Peak ground acceleration
||The examples and perspective in this article may not represent a worldwide view of the subject. (September 2011)|
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 accelerographs, 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 codes 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.
Earthquake energy is dispersed in waves from the epicentre, causing ground movement horizontally (in two directions) and vertically. PGA records the acceleration (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 (1 g = 9.81 m/s2); or in Gal, where 1 Gal is equal to 0.01 m/s² (1 g = 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, the maximum ground acceleration to which a building responds) 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 risk 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 earthquake (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 instruments. 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.
- 0.001 g (0.01 m/s²) – perceptible by people
- 0.02 g (0.2 m/s²) – people lose their balance
- 0.50 g – very high; well-designed buildings can survive if the duration is short.
Correlation with the Mercalli scale
The United States Geological Survey 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.
|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 scale, the highest intensity, Shindo 7, covers accelerations greater than 4 m/s² (0.41 g).
PGA hazard risks worldwide
In India, areas with expected PGA values higher than 0.36g are classed as "Zone 5", or "Very High Damage Risk Zone".
- Nuclear Power Plants and Earthquakes, accessed 2011-04-08
- "ShakeMap Scientific Background. Rapid Instrumental Intensity Maps". Earthquake Hazards Program. U. S. Geological Survey. Retrieved 22 March 2011.
- "Explanation of Parameters". Geologic Hazards Science Center. U.S. Geological Survey. Retrieved 22 March 2011.
- Lorant, Gabor (17 June 2010). "Seismic Design Principles". Whole Building Design Guide. National Institute of Building Sciences. Retrieved 15 March 2011.
- "Magnitude 6.6 – Near the west coast of Honshu, Japan". Earthquake summary. USGS. 16 July 2001. Retrieved 15 March 2011.
- "ShakeMap scientific background. Peak acceleration maps". Earthquake Hazards Program. U. S. Geological Survey. Retrieved 22 March 2011.
- "ShakeMap Scientific Background". Earthquake Hazards Program. U. S. Geological Survey. Retrieved 22 March 2011.
- Erol Kalkan, Volkan Sevilgen (17 March 2011). "March 11, 2011 M9.0 Tohoku, Japan Earthquake: Preliminary results". United States Geological Survey. Retrieved 22 March 2011.
- "2011 Off the Pacific Coast of Tohoku earthquake, Strong Ground Motion". National Research Institute for Earth Science and Disaster Prevention. Retrieved 18 March 2011.
- "Damage Situation and Police Countermeasures associated with 2011Tohoku District - off the Pacific Earthquake". Emergency Disaster Countermeasures Headquarters. National Police Agency of Japan.
- "Feb 22 2011 - Christchurch badly damaged by magnitude 6.3 earthquake". Geonet. GNS Science. 23 February 2011. Retrieved 24 February 2011.
- "PGA intensity map". Geonet. GNS Science. Retrieved 24 February 2011.
- "PGA intensity map". Geonet. GNS Science. Retrieved 14 June 2011.
- Masumi Yamada et al (July/August 2010). "Spatially Dense Velocity Structure Exploration in the Source Region of the Iwate-Miyagi Nairiku Earthquake". Seismological Research Letters v. 81; no. 4;. Seismological Society of America. pp. 597–604. Retrieved 21 March 2011.
- Lin, Rong-Gong; Allen, Sam (26 February 2011). "New Zealand quake raises questions about L.A. buildings". Los Angeles Times (Tribune). Retrieved 27 February 2011.
- Carter, Hamish (24 February 2011). "Technically it's just an aftershock". New Zealand Herald (APN Holdings). Retrieved 24 February 2011.
- "M 7.1, Darfield (Canterbury), September 4, 2010". GeoNet. GNS Science. Retrieved 7 March 2011.
- Katsuhiko, Ishibashi (11 August 2001). "Why Worry? Japan's Nuclear Plants at Grave Risk From Quake Damage". Japan Focus (Asia Pacific Journal). Retrieved 15 March 2011.
- Central Weather Bureau. (2 September 2004). . Retrieved 21 March 2011.
- NZ Herald Article - Violence of tremors stuns experts. (24 Dec 2011). . Retrieved 24 December 2011.
- "Informe Tecnico Terremoto Cauquenes 27 de Febrero de 2010 Actualizado 27 de Mayo 2010".
- Anastasiadis A. N., et al. "The Athens (Greece) Earthquake of September 7, 1999: Preliminary Report on Strong Motion Data and Structural Response". Institute of Engineering Seismology and Earthquake Engineering. MCEER. Retrieved 22 March 2011.
- "Earthquake Mw 6.3 in Iran on February 22nd, 2005 at 02:25 UTC". European-Mediterranean Seismological Centre. Retrieved 7 March 2011.
- Brady, A. Gerald (1980). An investigation of the Miyagi-ken-oki, Japan, earthquake of June 12, 1978. National Bureau of Standards. p. 123.
- Los terremotos paradojicos - Seismo mortal en Murcia
- Crustal deformation associated with the 1960 earthquake events in the south of Chile
- Webber, Jude (27 February 2010). "Massive earthquake batters Chile". Financial Times. Retrieved 18 March 2011.
- USGS Earthquake Hazards Program » Magnitude 6.4 - NEAR NORTH COAST OF MOROCCO
- National Research Council (U.S.). Committee on the Alaska Earthquake, The great Alaska earthquake of 1964, Volume 1, Part 1, National Academies, 1968 p. 285
- Murphy, J.R.; o'brien (1977). "The correlation of peak ground acceleration amplitude with seismic intensity and other physical parameters". Bulletin of the Seismological Society of America 67 (3): 877–915.
- Campbell, K.W. (1997). "Empirical near-source attenuation relationships for horizontal and vertical components of peak ground acceleration, peak ground velocity, and pseudo-absolute acceleration response spectra". Seismological Research Letters 68: 154–179. doi:10.1785/gssrl.68.1.154.
- Campbell, K.W.; Y. Bozorgnia (2003). "Updated near-source ground-motion (attenuation) relations for the horizontal and vertical components of peak ground acceleration and acceleration response spectra". Bulletin of the Seismological Society of America 93 (1): 314–331. Bibcode:2003BuSSA..93..314C. doi:10.1785/0120020029.