Tectonic plate movement is the primary cause of earthquakes. Tectonic plates do not have smooth edges, and generate a ton of friction when they slide/grind past each other at a transform boundary. The edges can become stuck for a while, too. If this happens, pressure builds up in the crust. The pressure can build and build until the rocks snap, releasing seismic waves. The sudden release of these seismic waves is called an earthquake.
1) P-waves (top): P-waves move in a horizontal, push-and-pull way. P-waves travel faster than S-waves, and can travel through liquids or solid materials.
2) S-waves (bottom): S-waves move in a side-to-side way, forming an S shape. S-waves travel slower than P-waves, but are also more destructive. S waves can only travel through solid materials.
P and S waves can be modeled with a slinky as shown below.
Because P and S waves travel at different speeds and through different types of materials (S waves cannot travel through solids), scientists were able to determine that the Earth had 4 layers. Since the S waves could not travel through the liquid outer core, the waves cannot travel through to the other side of the Earth.
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