www.iris.edu/earthquake for more animations
All buildings have a natural, period, or resonance, which is the number of seconds it takes for the building to naturally vibrate back and forth. The ground also has a specific resonant frequency. Hard bedrock has higher frequencies softer sediments. If the period of ground motion matches the natural resonance of a building, it will undergo the largest oscillations possible and suffer the greatest damage.
Dr. Robert Butler (Univ.Portland) gives demonstration of resonance.

Animation & graphics by Jenda Johnson, Earth Sciences Animated
Video demonstration by Dr. Robert Butler, University of Portland
Narrated by NappyT
Guitar solo by Dennis P. McNamara
Science editing by Dr. Robert Lillie (Oregon State University) and John Taber