Characterizing Earthquakes using Engineering Parameters
Parameters Describing Ground Motion

In order to accurately describe any earthquake, one must be able to describe the three major characteristics of the motion:

There are many ways of doing this, using a large variety of parameters. Only a limited number will be discussed here, and these and others can be found in the text by Kramer, 1996 [1]. These parameters describe characteristics of the recorded ground motion time history, which fully describes the ground motion at a particular site.


The most common measures of amplitude are:

  • peak ground acceleration (PGA, PHA or PVA)
  • peak ground velocity (PGV, PHV or PVV)
  • peak ground displacement (PGD)

Other measures of amplitude include effective acceleration (Newmark and Hall, 1982) and sustained maximum acceleration (Nuttli, 1979).

Frequency Content

The frequency content of a ground motion is usually described using some type of spectra such as Fourier, power, or response spectra. Spectral parameters can include predominant period, mean period, bandwidth, central frequency, shape factor, and vmax/amax.

Combined Parameters

There are also many parameters that describe both amplitude and frequency content. Some of these are:

Arias intensity (insert gif)
Housner intensity (insert gif)
rms acceleration (insert gif)


The duration of strong ground motion, rather than the duration of the entire time history, is what is of interest to engineers. Here are some common ways of defining this duration:

  • bracketed duration (Bolt, 1969): the time between the first and last exceedances of some threshold acceleration, usually 0.05g; most commonly used

  • D5-95 (Trifunac and Brady, 1975): the time interval between the points where 5% and 95% of the total energy has been recorded

  • corner period (Boore, 1983): the inverse of the corner frequency of the Fourier amplitude spectrum for the ground motion
  • number of equivalent cycles (Seed et al., 1975): ground motion is represented as a series of harmonic stress cycles; used to evaluate liquefaction potential