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Seismic hazard

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Surface motion map for a hypothetical earthquake on the northern portion of theHayward Fault Zoneand its presumed northern extension, theRodgers Creek Fault Zone

Aseismic hazardis the probability that anearthquakewill occur in a given geographic area, within a given window of time, and withground motionintensity exceeding a given threshold.[1][2]With a hazard thus estimated,riskcan be assessed and included in such areas asbuilding codesfor standard buildings, designing larger buildings and infrastructure projects,land use planningand determining insurance rates. The seismic hazard studies also may generate two standard measures of anticipated ground motion, both confusingly abbreviated MCE; the simplerprobabilisticMaximum Considered Earthquake (or Event[3]), used in standard building codes, and the more detailed anddeterministicMaximum 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 byC. Allin Cornellin 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, bothin depthandat the surfacefromseismometerrecords; 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 knownSan Andreas Faultis illustrated as a long narrow elliptical zone of greater potential motion, like many areas along continental margins associated with thePacific ring of fire.Zones of higher seismicity in the continental interior may be the site forintraplate earthquakes) and tend to be drawn as broad areas, based on historic records, like the1812 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 usepeak 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 throughpaleoseismology.The results may be in the form of a groundresponse spectrumfor use inseismic analysis.

More elaborate variations on the theme also look at the soil conditions.[7]Higher ground motions are likely to be experienced on a soft swamp compared to a hard rock site. The standard seismic hazard calculations become adjusted upwards when postulatingcharacteristic earthquakes.Areas with high ground motion due to soil conditions are also often subject tosoil failure due to liquefaction.Soil failure can also occur due to earthquake-inducedlandslidesin steep terrain. Large area landsliding can also occur on rather gentle slopes as was seen in theGood Friday earthquakeinAnchorage, Alaska,March 28, 1964.

MCEs[edit]

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]

Map of peak ground acceleration with 2%probability of exceedancein 50 years

US seismic hazard maps[edit]

Some maps released by theUSGSare shown withpeak ground accelerationwith a 10%probability of exceedancein 50 years, measured inMetre 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 ofgravity) 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]

Color-blind friendly global map of peak ground acceleration with 10%probability of exceedancein 50 years

Global seismic hazard maps[edit]

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[edit]

References[edit]

  1. ^Baker, Jack; Bradley, Brendon; Stafford, Peter (2021).Seismic Hazard and Risk Analysis.Cambridge University Press.ISBN9781108425056.RetrievedJanuary 14,2021.
  2. ^Natural Resources Canada page on Seismic Hazard CalculationsArchived2008-06-02 at theWayback Machine
  3. ^Craig Taylor and Erik VanMarcke, ed. (2002).Acceptable Risk Processes: Lifeline and Natural Hazards.Reston, VA: ASCE, TCLEE.ISBN9780784406236.
  4. ^["Archived copy"(PDF).Archived fromthe original(PDF)on 2012-03-30.Retrieved2011-10-06.{{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, 329–338Archived2009-02-27 at theWayback 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. ^Earthquake Design and Evaluation for civil Works ProjectsEngineering and Design, Department of the Army,U.S. Army Corps of Engineers.Regulation No. 1110-2-1806, 31 July 1995
  9. ^Maximum credible earthquakeArchived2007-12-25 at theWayback Machine,United States Bureau of Reclamation
  10. ^"Maximum design earthquake".Archived fromthe originalon 2007-12-25.Retrieved2011-10-06.
  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[edit]

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