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 accelerogram at a site during a particular earthquake. Earthquake shaking generally occurs in all three directions. Therefore, PGA is often split into the horizontal and vertical components. Horizontal PGAs are generally larger than those in the vertical direction but this is not always true, especially close to large earthquakes. PGA is an important parameter (also known as an intensity measure) for earthquake engineering, The design basis earthquake ground motion (DBEGM) is often defined in terms of PGA.
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 at a given geographic point. The Mercalli intensity scale uses personal reports and observations to measure earthquake intensity but PGA is measured by instruments, such as accelerographs. It can be correlated to macroseismic intensities on the Mercalli scale but these correlations are associated with large uncertainty. See also seismic scale.
The peak horizontal acceleration (PHA) is the most commonly used type of ground acceleration in engineering applications. It is often used within earthquake engineering (including seismic building codes) and it is commonly plotted on seismic hazard maps. In an earthquake, damage to buildings and infrastructure is related more closely to ground motion, of which PGA is a measure, rather than the magnitude of the earthquake itself. For moderate earthquakes, PGA is a reasonably good determinant of damage; in severe earthquakes, damage is more often correlated with peak ground velocity.
Earthquake energy is dispersed in waves from the hypocentre, causing ground movement omnidirectionally but typically modelled 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 fractions of g (the standard 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 multiples of 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 be 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 (PGV) 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".
- Douglas, J (2003-04-01). "Earthquake ground motion estimation using strong-motion records: a review of equations for the estimation of peak ground acceleration and response spectral ordinates" (PDF). Earth-Science Reviews. 61 (1–2): 43–104. Bibcode:2003ESRv...61...43D. doi:10.1016/S0012-8252(02)00112-5.
- Nuclear Power Plants and Earthquakes, accessed 2011-04-08
- "ShakeMap Scientific Background. Rapid Instrumental Intensity Maps". Earthquake Hazards Program. U. S. Geological Survey. Archived from the original on 23 June 2011. Retrieved 22 March 2011.
- Cua, G.; et al. (2010). "Best Practices" for Using Macroseismic Intensity and Ground Motion Intensity Conversion Equations for Hazard and Loss Models in GEM1 (PDF). Global Earthquake Model. Archived from the original (PDF) on 2015-12-27. Retrieved 2015-11-11.
- European Facilities for Earthquake Hazard & Risk (2013). "The 2013 European Seismic Hazard Model (ESHM13)". EFEHR. Archived from the original on 2015-12-27. Retrieved 2015-11-11.
- "Explanation of Parameters". Geologic Hazards Science Center. U.S. Geological Survey. Archived from the original on 21 July 2011. 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. Archived from the original on 14 March 2011. Retrieved 15 March 2011.
- "ShakeMap scientific background. Peak acceleration maps". Earthquake Hazards Program. U. S. Geological Survey. Archived from the original on 23 June 2011. Retrieved 22 March 2011.
- "ShakeMap Scientific Background". Earthquake Hazards Program. U. S. Geological Survey. Archived from the original on 23 June 2011. Retrieved 22 March 2011.
- Anna Kaiser (9 April 2017). "Kaikoura quake produced strongest ground shaking in NZ, new research shows - 10/04/2017". GNS Science. Retrieved 1 September 2017.
- Erol Kalkan, Volkan Sevilgen (17 March 2011). "March 11, 2011 M9.0 Tohoku, Japan Earthquake: Preliminary results". United States Geological Survey. Archived from the original on 24 March 2011. Retrieved 22 March 2011.
- "2011 Off the Pacific Coast of Tohoku earthquake, Strong Ground Motion" (PDF). National Research Institute for Earth Science and Disaster Prevention. Archived from the original (PDF) on 24 March 2011. Retrieved 18 March 2011.
- "Archived copy". Archived from the original on 2016-04-13. Retrieved 2017-09-08.CS1 maint: archived copy as title (link)
- "Damage Situation and Police Countermeasures associated with 2011Tohoku District – off the Pacific Earthquake" (PDF). 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. Archived from the original on 4 March 2011. Retrieved 24 February 2011.
- "PGA intensity map". Geonet. GNS Science. Archived from the original on 31 May 2012. Retrieved 24 February 2011.
- "New Zealand Earthquake Report – Feb 22 2011 at 12:51 pm (NZDT)". Geonet. GNS Science. 22 February 2011. Archived from the original on 25 February 2011. Retrieved 24 February 2011.
- "Jun 13 2011 – Large earthquakes strike south-east of Christchurch". Geonet. GNS Science. 13 June 2011. Archived from the original on 14 June 2011. Retrieved 14 June 2011.
- "PGA intensity map". Geonet. GNS Science. Archived from the original on 20 March 2012. 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.CS1 maint: extra punctuation (link)
- "Significant Earthquakes of the World". 2011-07-14. Archived from the original on 2011-07-14. Retrieved 2019-04-16.
- "Archived copy". Archived from the original on 2016-03-04. Retrieved 2017-09-08.CS1 maint: archived copy as title (link)
- 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. Archived from the original on 2 March 2011. 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). [permanent dead link]. 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" (PDF).[permanent dead link]
- "Archived copy". Archived from the original on 2010-05-13. Retrieved 2010-07-12.CS1 maint: archived copy as title (link)
- 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.
- 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.
- Brady, A. Gerald (1980). An investigation of the Miyagi-ken-oki, Japan, earthquake of June 12, 1978. National Bureau of Standards. p. 123.
- "Large quake off the coast of Christchurch". info.geonet.org.nz. Retrieved 2016-02-18.
- 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.
- 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" (PDF). Bulletin of the Seismological Society of America. 93 (1): 314–331. Bibcode:2003BuSSA..93..314C. doi:10.1785/0120020029.