Peak ground acceleration explained

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.[1] 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)[2] 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 much 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[3] but these correlations are associated with large uncertainty.[4] [5]

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.[6] 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.

Geophysics

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.[7] 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.[8]

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/s (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.[9] The varying PGA results from an earthquake can be displayed on a shake map.[10] 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.

Generally speaking,

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.

Instrumental
Intensity		Acceleration
(g)		Velocity
(cm/s)		Perceived shakingPotential damage
I< 0.000464< 0.0215Not feltNone
II–III0.000464 – 0.002970.135 – 1.41WeakNone
IV0.00297 – 0.02761.41 – 4.65LightNone
V0.0276 – 0.1154.65 – 9.64ModerateVery light
VI0.115 – 0.2159.64 – 20StrongLight
VII0.215 – 0.40120 – 41.4Very strongModerate
VIII0.401 – 0.74741.4 – 85.8SevereModerate to heavy
IX0.747 – 1.3985.8 – 178ViolentHeavy
X+> 1.39> 178ExtremeVery 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/s (0.41 g).

PGA hazard risks worldwide

In India, areas with expected PGA values higher than 0.36 g 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)		Magnitude MwDepthFatalitiesEarthquake
3.23 g [12] 7.8 15 km 2 2016 Kaikōura earthquake
2.93 g [13] 3.54 g 9.5 33 km 1,000–6000 1960 Valdivia earthquake
2.88 g [14] 7.516 km2602024 Noto earthquake
2.7 g [15] 2.99 g[16] [17] 9.1[18] 30 km[19] 19,759[20] 2011 Tōhoku earthquake and tsunami
4.36 g [21] 6.9 8 km 12 2008 Iwate–Miyagi Nairiku earthquake
2.88 g[22] 7.510 km782024 Noto Peninsula Earthquake
1.92 g [23] 7.7 8 km 2,415 1999 Jiji earthquake
1.82 g [24] 6.7 18 km [25] 57 1994 Northridge earthquake
1.62 g [26] 7.810 km57,658 2023 Turkey–Syria earthquake
1.51 g [27] [28] 6.2[29] 5 km 185 February 2011 Christchurch earthquake
1.26 g [30] [31] 7.1 10 km 0 2010 Canterbury earthquake
1.25 g 6.6 8.4 km 58–65 1971 Sylmar earthquake
1.04 g [32] 6.6 10 km 11 2007 Chūetsu offshore earthquake
0.98 g [33] 7.016.1 km1182020 Aegean Sea earthquake
0.91 g 6.917.6 km5,502–6,434
0.8 g [34] 7.212 km2222013 Bohol earthquake
0.65 [35] 6.919 km631989 Loma Prieta earthquake
0.5 g [36] 7.013 km100,000–316,0002010 Haiti earthquake
0.34 g 6.415 km5,7782006 Yogyakarta earthquake
0.18 g [37] 9.225 km1311964 Alaska earthquake

