Intensity Measure Type (IMT)
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).