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What Is an Earthquake?

The crustal plates of the Earth are being deformed by stresses from deep within the Earth. The ground first bends, then, upon reaching a certain limit, breaks and "snaps" to a new position. In the process of breaking or "faulting," vibrations are set up that are the earthquakes. Some of the vibrations are of very low frequency, with many seconds between waves, whereas other vibrations are of high enough frequency to be in the audible range.

The vibrations are of two basic types, compression waves and transverse or shear waves. Since the compression waves travel faster through the Earth, they arrive first at a distant point; they are known as primary or "P" waves. The transverse waves arriving later are referred to as shear or "S" waves. In an earthquake, people may note first a sharp thud, or blast-like shock, that marks the arrival of the P wave. A few seconds later, they may feel a swaying or rolling motion that marks the arrival of the S wave.

Cross Section of the Earth

Cross section of the Earth showing the paths of some compression ("P," or primary) and transverse ("S," or shear) waves generated by earthquakes.

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Page Last Modified: Wednesday, November 30, 2016, 12:12:24 PM