Seismic hazard
Encyclopedia
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. With hazards thus determined, their risks
are assessed and included in such areas as building code
s 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.
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 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 earthquake
s) 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. 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.
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.
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.
The Global Seismic Hazard map: http://www.seismo.ethz.ch/GSHAP/
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
ground motions at the earth's surface, and its likely effects on existing natural conditions and man-made structures for public safety
Public Safety
Public safety involves the prevention of and protection from events that could endanger the safety of the general public from significant danger, injury/harm, or damage, such as crimes or disasters .-See also:* By nation...
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. With hazards thus determined, their risks
Seismic risk
Seismic risk uses the results of a seismic hazard analysis, and includes both consequence and probability. Seismic risk has been defined, for most management purposes, as the potential economic, social and environmental consequences of hazardous events that may occur in a specified period of time...
are assessed and included in such areas as building code
Building code
A building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the...
s for standard buildings, designing larger buildings and infrastructure projects, land use planning
Land use planning
Land-use planning is the term used for a branch of public policy encompassing various disciplines which seek to order and regulate land use in an efficient and ethical way, thus preventing land-use conflicts. Governments use land-use planning to manage the development of land within their...
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
Probability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...
Maximum Considered Earthquake, used in standard building codes, and the more detailed and deterministic
Determinism
Determinism is the general philosophical thesis that states that for everything that happens there are conditions such that, given them, nothing else could happen. There are many versions of this thesis. Each of them rests upon various alleged connections, and interdependencies of things and...
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.
C. Allin Cornell
C. Allin Cornell is a civil engineer who made important contributions to reliability theory and earthquake engineering and, along with Dr. Luis Esteva, developed the field of Probabilistic Seismic Hazard Analysis in 1968....
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
Hypocenter
The hypocenter refers to the site of an earthquake or a nuclear explosion...
and at the at the surface
Epicenter
The epicenter or epicentre is the point on the Earth's surface that is directly above the hypocenter or focus, the point where an earthquake or underground explosion originates...
from seismometer
Seismometer
Seismometers are instruments that measure motions of the ground, including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources...
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
San Andreas Fault
The San Andreas Fault is a continental strike-slip fault that runs a length of roughly through California in the United States. The fault's motion is right-lateral strike-slip...
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
Pacific Ring of Fire
The Pacific Ring of Fire is an area where large numbers of earthquakes and volcanic eruptions occur in the basin of the Pacific Ocean. In a horseshoe shape, it is associated with a nearly continuous series of oceanic trenches, volcanic arcs, and volcanic belts and/or plate movements...
. Zones of higher seismicity in the continental interior may be the site for intraplate earthquake
Intraplate earthquake
An intraplate earthquake is an earthquake that occurs in the interior of a tectonic plate, whereas an interplate earthquake is one that occurs at a plate boundary....
s) 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
Maximum magnitude
An important parameter in the calculation of seismic hazard, maximum magnitude is also one of the more contentious...
), 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
Peak ground acceleration
Peak ground acceleration is a measure of earthquake acceleration on the ground and an important input parameter for earthquake engineering, also known as the design basis earthquake ground motion...
, 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
Paleoseismology
Paleoseismology looks at geologic sediments and rocks, for signs of ancient earthquakes. It is used to supplement seismic monitoring, for the calculation of seismic hazard...
. The results may be in the form of a ground response spectrum
Response spectrum
A response spectrum is simply a plot of the peak or steady-state response of a series of oscillators of varying natural frequency, that are forced into motion by the same base vibration or shock. The resulting plot can then be used to pick off the response of any linear system, given its natural...
for use in seismic analysis
Seismic analysis
Seismic Analysis is a subset of structural analysis and is the calculation of the response of a building structure to earthquakes...
.
More elaborate variations on the theme also look at the soil conditions. 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
Good Friday Earthquake
The 1964 Alaska earthquake, also known as the Great Alaskan Earthquake, the Portage Earthquake and the Good Friday Earthquake, was a megathrust earthquake that began at 5:36 P.M. AST on Good Friday, March 27, 1964...
" in Anchorage, Alaska
Anchorage, Alaska
Anchorage is a unified home rule municipality in the southcentral part of the U.S. state of Alaska. It is the northernmost major city in the United States...
, 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 are shown with peak ground accelerationPeak ground acceleration
Peak ground acceleration is a measure of earthquake acceleration on the ground and an important input parameter for earthquake engineering, also known as the design basis earthquake ground motion...
with a 10% probability of exceedance in 50 years, measured in Metre per second squared
Metre per second squared
The metre per second squared is the unit of acceleration in the International System of Units . As a derived unit it is composed from the SI base units of length, the metre, and the standard unit of time, the second...
. 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
- C. Allin CornellC. Allin CornellC. Allin Cornell is a civil engineer who made important contributions to reliability theory and earthquake engineering and, along with Dr. Luis Esteva, developed the field of Probabilistic Seismic Hazard Analysis in 1968....
- Earthquake engineeringEarthquake engineeringEarthquake engineering is the scientific field concerned with protecting society, the natural and the man-made environment from earthquakes by limiting the seismic risk to socio-economically acceptable levels...
- Mitigation of seismic motionMitigation of seismic motionMitigation 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 or indirect .Knowledge of local amplification of the seismic motion from...
- NeotectonicsNeotectonicsNeotectonics is a subdiscipline of tectonics. It is the study of the motions and deformations of the Earth's crust which are current or recent in geologic time. The term may also refer to the motions/deformations in question themselves. The corresponding time frame is referred to as the...
- Seismic performance
- Vibration controlVibration controlIn earthquake engineering, vibration control is a set of technical means aimed to mitigate seismic impacts in building and non-building structures.All seismic vibration control devices may be classified as passive, active or hybrid where:...
External links
U.S. Geological Survey National Seismic Hazard Maps: http://earthquake.usgs.gov/research/hazmaps/The Global Seismic Hazard map: http://www.seismo.ethz.ch/GSHAP/