seismic hazard

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A seismic hazard is the probability that an
earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth Earth is the third planet from the Sun and the only astronomical object ...
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 earth a ...
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 environme ...
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 constructed objects such as buildings and non-building structures. Buildings must conform to the code to obtain planning permission ...
s for standard buildings, designing larger buildings and infrastructure projects,
land use planning Land use planning 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 use of resources. More specifically, ...
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 the branch of mathematics concerning numerical descriptions of how likely an Event (probability theory), event is to occur, or how likely it is that a proposition is true. The probability of an event is a number between 0 and ...
Maximum Considered Earthquake (or Event ), used in standard building codes, and the more detailed and
deterministic Determinism is a Philosophy, philosophical view, where all events are determined completely by previously existing causes. Deterministic theories throughout the history of philosophy have developed from diverse and sometimes overlapping motive ...
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 noises and shaking such as caused by earthquakes, Types of volcanic eruptions, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to f ...
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 transform fault that extends roughly through California. It forms the tectonics, tectonic boundary between the Pacific Plate and the North American Plate, and its motion is Fault (geology)#Strike-slip fau ...
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 region around much of the rim of the Pacific Ocean where many Types of volcanic eruptions, volcanic eruptions and ...
. Zones of higher seismicity in the continental interior may be the site for intraplate earthquakes) and tend to be drawn as broad areas, based on historic records, like the
1812 New Madrid earthquake Year 181 (Roman numerals, CLXXXI) was a common year starting on Sunday (link will display the full calendar) 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 ' ...
, 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 looks at geologic sediments and rock (geology), rocks, for signs of ancient earthquakes. It is used to supplement seismology, seismic monitoring, for the calculation of seismic hazard. Paleoseismology is usually restricted to g ...
. The results may be in the form of a ground response spectrum for use in
seismic analysis Seismic analysis is a subset of structural analysis Structural analysis is a branch of Solid Mechanics which uses simplified models for solids like bars, beams and shells for engineering decision making. Its main objective is to determine ...
. 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 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 Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated grade (slope), slope failures, mudflows, and debris flows. Landslides occur in a variety of ...
in steep terrain. Large area landsliding can also occur on rather gentle slopes as was seen in the
Good Friday earthquake The 1964 Alaskan earthquake, also known as the Great Alaskan earthquake and Good Friday earthquake, occurred at 5:36 PM AKST on Good Friday Good Friday is a Christian holiday commemorating the crucifixion of Jesus and his death at Calvary. ...
in
Anchorage, Alaska Anchorage () is the largest city in the U.S. state of Alaska by population. With a population of 291,247 in 2020, it contains nearly 40% of the state's population. The Anchorage metropolitan area, which includes Anchorage and the neighboring Ma ...
, 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), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States The United Stat ...
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 is the Units of measurement, unit of acceleration in the International System of Units (SI). As a SI derived unit, derived unit, it is composed from the SI base units of length, the metre, and time, the second. Its sy ...
. 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 () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...
) with a 2% probability of exceedance in 50 years. Temblor, a company founded in 2014, offers a seismic hazard rank for the 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."