Peak ground acceleration
Encyclopedia
Peak ground acceleration (PGA) 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 (DBEGM)
Unlike the Richter
and moment magnitude
scales, it is not a measure of the total energy (magnitude, or size) of an earthquake, but rather of how hard the earth shakes in a given geographic area (the intensity). The Mercalli intensity scale
uses personal reports and observations to measure earthquake intensity but PGA is measured by instruments, such as accelerograph
s, and it generally correlates well with the Mercalli scale. See also seismic scale
.
The peak horizontal acceleration (PHA) is the most commonly used type of ground acceleration in engineering applications, and is used to set building code
s and design hazard risks. In an earthquake, damage to buildings and infrastructure is related more closely to ground motion, rather than the magnitude of the earthquake. For moderate earthquakes, PGA is the best determinate of damage; in severe earthquakes, damage is more often correlated with peak ground velocity.
(rate of change of speed) of these movements, while peak ground velocity is the greatest speed (rate of movement) reached by the ground, and peak displacement is the distance moved. These values vary in different earthquakes, and in differing sites within one earthquake event, depending on a number of factors. These include the length of the fault, magnitude, the depth of the quake, the distance from the epicentre, the duration (length of the shake cycle), and the geology of the ground (subsurface). Shallow-focused earthquakes generate stronger shaking (acceleration) than intermediate and deep quakes, since the energy is released closer to the surface.
Peak ground acceleration can be expressed in g (the acceleration due to Earth's gravity
, equivalent to g-force
) as either a decimal or percentage; in m/s2 (1g=9.81 m/s2); or in Gal
, where 1 Gal is equal to 0.01 m/s² (1g=981 Gal).
The ground type can significantly influence ground acceleration, so PGA values can display extreme variability over distances of a few kilometers, particularly with moderate to large earthquakes. The varying PGA results from an earthquake can be displayed on a shake map.
Due to the complex conditions affecting PGA, earthquakes of similar magnitude can offer disparate results, with many moderate magnitude earthquakes generating significantly larger PGA values than larger magnitude quakes.
During an earthquake, ground acceleration is measured in three directions: vertically (V or UD, for up-down) and two perpendicular horizontal directions (H1 and H2), often north-south (NS) and east-west (EW). The peak acceleration in each of these directions is recorded, with the highest individual value often reported. Alternatively, a combined value for a given station can be noted. The peak horizontal ground acceleration (PHA or PHGA) can reached by selecting the higher individual recording, taking the mean
of the two values, or calculating a vector sum of the two components. A three-component value can also be reached, by taking the vertical component into consideration also.
In seismic engineering, the effective peak acceleration (EPA) is often used, which tends to be ⅔ - ¾ the PGA.
and to create seismic hazard maps, which show the likely PGA values to be experienced in a region during an earthquake, with a probability of exceedance (PE). Seismic engineers and government planning departments use these values to determine the appropriate earthquake loading for buildings in each zone, with key identified structures (such as hospitals, bridges, power plants) needing to survive the maximum considered event (MCE).
Damage to buildings is related to both peak ground velocity and PGA, and the duration of the earthquake – the longer high-level shaking persists, the greater the likelihood of damage.
. Other intensity scales measure felt intensity, based on eyewitness reports, felt shaking, and observed damage. There is correlation between these scales, but not always absolute agreement since experiences and damage can be affected by many other factors, including the quality of earthquake engineering.
Generally speaking,
developed an Instrumental Intensity scale which maps peak ground acceleration and peak ground velocity on an intensity scale similar to the felt Mercalli scale
. These values are used to create shake maps by seismologists around the world.
, the highest intensity earthquake, Shindo 7, generally covers accelerations greater than 4 m/s² (0.41 g).
, areas with expected PGA values higher than 0.36g are classed as "Zone 5", or "Very High Damage Risk Zone".
|}
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...
acceleration on the ground and an important input parameter for earthquake engineering
Earthquake engineering
Earthquake 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...
, also known as the design basis earthquake ground motion (DBEGM)
Unlike the Richter
Richter magnitude scale
The expression Richter magnitude scale refers to a number of ways to assign a single number to quantify the energy contained in an earthquake....
and moment magnitude
Moment magnitude scale
The moment magnitude scale is used by seismologists to measure the size of earthquakes in terms of the energy released. The magnitude is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of...
scales, it is not a measure of the total energy (magnitude, or size) of an earthquake, but rather of how hard the earth shakes in a given geographic area (the intensity). The Mercalli intensity scale
Mercalli intensity scale
The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake. It measures the effects of an earthquake, and is distinct from the moment magnitude M_w usually reported for an earthquake , which is a measure of the energy released...
uses personal reports and observations to measure earthquake intensity but PGA is measured by instruments, such as accelerograph
Accelerograph
An accelerograph can be referred to as a strong motion seismograph, or simply as an earthquake accelerometer. They are usually constructed as a self-contained box, more commonly now being connected directly to the Internet....
s, and it generally correlates well with the Mercalli scale. See also seismic scale
Seismic scale
A seismic scale is used to calculate and compare the severity of earthquakes....