See also

Bibliography

Notes and References

  1. Earthquake ground motion estimation using strong-motion records: a review of equations for the estimation of peak ground acceleration and response spectral ordinates. Earth-Science Reviews. 2003-04-01. 43–104. 61. 1–2. 10.1016/S0012-8252(02)00112-5. J. Douglas. 2003ESRv...61...43D.
  2. http://www.world-nuclear.org/info/inf18.html Nuclear Power Plants and Earthquakes
  3. Web site: ShakeMap Scientific Background. Rapid Instrumental Intensity Maps. Earthquake Hazards Program. U. S. Geological Survey. 22 March 2011. https://web.archive.org/web/20110623092131/http://earthquake.usgs.gov/earthquakes/shakemap/background.php#intmaps. 23 June 2011. dmy-all.
  4. Book: Cua, G.. "Best Practices" for Using Macroseismic Intensity and Ground Motion Intensity Conversion Equations for Hazard and Loss Models in GEM1. Global Earthquake Model. 2010. etal. 11 November 2015. https://web.archive.org/web/20151227050149/http://www.globalquakemodel.org/media/publication/GEM-TechnicalReport_2010-4.pdf. 27 December 2015.
  5. See also: Seismic magnitude scales
  6. Web site: The 2013 European Seismic Hazard Model (ESHM13). 2013. 11 November 2015. EFEHR. European Facilities for Earthquake Hazard & Risk. https://web.archive.org/web/20151227080447/http://www.efehr.org:8080/jetspeed/portal/hazard.psml. 27 December 2015.
  7. Web site: Explanation of Parameters. Geologic Hazards Science Center. U.S. Geological Survey. 22 March 2011. https://web.archive.org/web/20110721070322/https://geohazards.usgs.gov/deaggint/2002/documentation/parm.php. 21 July 2011.
  8. Web site: Magnitude 6.6 – Near the west coast of Honshu, Japan. Earthquake summary. USGS. 16 July 2001. 15 March 2011. https://web.archive.org/web/20110314201821/http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/us2007ewac.php#summary. 14 March 2011. dmy-all.
  9. Web site: ShakeMap scientific background. Peak acceleration maps. Earthquake Hazards Program. U. S. Geological Survey. 22 March 2011. https://web.archive.org/web/20110623092131/http://earthquake.usgs.gov/earthquakes/shakemap/background.php#accmaps. 23 June 2011. dmy-all.
  10. Web site: ShakeMap Scientific Background. Earthquake Hazards Program. U. S. Geological Survey. 22 March 2011. https://web.archive.org/web/20110623092131/http://earthquake.usgs.gov/earthquakes/shakemap/background.php. 23 June 2011. dmy-all.
  11. Web site: Seismic Design Principles. Lorant, Gabor. Whole Building Design Guide. National Institute of Building Sciences. 17 June 2010. 15 March 2011.
  12. Goto . Hiroyuki . Kaneko . Yoshihiro . Young . John . Avery . Hamish . Damiano . Len . Extreme Accelerations During Earthquakes Caused by Elastic Flapping Effect . Scientific Reports . 4 February 2019 . 9 . 1 . 1117 . 10.1038/s41598-018-37716-y . 30718810 . 6361895 . 2019NatSR...9.1117G .
  13. Web site: M 9.5 - 1960 Great Chilean Earthquake (Valdivia Earthquake). USGS. 21 September 2023.
  14. Schäfer, Andreas, et al. Center for Disaster Management and Risk Reduction Technology CEDIM Forensic Disaster Analysis Group (FDA). 2024, www.cedim.kit.edu/download/FDA_EQ_Japan2024.pdf, https://doi.org/10.5445/IR/1000166937. Accessed 3 Apr. 2024.
  15. Web site: March 11, 2011 M9.0 Tohoku, Japan Earthquake: Preliminary results. Erol Kalkan. Volkan Sevilgen. Volkan Sevilgen. United States Geological Survey. 17 March 2011. 22 March 2011. https://web.archive.org/web/20110324003222/http://nsmp.wr.usgs.gov/ekalkan/Tohoku/index.html. 24 March 2011.
  16. Web site: 平成23年(2011年)東北地方太平洋沖地震による強震動. About strong ground motion caused by the 2011 off the Pacific coast of Tohoku Earthquake . Kyoshin Bosai . 10 November 2021 .
  17. Web site: 2011 Off the Pacific Coast of Tohoku earthquake, Strong Ground Motion. National Research Institute for Earth Science and Disaster Prevention. 18 March 2011. https://web.archive.org/web/20110324003616/http://www.k-net.bosai.go.jp/k-net/topics/TohokuTaiheiyo_20110311/nied_kyoshin1e.pdf. 24 March 2011.
  18. Web site: M 9.1 - 2011 Great Tohoku Earthquake, Japan - Origin. USGS. 10 November 2021.
