# Intensity Measure Type (IMT)

The **Intensity Measure Types** supported in OpenSHA include:

The peak ground acceleration during an earthquake measured in units of 'g' (gravity).

The peak ground velocity during an earthquake measured in units of 'cm/sec' (cgs)

The peak ground displacement during an earthquake measured in units of 'cm' (cgs).

The maximum acceleration of a damped, single-degree-of-freedom harmonic oscillator, measured in units of 'g' (gravity). Spectral acceleration is an approximate measure of what is experienced by a building during an earthquake and is further specified by:

*Spectral Period:*the natural period (seconds) of the oscillator*Spectral Damping:*the degree of damping for the oscillator (usually 5%)

More information on SA is available at the USGS.

A Roman numeral describing the severity of an earthquake in terms of its effects on the earth's surface and on humans and their structures (see also Wikipedia).