Stress Fields in Focal Regions

Abstract

Based on the assumption that the double couple model is the most adequate representation of an earthquake, and with the aid of the dislocation theory and of the results in true triaxial experiments of rock fracture, it is pointed out that the directions of the maximum and minimum principal stresses released on the occurence of an earthquake make an angle of (45-θ)°, where θ is the fault angle, to those of tectonic principal maximum and minimum stresses, respectively. It is also pointed out that the released stress deduced from the analysis of a focal mechanism solution is the variation before and after the occurrence of an earthquake, and not the tectonic stress itself in a focal region. So that the discrepancy between both stresses is reasonably understood. Other related problems such as the difference between both focal mechanism solutions obtained from data of body waves and surface waves are briefly discussed.