.
The peak horizontal acceleration (PHA) is the most commonly used type of ground acceleration in engineering applications, and is used to set 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 and design hazard risks. In an earthquake, damage to buildings and infrastructure is related more closely to ground motion, rather than the magnitude of the earthquake. For moderate earthquakes, PGA is the best determinate of damage; in severe earthquakes, damage is more often correlated with peak ground velocity.
Geophysics
Earthquake energy is dispersed in waves from the epicentre, causing ground movement horizontally (in two directions) and vertically. PGA records the accelerationAcceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...
(rate of change of speed) of these movements, while peak ground velocity is the greatest speed (rate of movement) reached by the ground, and peak displacement is the distance moved. These values vary in different earthquakes, and in differing sites within one earthquake event, depending on a number of factors. These include the length of the fault, magnitude, the depth of the quake, the distance from the epicentre, the duration (length of the shake cycle), and the geology of the ground (subsurface). Shallow-focused earthquakes generate stronger shaking (acceleration) than intermediate and deep quakes, since the energy is released closer to the surface.
Peak ground acceleration can be expressed in g (the acceleration due to Earth's gravity
Earth's gravity
The gravity of Earth, denoted g, refers to the acceleration that the Earth imparts to objects on or near its surface. In SI units this acceleration is measured in metres per second per second or equivalently in newtons per kilogram...
, equivalent to g-force
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...
) as either a decimal or percentage; in m/s2 (1g=9.81 m/s2); or in Gal
Gal (unit)
The gal, sometimes called galileo, is a unit of acceleration used extensively in the science of gravimetry. The gal is defined as 1 centimeter per second squared ....
, where 1 Gal is equal to 0.01 m/s² (1g=981 Gal).
The ground type can significantly influence ground acceleration, so PGA values can display extreme variability over distances of a few kilometers, particularly with moderate to large earthquakes. The varying PGA results from an earthquake can be displayed on a shake map.
Due to the complex conditions affecting PGA, earthquakes of similar magnitude can offer disparate results, with many moderate magnitude earthquakes generating significantly larger PGA values than larger magnitude quakes.
During an earthquake, ground acceleration is measured in three directions: vertically (V or UD, for up-down) and two perpendicular horizontal directions (H1 and H2), often north-south (NS) and east-west (EW). The peak acceleration in each of these directions is recorded, with the highest individual value often reported. Alternatively, a combined value for a given station can be noted. The peak horizontal ground acceleration (PHA or PHGA) can reached by selecting the higher individual recording, taking the mean
Mean
In statistics, mean has two related meanings:* the arithmetic mean .* the expected value of a random variable, which is also called the population mean....
of the two values, or calculating a vector sum of the two components. A three-component value can also be reached, by taking the vertical component into consideration also.
In seismic engineering, the effective peak acceleration (EPA) is often used, which tends to be ⅔ - ¾ the PGA.
Seismic risk and engineering
Study of geographic areas combined with an assessment of historical earthquakes allows geologists to determine seismic riskSeismic 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...
and to create seismic hazard maps, which show the likely PGA values to be experienced in a region during an earthquake, with a probability of exceedance (PE). Seismic engineers and government planning departments use these values to determine the appropriate earthquake loading for buildings in each zone, with key identified structures (such as hospitals, bridges, power plants) needing to survive the maximum considered event (MCE).
Damage to buildings is related to both peak ground velocity and PGA, and the duration of the earthquake – the longer high-level shaking persists, the greater the likelihood of damage.
Comparison of instrumental and felt intensity
Peak ground acceleration provides a measurement of instrumental intensity, that is, ground shaking recorded by seismic instrumentsSeismometer
Seismometers are instruments that measure motions of the ground, including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources...
. Other intensity scales measure felt intensity, based on eyewitness reports, felt shaking, and observed damage. There is correlation between these scales, but not always absolute agreement since experiences and damage can be affected by many other factors, including the quality of earthquake engineering.
Generally speaking,
- 0.001g (0.01 m/s²) – perceptible by people
- 0.02 g (0.2 m/s²) – people lose their balance
- 0.50g – very high; well-designed buildings can survive if the duration is short.
