In this cutaway of Earth, variations in the seismic velocity are depicted by warm (lower seismic wavespeeds) or cool (high seismic wavespeeds) colors (courtesy of A. Dziewonski & coworkers). Large scale patterns of heterogeneity are present, and correlate geographically with features at Earth's surface. Lower velocities are seen beneath mid-ocean ridges, high velocities are detected beneath regions of past or present subduction.
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