PGA is equal to the amplitude of the largest absolute acceleration recorded on an accelerogram at a site during a particular earthquake.
Horizontal PGAs are generally larger than those in the vertical direction but this is not always true, especially close to large earthquakes.
[3] Earthquake energy is dispersed in waves from the hypocentre, causing ground movement omnidirectionally but typically modelled horizontally (in two directions) and vertically.
Shallow-focused earthquakes generate stronger shaking (acceleration) than intermediate and deep quakes, since the energy is released closer to the surface.
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.
[citation needed] 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).
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.