Correlation with the Mercalli scale
The United States Geological SurveyUnited States Geological Survey
The United States Geological Survey is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and the natural hazards that threaten it. The organization has four major science disciplines, concerning biology,...
developed an Instrumental Intensity scale which maps peak ground acceleration and peak ground velocity on an intensity scale similar to the felt Mercalli scale
Mercalli intensity scale
The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake. It measures the effects of an earthquake, and is distinct from the moment magnitude M_w usually reported for an earthquake , which is a measure of the energy released...
. These values are used to create shake maps by seismologists around the world.
| Instrumental Intensity |
Acceleration (g) |
Velocity (cm/s) |
Perceived Shaking | Potential Damage |
|---|---|---|---|---|
| I | < 0.0017 | < 0.1 | Not Felt | None |
| II-III | 0.0017 - 0.014 | 0.1 - 1.1 | Weak | None |
| IV | 0.014 - 0.039 | 1.1 - 3.4 | Light | None |
| V | 0.039 - 0.092 | 3.4 - 8.1 | Moderate | Very light |
| VI | 0.092 - 0.18 | 8.1 - 16 | Strong | Light |
| VII | 0.18 - 0.34 | 16 - 31 | Very Strong | Moderate |
| VIII | 0.34 - 0.65 | 31 - 60 | Severe | Moderate to Heavy |
| IX | 0.65 - 1.24 | 60 - 116 | Violent | Heavy |
| X+ | > 1.24 | > 116 | Extreme | Very Heavy |
Other intensity scales
In the 7-class Japan Meteorological Agency seismic intensity scaleJapan Meteorological Agency seismic intensity scale
The Japan Meteorological Agency seismic intensity scale is a seismic scale used in Japan and Taiwan to measure the intensity of earthquakes. It is measured in units of...
, the highest intensity earthquake, Shindo 7, generally covers accelerations greater than 4 m/s² (0.41 g).
PGA hazard risks worldwide
In IndiaEarthquake hazard zoning of India
The Indian subcontinent has a history of devastating earthquakes. The major reason for the high frequency and intensity of the earthquakes is that India is driving into Asia at a rate of approximately 47 mm/year. Geographical statistics of India show that almost 54% of the land is vulnerable...
, areas with expected PGA values higher than 0.36g are classed as "Zone 5", or "Very High Damage Risk Zone".
Notable earthquakes
| PGA single direction (max recorded) |
PGA vector sum (H1, H2, V) (max recorded) |
Mag | Depth | Fatalities | Earthquake |
|---|---|---|---|---|---|
| 2.7g | 2.99 g | 9.0 | 30 km | >15000 | 2011 Tōhoku earthquake and tsunami 2011 Tōhoku earthquake and tsunami The 2011 earthquake off the Pacific coast of Tohoku, also known as the 2011 Tohoku earthquake, or the Great East Japan Earthquake, was a magnitude 9.0 undersea megathrust earthquake off the coast of Japan that occurred at 14:46 JST on Friday, 11 March 2011, with the epicenter approximately east... |
| 2.2g | 6.3 | 5 km | 181 | February 2011 Christchurch earthquake | |
| 2.13g | 6.3 | 6 km | 1 | June 2011 Christchurch earthquake June 2011 Christchurch earthquake The June 2011 Christchurch earthquake was a shallow magnitude 6.3 ML earthquake that occurred on 13 June 2011 at 14:20 NZST . It was centred at a depth of 6.0 km , about 13 km from Christchurch, which had previously been devastated by the February 2011 magnitude 6.3 ML earthquake... |
|
| 4.36g | 6.9/7.2 | 8 km | 12 | 2008 Iwate-Miyagi Nairiku earthquake 2008 Iwate-Miyagi Nairiku earthquake The 2008 Iwate earthquake on June 14, 2008 mainly struck mid Tōhoku region, northeastern Honshū, Japan. Japan Meteorological Agency officially named this earthquake .... |
|
| 1.7g | 6.7 | 19 km | 57 | 1994 California earthquake | |
| 1.47g | 7.1 | 42km | 4 | April 2011 Miyagi earthquake April 2011 Miyagi earthquake The April 2011 Miyagi earthquake was a magnitude 7.1 Mw earthquake, with an epicentre off the coast of Miyagi Prefecture, approximately east of Sendai, Japan... |
|
| 1.26g | 7.1 | 10 km | 0 | 2010 Canterbury earthquake 2010 Canterbury earthquake The 2010 Canterbury earthquake was a 7.1 magnitude earthquake, which struck the South Island of New Zealand at 4:35 am on local time .... |
|
