Long-period ground motion

Long-period ground motion (LPGM) refers to seismic waves that occur during a seismic event, like an earthquake or nuclear explosion, that has a frequency typically defined as less than 1 Hertz (i.e. a period of more than 1 second). These long-period waves are particularly significant in the context of tall or long structures, 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). Furthermore, a frequency of around 1 Hz or less is also a frequency 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.