Long period ground motion (LPGM) refers to any seismic waves that occur during a seismic event, like an earthquake or nuclear explosions, that has a period typically defined as less than 1 Hz.
These long-period waves are particularly significant in the context of tall or long buildings, like bridges and high-rise buildings, as their nature causes short-period waves to dampen significantly before reaching the points more distant from the seismic event (e.g. from the ground).
The Japan Meteorological Agency (JMA) classifies long-period seismic intensity under 4 classes.
The higher the class the more difficult it is to perform regular activity and the greater the expected damage is to buildings.
In order to classify the intensity, the JMA looks at the maximum value of the absolute velocity response spectrum (Sva) with a damping constant of 5% over a period range from 1.6 to 7.8 seconds with calculation increments of 0.2 second.