Seismic model for the EarthA seismic model for the Earth based on observations of the arrival time of seismic energy at receiver stations distibuted around the Earth's surface. Selected raypaths are shown for seismic P-waves transmitted through the Earth from a single earthquake source. The raypaths are curved because of refraction through rocks whose ability to transmit seismic energy (velocity) increases steadily with depth. The effect is to create a shadow zone for P-waves as shown. However, rare P-waves are reflected off the inner core to arrive in this shadow zone. Models such as this are used to build a general uniform view of how seismic velocity varies in the Earth and are based on the analysis of hundreds of earthquakes from many different sources (examples include a version called PREM, the Preliminary Reference Earth Model). It averages out lateral variations to give a classic picture of an Earth with "onion-skin" like layering. Detailed analysis of a few earthquakes and the seismic travel time (for P-waves or S-waves) to a few closely spaced receivers can be used to measure small (a few %) discrepancies of local seismic velocity within small volumes of the Earth, in comparison with the general model. This method is called seismic tomography, the geophysical equivalent of a brain scan.

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