

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.

Temblor, a company founded in 2014, offers a seismic hazard rank for all of the conterminous US. This service is free and ad-free for the public. The hazard rank "is made for the likelihood of experiencing strong shaking (0.4g peak ground acceleration) in 30 years, based on the 2014 USGS NSHMP hazard model."[11]

probability of exceedance
in 50 years

Global seismic hazard maps

Global seismic hazard maps exist too, which similarly present the level of certain ground motions that have a 10%

probability of exceedance (or a 90% chance of non-exceedance) during a 50-year time span (that corresponds to a return period of 475 years).[12]

See also

References

  1. ISBN 9781108425056
    . Retrieved January 14, 2021.
  2. ^ Natural Resources Canada page on Seismic Hazard Calculations Archived 2008-06-02 at the Wayback Machine
  3. ISBN 9780784406236
    .
  4. ^ ["Archived copy" (PDF). Archived from the original (PDF) on 2012-03-30. Retrieved 2011-10-06.{{cite web}}: CS1 maint: archived copy as title (link) Earthquake Definitions, Oregon State University
  5. ^ Cornell, C.A. 1968, Engineering seismic risk analysis, Bulletin of the Seismological Society of America, 58, 1583-1606
  6. ^ McGuire, R. 2008, Probabilistic seismic hazard analysis: Early history, Earthquake Engng Struct. Dyn., 37, 329–338 Archived 2009-02-27 at the Wayback Machine
  7. ^ Wang, Z. 2008. A technical note on seismic microzonation in the central United States, J. Earth Syst. Sci. 117, S2, pp. 749–756
  8. U.S. Army Corps of Engineers
    . Regulation No. 1110-2-1806, 31 July 1995
  9. ^ Maximum credible earthquake Archived 2007-12-25 at the Wayback Machine, United States Bureau of Reclamation
  10. ^ "Maximum design earthquake". Archived from the original on 2007-12-25. Retrieved 2011-10-06.
  11. ^ "Temblor website". Temblor, Inc.
  12. ^ Giardini, D., Grünthal, G., Shedlock, K. M. and Zhang, P.: The GSHAP Global Seismic Hazard Map. In: Lee, W., Kanamori, H., Jennings, P. and Kisslinger, C. (eds.): International Handbook of Earthquake & Engineering Seismology, International Geophysics Series 81 B, Academic Press, Amsterdam, 1233-1239, 2003.

External links

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