 In their conception, the base of the asthenosphere extended as deep as the deepest (650–700 km) earthquakes in the inclined seismic zones where descending lithospheric plates penetrate the upper mantle.The (spherical) lithospheric plates of plate tectonics are so defined because they behave in a kinematically rigid manner.That is, any three points on the same plate do not move relative to one another, while the plate itself (and all points it contains) may move relative to other plates or other internal reference frames (e.g., the earth's spin axis or geomagnetic poles).