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Unlike body waves, surface waves (also known as long waves, or simply L waves) move along the surface of the Earth. Surface waves are to blame for most of an earthquake's carnage. They move up and down the surface of the Earth, rocking the foundations of man-made structures. Surface waves are the slowest moving of all waves, which means they arrive the last.
Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency.