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). Further more, a frequency of around 1 Hz or less is also a frequency that often found to resonate with these buildings. Although there is no official lower limit to the wave-frequency in LPGM, in terms of earthquakes, it is usually limited in scope between a 1 and 10 second period.
The significance of resonance in buildings can be seen in buildings like the Millennium Bridge, which had to be temporarily closed due to people walking in resonance with their stepping frequency being around 1 Hz.
The Japan Meteorological Agency (JMA) classifies long-period seismic intensity under 4 classes.[1] These classes are particularly relevant for estimating the effects of LPGM on high-rise buildings. 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. This then converts to the following classes:
| Class 1 | 5 cm/s ≤ Sva < 15 cm/s | Felt by most. | Hanging objects move significantly. | ||
| Class 2 | 15 cm/s ≤ Sva < 50 cm/s | Strong shaking felt; difficult to move by foot without support. | Furniture with casters moves slightly. Objects may fall from shelves. | ||
| Class 3 | 50 cm/s ≤ Sva < 100 cm/s | It is difficult to remain standing. | Furniture on casters moves significantly. Cracks may form in walls. | ||
| Class 4 | 100 cm/s ≤ Sva | It is impossible to remain standing. | Most unsecured furniture shifts significantly or falls over. Cracks in walls are common. |