The role of shear cracks in the formation of electromagnetic noise preceding some earthquakesстатья
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Аннотация:The increase of ultralow frequencies (ULF) of electromagnetic noises (EMN) was observed several hours and days before the onset of earthquakes in Spitak (Armenia, 1988, M=6.9), Loma Prieta (USA, 1989, M=7.1), Northridge (1994, M=6.7), and a number of other cases [1-4]. The amplitude of magnetic disturbances exceeded the background variations by one order of magnitude at distances of several tens of kilometers from earthquake epicenter. At the same time, no noticable disturbances were observed in the upper atmosphere. Therefore, we can regard the ULF noise as a possible electromagnetic forerunner of earthquakes. The theory explaining this effect by magnetohydrodynamic (MHD) phenomena, which appear during the opening of cracks at the earthquake source, has been recently proposed [5,6]. The motion of conducting layers of the Earth in the acoustic wave radiated by an isolated crack results in a geomagnetic field disturbance. These disturbances and electrical currents propagate within the conducting medium through diffusion and run ahead of the acoustic wave front. It was shown in [5,6] that the disturbance fronts have the same sign and polarity for all opening cracks regardless of their orientations. This effect is retained from the moment of elastic wave radiation by the crack to the moment of its arrival at the observation point. The summation of frontal parts of electromagnetic signals from individual cracks results in the appearance of peculiar effec tof the coherent amplification of micrifields in the ULF region. This effect appears due to the fact that magnetic moments of currents generated by cracks are directed opposite to the geomagnetic field vector; i.e., the magnetic moments always have the same direction. Thus, the amplitude and temporal parameters of EMN that appear in a number of cases prior to an earthquake can be explained.In the model prposed in [5,6], the anticipated effects is proportional to the change of the total volume of voids appearing at the earthquake source. Hence, the basic role is assigned to tensile cracks and dilatancy effects in the underground destruction zone. Meanwhile, many authors [9,10] believe that the majority of cracks within fracture zone are shear cracks. Therefore, the purpose of the present work is to clarify the role of shear cracks in EMN formation. The results of this work provide new insight into the relationship of EMN to the processes of dilatancy in the earthquake source.