On the nature of the ULF electromagnetic emission preceding some earthquakes

A theory describing the appearance of a ULF electromagnetic emission prior to earthquakes is developed on the basis of the generation mechanism of geomagnetic disturbances due to the opening of fractures in the source of a forthcoming earthquake. This phenomenon is caused by the fact that the acoustic waves radiated by opening fractures generate currents in Earth's conducting layers. Effective magnetic moments of all fractures are directed opposite to the geomagnetic field induction vector irrespective of the orientation of the fractures. This uniformity in the directions of the magnetic moments of fractures results in the coherent enhancement of the ULF fields of individual fractures. At the same time, the resultant acoustic signal from the earthquake source can be very weak owing to the incoherence of the radiation sources. The magnetic dipole approximation used to solve the problem is described, the total electromagnetic signal from all fractures is estimated, and the estimates are compared with observed values.