# Intensity Measure Type (IMT)

Peak Ground Acceleration (PGA)

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

Peak Ground Velocity (PGV)

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

Peak Ground Displacement (PGD)

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

Spectral Acceleration (SA)

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.

Modified Mercalli Intensity (MMI)

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).

See also Intensity Measure