Editing Seismology (section)

===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.