GARY HINCKS / SCIENCE PHOTO LIBRARY GARY HINCKS / SCIENCE PHOTO LIBRARY
Primary seismic waves. Artwork showing the form of primary (P) seismic (shock) waves during an earthquake. An earthquake is caused by the sudden movement of rocks in the Earth's crust. The initial tremor of an earthquake can be likened to pushing a coiled metal spring. This sets up longitudinal waves, which alternately push and pull the spring as they travel along it (arrowed). P waves travel through rock in this way, changing its volume by expanding and compressing it. They are the first waves to arrive at the surface and can travel through the solid crust and mantle of the Earth, and through its liquid core. See E360/008 for a diagram of secondary (S) waves.
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