![]() ![]() Alternatively, a combined value for a given station can be noted. The peak acceleration in each of these directions is recorded, with the highest individual value often reported. ĭue 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.ĭuring 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 varying PGA results from an earthquake can be displayed on a shake map. 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. 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/s 2 (1 g = 9.81 m/s 2) or in multiples of Gal, where 1 Gal is equal to 0.01 m/s 2 (1 g = 981 Gal). Shallow-focused earthquakes generate stronger shaking (acceleration) than intermediate and deep quakes, since the energy is released closer to the surface. 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). These values vary in different earthquakes, and in differing sites within one earthquake event, depending on a number of factors. 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. Geophysics Įarthquake energy is dispersed in waves from the hypocentre, causing ground movement omnidirectionally but typically modelled horizontally (in two directions) and vertically. For moderate earthquakes, PGA is a reasonably good determinant of damage in severe earthquakes, damage is more often correlated with peak ground velocity. 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. It is often used within earthquake engineering (including seismic building codes) and it is commonly plotted on seismic hazard maps. ![]() ![]() The peak horizontal acceleration (PHA) is the most commonly used type of ground acceleration in engineering applications. It can be correlated to macroseismic intensities on the Mercalli scale but these correlations are associated with large uncertainty. The Mercalli intensity scale uses personal reports and observations to measure earthquake intensity but PGA is measured by instruments, such as accelerographs. 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. 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. Horizontal PGAs are generally larger than those in the vertical direction but this is not always true, especially close to large earthquakes. Therefore, PGA is often split into the horizontal and vertical components. Earthquake shaking generally occurs in all three directions. PGA is equal to the amplitude of the largest absolute acceleration recorded on an accelerogram at a site during a particular earthquake. Peak ground acceleration ( PGA) is equal to the maximum ground acceleration that occurred during earthquake shaking at a location. Maximum ground acceleration during an earthquake at a location ![]()
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