Find out more about Earthquakes and Resonance
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The energy of an earthquake produces seismic waves, which can cause great damage.- There is a number of factors that affect the amount of damage done by an earthquake, including the type of subsurface material as well as the quality, height, and structure of buildings.
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In 1989, the magnitude 7.1 Loma Prieta earthquake south of San Francisco, California, caused the upper roadway sections to collapse onto lower roadways. |
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In the building above, one side failed while the other held in a September 1999 earthquake of magnitude 7.6 in Taiwan. |
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Army barracks collapsed in West Bengal, India, in a 7.6 earthquake in 2005. |
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The levels of a parking garage in Mexico City collapsed like playing cards after an 8.1 earthquake in 1985. |
Earthquake hazards
When a major earthquake strikes, the severe shaking produced by seismic waves can destroy buildings and bridges. The problems are made worse when utility poles topple over or gas and water mains break. In addition, roads are sometimes damaged, disrupting transportation. Earthquakes can also trigger landslides and avalanches. In coastal regions, earthquakes sometimes trigger tsunamis, which can cause tremendous destruction.
The duration of an earthquake can affect the extent of damage. For example, buildings made of reinforced concrete may withstand a 20-second main shock yet collapse in a smaller-magnitude earthquake that lasts longer.
Earthquake damage is also determined by the direction of ground motion. Some seismic waves vibrate up and down; most structures can withstand this type of motion. Sometimes the largest vibrations at the Earth’s surface are side-to-side motions. Few buildings can survive this type of motion, with many collapsing as a result.
Underlying Earth materials strongly influence the motions of structures during an earthquake. When seismic waves move from hard, dense rock to loosely packed soil, they transmit their energy to the soil. The loose soil shakes more violently than the surrounding rock. The thicker the layer of soil, the more violent the shaking will be. This means that a building built on hard, resistant rock will shake less than a building built on soft soil.
Earthquake damage is directly related as well to the strength or quality of the structures involved. Severe damage often occurs to buildings made of stone, concrete, or other brittle building materials. Wooden structures, on the other hand, are much more resilient and generally sustain significantly less damage. Many high-rise, steel-frame buildings also sustain little damage during an earthquake because they are reinforced, which makes them more able to withstand ground motion.
Building height also affects whether a structure survives an earthquake. As you determined in the activity, all structures have a natural period of oscillation, or swaying back and forth. When the shaking of the ground during an earthquake happens to be close to the natural oscillation period of a building, the swaying of the building is at its maximum and contributes to the collapse. When the oscillation that is imposed on the building is nearly the same as the natural oscillation period of the building, the swaying is greatly intensified. This is called resonance. A common example of resonance occurs with a child on a swing. The swing has a natural period of oscillation, so when the child “pumps” to go higher, he or she pumps at the same period as the natural period. Pumping at a different period gets the child nowhere.