Earthquake (seismic) waves |
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Earthquakes are displacements, usually (but not always) along tectonic plate boundaries, and/or fault lines. Primary (longitudinal) and secondary (transverse) waves penetrate the inner layers of the Earth. Because both types of waves travel faster deeper within the Earth, they return to the surface. [Rays bend, as they do in a ripple tank that is deep on one side and shallow on the other.] Because longitudinal waves travel faster than transverse waves they are recorded first on a distant seismograph. The delay between the arrival of P and S waves is a measure of how far away the earthquake was.
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