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Crust

The outmost layer of the Earth above the Mohorovičić discontinuity (“Moho” for short). The crust in continental regions is about 25-75 km thick, and that in oceanic areas is about 5-10 km thick. (For global cross section see Fig. 2.10 in Chapter 2 of this Manual). It represents less than 0.1% of the Earth’s volume, with rocks that are chemically different from those in the mantle. The uppermost 15-35 km of the crust is brittle enough to produce earthquakes. The seismogenic crust is separated from the lower crust by the brittle-ductile boundary. The crust is usually characterized by P-wave velocities below 8 km/s (average velocity of about 6 km/s).

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