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Wave propagation through the Earth's interior. "P" and "S" waves are deflected and reflected as they move from the earthquake epicenter towards the interior, due to changing density, composition and/or matter state of different strata below.

Sonar, seismology and radar are similar technologies, based on the principle of sending a signal from a source, then timing how long it takes the signal to return to the source after it has reflected off of some object at a distance. In the case of sonar, which is used to map distance through water, a pulse of sound is sent from a ship over a specific beam width in a specific direction. Detectors that are very sensitive to sound, measure the sound waves as they return and use the strength and return time to reconstruct an image. In this manner, instruments on a ship can be used to map the sea floor. Bats, whales and porpoises all navigate using sonar.

Seismology uses a similar principle to map the structure of rocks beneath the Earth's surface. In the case of seismology, a powerful pulse is generated at the surface either by an explosion or a device designed to periodically "thump" the ground. Sensors designed to pick up subtle ground vibrations measure the intensity and timing of return vibrations. Based on distance and strength, much of the subsurface structure can be mapped. Seismology has been particularly important in helping us understand the structure of the deep Earth's interior. Seismographs, originally installed to help monitor nuclear explosions and earthquakes, have been used to map the mantle and core based on the types of vibrational waves they measure and the length of time it takes them to travel from the source of the vibration (the epicenter), to the detector. Seismographs also enable us to determine where an earthquake occurs and how strong it was. [Check out an earthquake wave-propagation time-travel map of this major earthquake] Our understanding of how plates interact at subduction zones, was in large part a result of mapping sub-surface earthquakes along these faults.

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