  19. Web site: Archived copy of USGS Magnitude 7 and Greater Earthquakes in 2011 . 8 September 2017 . https://web.archive.org/web/20160413163214/http://earthquake.usgs.gov/earthquakes/eqarchives/year/2011/2011_stats.php . 13 April 2016 .
  20. Web site: 平成23年（2011年）東北地方太平洋沖地震（東日本大震災）について（第162報）（令和4年3月8日）. Press release no. 162 of the 2011 Tohuku earthquake. 総務省消防庁災害対策本部. Fire and Disaster Management Agency. https://web.archive.org/web/20220827123449/https://www.fdma.go.jp/disaster/higashinihon/items/162.pdf. 2022-08-27 . 2022-09-23. Page 31 of the PDF file.
  21. Web site: Spatially Dense Velocity Structure Exploration in the Source Region of the Iwate-Miyagi Nairiku Earthquake. Masumi Yamada. Seismological Research Letters v. 81; no. 4. Seismological Society of America. July–August 2010. 597–604. 21 March 2011. etal.
  22. Web site: 2 January 2024 . 【速報】揺れの最大加速度、東日本大震災に匹敵 . 4 January 2024 . 47news.
  23. Web site: M 7.7 - 21 km S of Puli, Taiwan. USGS. 10 November 2021.
  24. Web site: The Northridge Earthquake of 1994: Ground Motions and Geotechnical Aspects. 7 April 2021. Yegian. M.K.. Ghahraman. Gazetas. G.. Dakoulas. P.. Makris. N.. April 1995. 1384. Third International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics. Northeastern University College of Engineering. https://web.archive.org/web/20130506100941/http://www.coe.neu.edu/Depts/CIV/faculty/myegian/library/Thenorthridge%20Earthquake%20of%201994%20Ground%20Motions%20and%20Geotechnical%20Aspects.pdf. 6 May 2013.
  25. Web site: M 6.7 - 1km NNW of Reseda, CA. USGS. 10 November 2021.
  26. Web site: M 7.8 - Pazarcik earthquake, Kahramanmaras earthquake sequence . 2023-04-07 . https://web.archive.org/web/20230302171403/https://earthquake.usgs.gov/earthquakes/eventpage/us6000jllz/map?shakemap-code=us6000jllz&shakemap-source=us&shakemap-intensity=true&shakemap-mmi-contours=false&shakemap-stations=true . 2023-03-02 .
  27. Web site: Feb 22 2011 – Christchurch badly damaged by magnitude 6.3 earthquake. Geonet. GNS Science. 23 February 2011. 24 February 2011. https://web.archive.org/web/20110304221355/http://www.geonet.org.nz/news/feb-2011-christchurch-badly-damaged-by-magnitude-6-3-earthquake.html. 4 March 2011.
  28. Web site: PGA intensity map. Geonet. GNS Science . 24 February 2011. https://web.archive.org/web/20120531202123/http://geonet.org.nz/var/storage/images/media/images/news/2011/lyttelton_pga/57159-2-eng-GB/lyttelton_pga.png. 31 May 2012.
  29. Web site: New Zealand Earthquake Report – Feb 22 2011 at 12:51 pm (NZDT). Geonet. GNS Science. 22 February 2011. 24 February 2011. https://web.archive.org/web/20110225224013/http://www.geonet.org.nz/earthquake/quakes/3468575g.html. 25 February 2011.
  30. News: Technically it's just an aftershock. Carter . Hamish. New Zealand Herald. APN Holdings. 24 February 2011. 24 February 2011.
  31. Web site: M 7.1, Darfield (Canterbury), September 4, 2010. GeoNet. GNS Science. 7 March 2011. https://web.archive.org/web/20110302094331/http://www.geonet.org.nz/earthquake/historic-earthquakes/top-nz/quake-13.html. 2 March 2011.
  32. News: Why Worry? Japan's Nuclear Plants at Grave Risk From Quake Damage. Katsuhiko, Ishibashi. Japan Focus. Asia Pacific Journal. 11 August 2001. 15 March 2011.
  33. Web site: Mauricio Morales . Oguz C. Celik . EERI PERW 2021 – Part 1: Aegean Sea Earthquake . 12 October 2021 . slc.eeri.org . Earthquake Engineering Research Institute . 27 June 2022 . https://web.archive.org/web/20220627065554/https://slc.eeri.org/2021-sdc/perw/ . dead .
  34. The Mw7.2 15 October 2013 Bohol, Philippines Earthquake . Emi-megacities.org . 17 December 2022 . 13 October 2022 . https://web.archive.org/web/20221013201102/https://emi-megacities.org/wp-content/uploads/2014/10/TR-14-01.pdf . dead .
  35. Book: Clough . G. W. . Practical lessons from the Loma Prieta earthquake . Martin . J. R. . Chameau . J. L. II . . 1994 . 978-0309050302 . 29–46 . The geotechnical aspects . Ray W. Clough.
  36. News: Lin . Rong-Gong . Allen . Sam . 26 February 2011 . New Zealand quake raises questions about L.A. buildings . Los Angeles Times . 27 February 2011.
  37. 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

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