Visualising a Building’s Response to Earthquake

Here’s an animation of the motion of a building, recorded on three floors during a magnitude 6.1 earthquake whose hypocentre was around 130km away. The blue dot represents a high resolution strong motion accelerograph located in the lowest basement (6 levels underground), the green dot an accelerograph on the 9th floor, and the red dot an accelerograph on the 18th floor, just below roof level.

You can imagine that you’re looking down on the building from above, and the moving dots are just the horizontal motion of the floors. The time series traces on the right show the displacement over time for the north-south and east-west channels. Note that the video is playing at roughly 4x actual speed, and that the scale is millimetres of displacement.

As you can clearly see, the upper floors move significantly further and faster than the ground. Although the total displacement in this case is only relatively small, the use of low noise instruments allows these tiny motions to be seen. Ambient vibrations show reveal the building’s natural frequency, and we can then see if the building resonates with the earthquake frequencies. These recordings also give a preview of the level of amplification that the building may be subjected to during larger or closer earthquakes.