Peak ground acceleration

Source: Wikipedia, the free encyclopedia.

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

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

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][8] 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.[9]

Peak ground acceleration can be expressed in fractions of

Earth's gravity, equivalent to g-force) as either a decimal or percentage; in m/s2 (1 g = 9.81 m/s2);[7] 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.

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

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[citation needed].

Seismic risk and engineering

Study of geographic areas combined with an assessment of historical earthquakes allows geologists to determine

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,

  • 0.001 g (0.01 m/s2) – perceptible by people
  • 0.02  g (0.2  m/s2) – people lose their balance
  • 0.50  g (5  m/s2) – very high; well-designed buildings can survive if the duration is short.[8]

Correlation with the Mercalli scale

The

Mercalli scale. These values are used to create shake maps by seismologists around the world.[3]

Instrumental
Intensity
Acceleration
(g)
Velocity
(cm/s)
Perceived shaking Potential damage
I < 0.000464 < 0.0215 Not felt None
II–III 0.000464 – 0.00297 0.135 – 1.41 Weak None
IV 0.00297 – 0.0276 1.41 – 4.65 Light None
V 0.0276 – 0.115 4.65 – 9.64 Moderate Very light
VI 0.115 – 0.215 9.64 – 20 Strong Light
VII 0.215 – 0.401 20 – 41.4 Very strong Moderate
VIII 0.401 – 0.747 41.4 – 85.8 Severe Moderate to heavy
IX 0.747 – 1.39 85.8 – 178 Violent Heavy
X+ > 1.39 > 178 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 0.36 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 Mw Depth Fatalities Earthquake
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
2.7 g [14] 2.99 g[15][16] 9.1[17] 30 km[18] 19,759[19] 2011 Tōhoku earthquake and tsunami
4.36 g [20] 6.9 8 km 12 2008 Iwate–Miyagi Nairiku earthquake
2.88 g[21] 7.5 10 km 78
2024 Noto Peninsula Earthquake
1.92 g [22] 7.7 8 km 2,415 1999 Jiji earthquake
1.82 g [23] 6.7 18 km [24] 57 1994 Northridge earthquake
1.62 g [25] 7.8 10 km 57,658
2023 Turkey–Syria earthquake
1.51 g [26][27] 6.2[28] 5 km 185 February 2011 Christchurch earthquake
1.26 g [29][30] 7.1 10 km 0 2010 Canterbury earthquake
1.25 g[31] 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] 7.0 16.1 km 118 2020 Aegean Sea earthquake
0.91 g 6.9 17.6 km 5,502–6,434 1995 Great Hanshin earthquake
0.8 g [34] 7.2 12 km 222 2013 Bohol earthquake
0.65 [35] 6.9 19 km 63 1989 Loma Prieta earthquake
0.5 g [36] 7.0 13 km 100,000–316,000 2010 Haiti earthquake
0.34 g [37] 6.4 15 km 5,778 2006 Yogyakarta earthquake
0.18 g [38] 9.2 25 km 131 1964 Alaska earthquake

See also

References

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Bibliography