A 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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