Evolution of the Downgoing Lithosphere and the Mechanisms of Deep Focus Earthquakes

Abstract

The thermal evolution of the lithospheric slab at subduction zones and its geophysical effects are numerically calculated. An alternating‐direction, implicit, finite‐difference scheme is used to compute the thermal models taking into account all heating sources and phase boundaries. These models, with the appropriate spreading rates and dip angles, are compared with different island arc systems. Temperatures inside the slab are strongly controlled by the conductivity and by the time elapsed since the initiation of descent. The depth to which temperature anomalies persist is generally about 700 km or less. The thermal results are used to construct the seismic velocities and ray paths, density anomalies, and the resulting stress distribution. Comparing the theoretical stress distribution and the focal mechanism studies of intermediate‐ and deep‐focus earthquakes indicates the importance of both the mantle'srheology and the temperature dependence of the slab'selastic properties. The intermediate and deep focus earthquakes are located along the coolest region of the slab. The theoretical results explain the source mechanisms and the orientation of principal stresses under major island arcs. Copyright © 1973, Wiley Blackwell. All rights reserved