A seismic hazard is the probability that an

earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...

will occur in a given geographic area, within a given window of time, and with

ground motion Ground motion is the movement of the Earth’s surface from earthquakes or explosions. Ground motion is produced by seismic waves that are generated by sudden slip on a fault or sudden pressure at the explosive source and travel through the Eart ...

intensity exceeding a given threshold. With a hazard thus estimated,

risk In simple terms, risk is the possibility of something bad happening. Risk involves uncertainty about the effects/implications of an activity with respect to something that humans value (such as health, well-being, wealth, property or the environ ...

can be assessed and included in such areas as

building code A building code (also building control or building regulations) is a set of rules that specify the standards for construction objects such as buildings and non-building structures. Buildings must conform to the code to obtain planning permis ...

s for standard buildings, designing larger buildings and infrastructure projects,

land use planning Land use planning or ''Land-use regulation'' is the process of regulating the Land use, use of land by a central authority. Usually, this is done to promote more desirable social and environmental outcomes as well as a more efficient resource u ...

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 Probability is a branch of mathematics and statistics concerning events and numerical descriptions of how likely they are to occur. The probability of an event is a number between 0 and 1; the larger the probability, the more likely an e ...

Maximum Considered Earthquake (or Event ), used in standard building codes, and the more detailed and

deterministic Determinism is the metaphysical view that all events within the universe (or multiverse) can occur only in one possible way. Deterministic theories throughout the history of philosophy have developed from diverse and sometimes overlapping mo ...

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

seismometer A seismometer is an instrument that responds to ground displacement and shaking such as caused by quakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The out ...

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 The San Andreas Fault is a continental Fault (geology)#Strike-slip faults, right-lateral strike-slip transform fault that extends roughly through the U.S. state of California. It forms part of the tectonics, tectonic boundary between the Paci ...

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 The Ring of Fire (also known as the Pacific Ring of Fire, the Rim of Fire, the Girdle of Fire or the Circum-Pacific belt) is a tectonic belt of volcanoes and earthquakes. It is about long and up to about wide, and surrounds most of the Pa ...

. Zones of higher seismicity in the continental interior may be the site for

intraplate earthquake An intraplate earthquake occurs in the ''interior'' of a Plate tectonics, tectonic plate, in contrast to an interplate earthquake on the ''boundary'' of a tectonic plate. They are relatively rare compared to the more familiar interplate earthqu ...

s) and tend to be drawn as broad areas, based on historic records, like the

1812 New Madrid earthquake Year 181 ( CLXXXI) was a common year starting on Sunday of the Julian calendar. At the time, it was known as the Year of the Consulship of Aurelius and Burrus (or, less frequently, year 934 ''Ab urbe condita''). The denomination 181 for this ye ...

, 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 Paleoseismology is the study of ancient earthquakes using geologic evidence, such as geologic sediments and rocks. It is used to supplement seismic monitoring to calculate seismic hazard. Paleoseismology is usually restricted to geologic reg ...

. The results may be in the form of a ground

response spectrum A response spectrum is a plot of the peak or steady-state response (displacement, velocity or acceleration) of a series of oscillators of varying natural frequency, that are forced into motion by the same base vibration or shock. The resulting plo ...

for use in

seismic analysis Seismic analysis is a subset of structural analysis and is the calculation of the response of a building (or nonbuilding) structure to earthquakes. It is part of the process of structural design, earthquake engineering or structural assessment ...

. More elaborate variations on the theme also look at the soil conditions. 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 postulating

characteristic earthquake Earthquake prediction is a branch of the science of geophysics, primarily seismology, concerned with the specification of the time, location, and magnitude of future earthquakes within stated limits, and particularly "the determination of para ...

s. 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 Landslides, also known as landslips, rockslips or rockslides, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, mudflows, shallow or deep-seated slope failures and debris flows. Landslide ...

in steep terrain. Large area landsliding can also occur on rather gentle slopes as was seen in the

Good Friday earthquake The 1964 Alaska earthquake, also known as the Great Alaska earthquake and Good Friday earthquake, occurred at 5:36 PM AKST on Good Friday, March 27, 1964.

Anchorage, Alaska Anchorage, officially the Municipality of Anchorage, is the List of cities in Alaska, most populous city in the U.S. state of Alaska. With a population of 291,247 at the 2020 United States census, 2020 census, it contains nearly 40 percent of ...

, 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", 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.

US seismic hazard maps

Some maps released by the

USGS The United States Geological Survey (USGS), founded as the Geological Survey, is an government agency, agency of the United States Department of the Interior, U.S. Department of the Interior whose work spans the disciplines of biology, geograp ...

are shown with

peak ground acceleration Peak ground acceleration (PGA) is equal to the maximum ground acceleration that occurred during earthquake shaking at a location. PGA is equal to the amplitude of the largest absolute acceleration recorded on an wikt:accelerogram, accelerogram at a ...

with a 10%

probability of exceedance The frequency of exceedance, sometimes called the annual rate of exceedance, is the frequency with which a random process exceeds some critical value. Typically, the critical value is far from the mean. It is usually defined in terms of the number ...

in 50 years, measured in

Metre per second squared The metre per second squared or metre per square second is the unit of acceleration in the International System of Units (SI). As a derived unit, it is composed from the SI base units of length, the metre, and of time, the second. Its symbol i ...

. 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 In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

) 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."

Global seismic hazard maps

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

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

References

External links

Global Seismic Hazard Assessment ProgramInfrastructure Risk Research Project at The University of British Columbia, Vancouver, CanadaDiagnose the impact of global earthquakes from direct and indirect eyewitnesses contributions
{{DEFAULTSORT:Seismic Hazard Earthquake and seismic risk mitigation

MCEs

US seismic hazard maps

Global seismic hazard maps

See also

References

External links