On the connection between the critical value of the "vertical" electrical anisotropy and earthquake prediction

Summary. It is shown that at any orientation of a dipole of direct current at the surface of a uniform anisotropic halfspace there exists a unique critical value η_cr of the "vertical" electric anisotropy η. For η > η_cr not all field lines are closed, and three straight field lines pass through the centre of the dipole. Also there exists a unique value η_eq > η_cr, where equatorial η is equal to zero. If η > η_eq then equatorial ρ_app is negative since the electric vector is turned more than through 90° at the equator of dipole, and it is shown that η_cr= 1.79, η_eq= 1.93 if the axis of dipole is directed symmetrically with respect to the axes of anisotropy. Routine observations of variations of ρ_app with the aim of earthquake prediction should be carried out taking into account the possibility of great fluctuations on η far from the epicentre of an earthquake.