Deformation and Instability of Underthrusting Lithospheric Plates

Summary Recent confirmation of the relationship between seismology and tectonic plate movement has focused attention on the deformation and instability of lithospheric plates. In this paper models of the underthrusting litho‐sphere for the calculation of displacement and deflection are constructed. The first of the three contributions is a mathematical theory that rigorously demonstrates the elastic instability in the descending lithosphere. The theory states that lithospheric thrust beneath island arcs becomes unstable and suffers deflection as the compression increases. Thus, in the neighbour‐hood of the edges where the lithospheric plate plunges into the asthenosphere and mesosphere its shape will be contorted. The second contribution is the calculation of the lateral displacement. It is found that, before contortion, the plate will thicken and contract at different positions with the variation in thickness following a parabolic profile. Finally, the depth distribution and the origin of the intermediate and deep focus earthquakes are explained in terms of plate buckling and contortion.