2.6 Amplitude Parameters

2.6.1 Peak Ground Acceleration

The earthquake time history contains several engineering characteristics of ground motion and maximum amplitude of motion is one of the important parameter among them. The PGA is a measure of maximum amplitude of motion and is defined as the largest absolute value of acceleration time history.
The response of very stiff structures (i.e., with high frequency) is related to PGA. Though PGA is not a very good measure of damage potential of ground motion; due to its close relation with response spectrum and usability in scaling of response spectrum, PGA is extensively used in engineering applications.
Generally, at distances several source dimensions away vertical PGAs are found to be less than horizontal PGA though at near source distances it could be equal to higher than the corresponding horizontal PGA. For engineering purposes, vertical PGA is assumed to be two thirds of the horizontal PGA.