During the decade just past, developments in Seismology have played an active and central role in the development of the concept of Plate Tectonics. Observational Seismology has provided support for and verification of a number of the dynamic aspects of the hypotheses of continental drift, sea-floor spreading, transform faults and the underthrusting of the lithosphere at island arcs and some continental margins.

Those types of seismological evidence which bear on the question of the thickness of the lithosphere are either indirect or circumstantial, or both. As early as 1926, Gutenberg postulated the existence of a layer at a depth of 80 to 150 or 200 km, probably worldwide in extent, in which the velocities of seismic waves are slightly lower than in the immediately overlying layers. Some plate tectonics workers equate this low-velocity layer to the relatively-weak asthenosphere required by Plate Tectonics to underlie the stronger, more brittle lithosphere.

In this review, several lines of evidence are marshalled in support of a plate model of the continental crust in seismically active regions in which a layer of decoupling of an upper, lithospheric layer from the weaker substrate may lie in the crust itself at a depth of perhaps 10 to 15 km.

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