Peak ground acceleration

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.{{cite web|title=Explanation of Parameters|url=https://geohazards.usgs.gov/deaggint/2002/documentation/parm.php|department=Geologic Hazards Science Center|publisher=United States Geological Survey|access-date=22 March 2011|archive-url=https://web.archive.org/web/20110721070322/https://geohazards.usgs.gov/deaggint/2002/documentation/parm.php|archive-date=21 July 2011}} 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.{{cite web|url=https://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/us2007ewac.php#summary|title=Magnitude 6.6 – Near the west coast of Honshu, Japan|work=Earthquake summary|publisher=United States Geological Survey|date=16 July 2001|access-date=15 March 2011|archive-url=https://web.archive.org/web/20110314201821/http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/us2007ewac.php#summary|archive-date=14 March 2011}}

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{{sup|2}} (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.{{cite web|title=ShakeMap scientific background. Peak acceleration maps|url=https://earthquake.usgs.gov/earthquakes/shakemap/background.php#accmaps|department=Earthquake Hazards Program|publisher=United States Geological Survey|access-date=22 March 2011|archive-url=https://web.archive.org/web/20110623092131/http://earthquake.usgs.gov/earthquakes/shakemap/background.php#accmaps|archive-date=23 June 2011}} The varying PGA results from an earthquake can be displayed on a shake map.{{cite web|title=ShakeMap Scientific Background|url=https://earthquake.usgs.gov/earthquakes/shakemap/background.php|department=Earthquake Hazards Program|publisher=United States Geological Survey|access-date=22 March 2011|archive-url=https://web.archive.org/web/20110623092131/http://earthquake.usgs.gov/earthquakes/shakemap/background.php|archive-date=23 June 2011}}

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 {{frac|2|3}} – {{frac|3|4}} the PGA.{{Citation needed|date=September 2011}}

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.

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,

• 0.001 g (0.01 m/s{{sup|2}}) – perceptible by people
• 0.02  g (0.2  m/s{{sup|2}}) – people lose their balance
• 0.50  g (5  m/s{{sup|2}}) – very high; well-designed buildings can survive if the duration is short.{{cite web|url=http://www.wbdg.org/resources/seismic_design.php|title=Seismic Design Principles|author=Lorant, Gabor|work=Whole Building Design Guide|publisher=National Institute of Building Sciences|date=17 June 2010|access-date=15 March 2011}}

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.

In the 7-class Japan Meteorological Agency seismic intensity scale, the highest intensity, Shindo 7, covers accelerations greater than 4 m/s{{sup|2}} (0.41 g).

In India, areas with expected PGA values higher than 0.36 g are classed as "Zone 5", or "Very High Damage Risk Zone".

• Earthquake simulation
• Japan Meteorological Agency seismic intensity scale
• Spectral acceleration

{{reflist}}

• {{Cite journal

| last = Murphy

| first = J.R.

|author2=o'brien

| year = 1977

| title = The correlation of peak ground acceleration amplitude with seismic intensity and other physical parameters

| journal = Bulletin of the Seismological Society of America

| volume = 67

| issue = 3

| pages = 877–915

| doi = 10.1785/BSSA0670030877

| bibcode = 1977BuSSA..67..877M

| s2cid = 129134843

}}

• {{Cite journal

| last = Campbell

| first = K.W.

| year = 1997

| title = Empirical near-source attenuation relationships for horizontal and vertical components of peak ground acceleration, peak ground velocity, and pseudo-absolute acceleration response spectra

| journal = Seismological Research Letters

| volume = 68

| issue = 1

| pages = 154–179

| doi = 10.1785/gssrl.68.1.154

| bibcode = 1997SeiRL..68..154C

}}

• {{Cite journal

| last = Campbell

| first = K.W.

|author2=Y. Bozorgnia

| year = 2003

| title = Updated near-source ground-motion (attenuation) relations for the horizontal and vertical components of peak ground acceleration and acceleration response spectra

| journal = Bulletin of the Seismological Society of America

| volume = 93

| issue = 1

| pages = 314–331

| doi = 10.1785/0120020029

|bibcode = 2003BuSSA..93..314C | url = http://bssa.geoscienceworld.org/content/ssabull/93/4/1872.full.pdf

}}

• {{Cite journal

| last = Wald

| first = D.J. |author2=V. Quitoriano |author3=T.H. Heaton |author4=H. Kanamori

| year = 1999

| title = Relationships between peak ground acceleration, peak ground velocity, and modified Mercalli intensity in California

| journal = Earthquake Spectra

| volume = 15

| pages = 557

| doi = 10.1193/1.1586058

| issue = 3

| bibcode = 1999EarSp..15..557W | s2cid = 110698014 }}

{{DEFAULTSORT:Peak Ground Acceleration}}

Category:Seismology

Category:Earthquake engineering

Category:Acceleration