earthquake: Seismic Waves

There are several types of earthquake waves including P, or primary, waves, which are compressional, travel fastest, and are the least destructive; and S, or secondary, waves, which are transverse, i.e., they cause the earth to vibrate perpendicularly to the direction of their motion. P and S waves are known as body waves because they propograte within the body of the earth. Surface waves consist of two main types, Love and Rayleigh waves; they move more slowly than P and S waves. Love waves cause horizontal shifting at the surface, and Rayleigh waves, the slowest moving, have a rolling motion similar to that of an ocean wave and vertically displace the earth's surface. Since the velocities of the P and S waves are affected by changes in the density and rigidity of the material through which they pass, the boundaries between the regions of the earth known as the crust, mantle, and core have been discerned by seismologists, scientists who deal with the analysis and interpretation of earthquake waves (see earth). Seismographs (see seismology) are used to record seismic waves. The disappearance of S waves below depths of 1,800 mi (2,900 km) indicates that at least the outer part of the earth's core is liquid.

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