| 1.01g | 6.6 | 10 km | 11 | 2007 Chūetsu offshore earthquake 2007 Chuetsu offshore earthquake The Chūetsu Offshore Earthquake was a powerful magnitude 6.6 earthquake that occurred 10:13 a.m. local time on July 16, 2007, in the northwest Niigata region of Japan. The earthquake shook Niigata and neighbouring prefectures... |
|
| 1.01g | 7.3 | 8 km | 2,415 | 1999 Jiji earthquake | |
| 0.8g | 6.8 | 16 km | 6,434 | 1995 Kobe earthquake Great Hanshin earthquake The Great Hanshin earthquake, or Kobe earthquake, was an earthquake that occurred on Tuesday, January 17, 1995, at 05:46 JST in the southern part of Hyōgo Prefecture, Japan. It measured 6.8 on the moment magnitude scale , and Mj7.3 on JMA magnitude scale. The tremors lasted for approximately 20... |
|
| 0.78g | 8.8 | 23 km | 521 | 2010 Chile earthquake 2010 Chile earthquake The 2010 Chile earthquake occurred off the coast of central Chile on Saturday, 27 February 2010, at 03:34 local time , having a magnitude of 8.8 on the moment magnitude scale, with intense shaking lasting for about three minutes. It ranks as the sixth largest earthquake ever to be recorded by a... |
|
| 0.6g | 6.0 | 10 km | 143 | 1999 Athens earthquake 1999 Athens earthquake The 1999 Athens earthquake, registering a moment magnitude of 6.0, occurred on September 7, 1999, at 2:56:50 pm local time and lasted approximately 15 seconds. The tremor was epicentered approximately 17 km to the northwest of the city center, in a sparsely populated area between the... |
|
| 0.51g | 6.4 | 612 | 2005 Zarand earthquake 2005 Zarand earthquake 2005 Zarand earthquake hit the city of Zarand and several villages in Kerman province of Iran on February 22, 2005. The earthquake happened at 5:55 local time and was measured at 6.4 on the Richter scale. The quake lasted for 11 seconds and at least 61 aftershocks were reported ranging in... |
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| 0.5g | 7.0 | 13 km | 92,000-316,000 | 2010 Haiti earthquake 2010 Haiti earthquake The 2010 Haiti earthquake was a catastrophic magnitude 7.0 Mw earthquake, with an epicentre near the town of Léogâne, approximately west of Port-au-Prince, Haiti's capital. The earthquake occurred at 16:53 local time on Tuesday, 12 January 2010.By 24 January, at least 52 aftershocks... |
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| 0.438g | 7.7 | 44 km | 27 | 1978 Miyagi earthquake 1978 Miyagi earthquake The occurred at 17:14 local time on 12 June. It had a magnitude of 7.7, JMA magnitude 7.4, and triggered a small tsunami. The earthquake reached a maximum intensity of Shindo 5 in Sendai and caused 28 deaths and 1,325 injuries.-Geology:... (Sendai) |
|
| 0.367g | 5.2 | 1 km | 9 | 2011 Lorca earthquake 2011 Lorca earthquake The 2011 Lorca earthquake was a moderate magnitude 5.1 Mw earthquake that caused significant localized damage in the Region of Murcia, Spain. Centred at a very shallow depth of 1 km near the town of Lorca, it occurred at 18:47 CEST on 11 May 2011, causing panic among locals and displacing... |
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| 0.25 - 0.3g | 9.5 | 33 km | 1,655 | 1960 Valdivia earthquake | |
| 0.24g | 6.4 | 628 | 2004 Morocco earthquake 2004 Morocco earthquake The 2004 Morocco earthquake, at a magnitude 6.4Mw, occurred on 24 February 2004 near the coast of northern Morocco.At least 628 people were killed, 926 injured, 2,539 homes destroyed and more than 15,000 people homeless in the Al Hoceima-Imzourene-Beni Abdallah area, Morocco. Maximum intensity IX... |
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| 0.18g | 9.2 | 23 km | 143 |
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See also
- Spectral accelerationSpectral accelerationSpectral acceleration is a unit measured in g that describes the maximum acceleration in an earthquake on an object – specifically a damped, harmonic oscillator moving in one physical dimension...
- Japan Meteorological Agency seismic intensity scaleJapan Meteorological Agency seismic intensity scaleThe Japan Meteorological Agency seismic intensity scale is a seismic scale used in Japan and Taiwan to measure the intensity of earthquakes. It is measured in units of...
- Earthquake simulationEarthquake simulationEarthquake simulation applies a real or simulated vibrational input to a structure that possesses the essential features of a real seismic event...