Editing Seismology (section)

==Types of seismic wave==
{{Main|Seismic wave}}
[[Image:Seismogram.gif|thumb|320px|Seismogram records showing the three components of [[ground motion]]. The red line marks the first arrival of P waves; the green line, the later arrival of S waves. |alt=Three lines with frequent vertical excursions.]]
Seismic waves are [[elastic waves]] that propagate in solid or fluid materials. They can be divided into ''body waves'' that travel through the interior of the materials; ''surface waves'' that travel along surfaces or interfaces between materials; and ''normal modes'', a form of standing wave.

===Body waves===

There are two types of body waves, pressure waves or primary waves (P waves) and [[Shearing (physics)|shear]] or secondary waves ([[S wave]]s). P waves are [[longitudinal wave]]s associated with [[Compression (geology)|compression]] and [[rarefaction|expansion]], and involve particle motion parallel to the direction of wave propagation. P waves are always the first waves to appear on a seismogram as they are the waves that travel fastest through solids. [[S waves]] are [[transverse wave]]s associated with shear, and involve particle motion perpendicular to the direction of wave propagation. S waves travel more slowly than P waves so they appear later than P waves on a seismogram. Because of their low shear strength, fluids cannot support transverse elastic waves, so S waves travel only in solids.<ref name=Gubbins1990>{{harvnb|Gubbins|1990}}</ref>

===Surface waves===
Surface waves are the result of P and S waves interacting with the surface of the Earth. These waves are [[Dispersion (optics)|dispersive]], meaning that different frequencies have different velocities. The two main surface wave types are [[Rayleigh wave]]s, which have both compressional and shear motions, and [[Love wave]]s, which are purely shear. Rayleigh waves result from the interaction of P waves and vertically polarized S waves with the surface and can exist in any solid medium. Love waves are formed by horizontally polarized S waves interacting with the surface, and can only exist if there is a change in the elastic properties with depth in a solid medium, which is always the case in seismological applications. Surface waves travel more slowly than P waves and S waves because they are the result of these waves traveling along indirect paths to interact with Earth's surface. Because they travel along the surface of the Earth, their energy decays less rapidly than body waves (1/distance<sup>2</sup> vs. 1/distance<sup>3</sup>), and thus the shaking caused by surface waves is generally stronger than that of body waves, and the primary surface waves are often thus the largest signals on earthquake [[seismogram]]s. Surface waves are strongly excited when their source is close to the surface, as in a shallow earthquake or a near-surface explosion, and are much weaker for deep earthquake sources.<ref name=Gubbins1990/>

===Normal modes===
{{See also|seismic wave#Free oscillations of the Earth|label 1=Free oscillations of the Earth}}
Both body and surface waves are traveling waves; however, large earthquakes can also make the entire Earth "ring" like a resonant bell. This ringing is a mixture of [[normal modes]] with discrete frequencies and periods of approximately an hour or shorter.  Normal-mode motion caused by a very large earthquake can be observed for up to a month after the event.<ref name=Gubbins1990/> The first observations of normal modes were made in the 1960s as the advent of higher-fidelity instruments coincided with two of the largest earthquakes of the 20th century, the [[1960 Valdivia earthquake]] and the [[1964 Alaska earthquake]].  Since then, the normal modes of the Earth have given us some of the strongest constraints on the deep structure of the Earth.