The land on which we live is not as solid as we might expect. Deep inside the Earth, it is as hot as the surface of the Sun! The outer layer of the Earth is called the crust. This is broken into many separate pieces, called plates. Under this lies the mantle, a hot, fluid layer which makes the plates move over a long time. N.B. The mantle is a solid that can flow very slowly. Sometimes they shift slightly, leading to gigantic vibrations that pass both throughout the earth, and over its surface as earthquake waves.
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Seismic Waves