Deep-Focus Earthquakes and Their Geological Significance

About 10 per cent of all major earthquakes are now known to originate below the "crust" of the earth at depths ranging up to 700 km. After a brief resume of the evidence for the existence of deep earthquakes, the paper describes the environment in which they occur under the headings of structure and composition, density and pressure, rigidity and bulk modulus, temperature, viscosity, and strength. The characteristics of deep earthquakes are then described under the headings of distribution (geographic and depth), energy released, aftershocks, periodicities, correlations, patterns of initial motion at the surface, and stress and strain near the focal region. Analysis of these data leads first to the conclusion that the type of process involved is probably a normal fracturing response to external stress differences, although other possibilities are considered. The possible known sources of stress appear to be inadequate to function as primary causes of deep earthquakes but may be important as "triggers." The meaning of strength is analyzed, and the conclusion is reached that seismic evidence probably points to an increase in strength from the surface of the earth downward at least to the core. The paper concludes that the concept of a weak substratum, within which flow results from extremely slight stress differences, does not appear to be valid.