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 an hour or shorter . Motion caused by a large earthquake can be observed for up to a month after the event . 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
1960 Valdivia earthquake
1960 Valdivia earthquake
Seismology