These motions in turn induce forces which can produce damage. Here it's useful to compare the surface of the ground under an earthquake to the surface of a small body of water, like a pond. The first few stones create a series of circular waves, which soon begin to collide with one another. During an earthquake, the ground vibrates in a similarly complex manner, as waves of different frequencies and amplitude interact with one another. Once the seismic waves reach the building site they undergo further modification which are dependent upon the characteristics of the ground and soil beneath the building.
Frequency is defined as the number of complete cycles of vibration made by the wave per second.
Here, we can consider a complete vibration to be be the same as the distance between one crest of the wave and the next, in other words one full wavelength. Frequency is often measured in units called Hertz. Surface ground motion at the building site, then, is actually a complex superposition of vibrations of different frequencies. We should also mention that at any given site, some frequencies usually predominate. The distribution of frequencies in a ground motion is referred to as its frequency content .