Whereas the frequency is the number of times per second that the building will vibrate back and forth, the period is time it takes for the building to make one complete vibration. This means that a short building with a high natural frequency also has short natural period. Conversely, a very tall building with a low frequency has a long period. For example, it takes the Empire State Building a comparatively long time to sway back and forth during a strong gust of wind. When the frequency contents of the ground motion are center around the building's natural frequency, we say that the building and the ground motion are in resonance with one another.
Resonance tend to increase or amplify the building's response.
Because of this, buildings suffer the greatest damage from ground motion at a frequency close or equal to their own natural frequency. These 20 story buildings were in resonance with the frequency contents of 1985 earthquake. Other buildings, of different heights and with different vibrational characteristics, were often found undamaged even though they were located right next to the damaged 20 story buildings. As we've just seen, different buildings can respond in widely differing manners to the same earthquake ground motion. Conversely, any given building will act differently during different earthquakes, which gives rise to the need of concisely representing the building's range of responses to ground motion of different frequency contents.