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Of an earthquake. The seismic focus is the point of origin of the earthquake within the crust; see Deji (1999) Eng. Geol. 51, 3 on seismic foci around the Three Gorges Project. The velocity of seismic waves varies with the density of the material through which they travel. This property is used in seismic tomography—the investigation of the interior of the Earth; see W. K. Lee (2003).

Subjects: Earth Sciences and Geography — Environmental Science.

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