The relation between fault plane solutions for earthquakes and the directions of the principal stresses

TLDR

The stresses involved in shallow earthquakes and their occurrence along fault planes suggest failure on weak planes rather than brittle fracture of a homogeneous material. The study examines possible stress‑tensor orientations to determine how fault‑plane solutions constrain the orientation of the greatest principal stress. For a general triaxial stress, the only restriction is that the greatest principal stress must lie within the quadrant containing P, though it may be at right angles to the P direction. Shallow earthquakes impose only limited constraints on stress‑tensor orientation, whereas deep‑earth fault‑plane solutions indicate fracture of a homogeneous material with the greatest principal stress directed down the dip.

Abstract

abstract The stresses involved in shallow earthquakes and their occurrence along fault planes suggest that they occur by failure on weak planes, rather than by brittle fracture of a homogeneous material. Possible orientations of the stress tensor are examined to determine what limits fault plane solutions can place on the orientation of the greatest principal stress. For the general case of a triaxial stress, the only restriction is that this stress direction must lie in the quadrant containing P, but may be at right angles to the P direction. Thus shallow earthquakes impose a few limitations on the orientation of the stress tensor. In contrast the fault plane solutions from deep earthquakes are best explained by fracture of a homogeneous material, with the greatest principal stress directed down the dip of the earthquake zone.