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Khan Academy
Khan Academy: Why S Waves Only Travel in Solids
Explains why S-waves will only travel in solids.
Khan Academy
Khan Academy: Plate Tectonics: Structure of the Earth
Explains in more detail the different layers that make up the structure of the Earth. [9:33]
Khan Academy
Khan Academy: Seismic Waves and Earth's Structure: Refraction of Seismic Waves
Explains how any type of wave can be refracted, and how this applies to seismic waves and to our understanding of the Earth's structure. [9:21]