Long period ground motion

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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.