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Purpose: Demonstrates how buildings respond to seismic shaking
Supplies: Cardboard, stiff paper (such as postcards or computer cards)
Background and Demonstration:
Earthquakes generate seismic waves with a broad spectrum; that is, the waves can shake over a fairly broad range of frequencies. As these waves propagate away from the earthquake source, however, anelastic behavior of the rocks of the crust and uppermost mantle, cause the higher frequency components to be damped out. Thus, the farther a seismic wave propagates, the less high-frequency energy it will contain (this is called anelastic attenuation). The response of a building to shaking at its base due to seismic waves depends on a number of factors related to its design and construction. However, one of the most important factors is simply the height of the building, because this determines the frequency of resonance of the building. Short buildings have a high resonant frequency (short wavelength), while tall buildings have a low resonant frequency (long wavelength). In terms of seismic hazard, therefore, short buildings are susceptible to damage from high-frequency seismic waves from relatively near earthquakes. On the other hand, tall buildings are at risk due to low-frequency seismic waves, which may have originated at much greater distance.
A simple model to demonstrate of the effects of building resonance may be constructed using cardboard and stiff paper (we use postcards and old computer cards). Make two parallel folds in the paper so that you have an inverted "U" shape, then fold out the ends so that they may be attached (by tape or staples) to the cardboard base (a box in our case). Now attach more inverted "U"-shaped papers on top of the first to build up a tower. These should all be attached on the same sides so that the tower can oscillate in one direction (you should be able to see through it one way). Build two towers of different heights attached to the same base and oriented in the same direction. Now, if you shake the base slowly (low frequency), the taller building will sway back and forth, but the shorter building will remain upright and simply move along with the base. On the other hand, if you shake the base quickly (high frequency), the shorter building oscillates, and while the taller building may deform with a variety of S-shapes, it will not sway back and forth as a unit.
Appropriate modifications may be made for science fairs or more permanent exhibits. The buildings may be constructed out of sheet metal or any other flexible material, as long as it is free to move in one direction. A source of shaking may be constructed with a toy motor powered by a variable transformer, using an off-center arm-and-wheel arrangement to shake the base back and forth.
Jeffrey S. Barker (SUNY Binghamton) Demonstrations of Geophysical Principles Applicable to the Properties and Processes of the Earth's Interior, NE Section GSA Meeting, Binghamton, NY, March 28-30, 1994.
Questions or comments: jbarker@binghamton.edu
Last modified: March 18, 1996 (content), June 6, 2021 (reformatted and moved to Google sites)
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