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Comment: 13:44 - 14:21 (00:37)
Source: Annenberg/CPB Resources - Earth Revealed - 9. Earthquakes
Keywords: earthquake, "wave period", "natural period", vibration, "primary wave", "secondary wave", "surface wave", "ground motion", epicenter
Our transcription: There are many different types of ground vibrations.
The rapid vibrations of the primary and secondary waves and the slower rolling motion of what are called surface waves.
Near the epicenter where all these vibrations are concentrated at the same location ground motion is similar to the complex sea surface in an ocean storm with waves of all sizes mixed together, but the longer waves travel faster and may not die out for hundreds of kilometers.
The amount of time it takes two successive wave crests to pass a stationary point is called the wave's "period."
For longer seismic waves, this period may be several seconds.
Other materials including buildings have a period associated with them. In a strong gust of wind a skyscraper will flex and bend.
The time it takes for the skyscraper to oscillate back and forth once is called its natural period.
When the period of a seismic wave matches the natural period of a building, the seismic energy is simply added to the oscillation of the building, wave after wave.
As a result, the swaying of the building increases dramatically.
Other buildings of different heights will not be similarly affected because their natural periods are different.
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