Two parts of the research are distinguished in this paper. The first part is devoted to the structure of signals of Acoustic Emission (AE) and electromagnetic emission (EME) which accompany the inelastic straining of terrestrial materials. Special attention is paid to the similarity of waveform of EME signals at various scale lengths. The second (and main) part of the work involves the investigation of AE responses to the action of additional power fields over strained rocks. Our experimental investigations have revealed the interrelation of acoustic emission activity to power actions applied externally (impacts of electromagnetic field). Different modes of responses to electromagnetic impact have been specified. The characteristics of such responses and their variations depending on the material of specimens tested and of electric parameters of external pulses during power impacts have been considered. A general conclusion on possible electromagnetic triggering of AE has been drawn, the prospects of further studies being outlined.
Recently published results of field and laboratory experiments on the seismic/acoustic response to injection of direct current (DC) pulses into the Earth crust or stressed rock samples raised a question on a possibility of electrical earthquake triggering. A physical mechanism of the considered phenomenon is not clear yet in view of the very low current density (10 -7 -10 -8 A/m 2 ) generated by the pulsed power systems at the epicenter depth (5-10 km) of local earthquakes occurred just after the current injection. The paper describes results of laboratory ''earthquake'' triggering by DC pulses under conditions of a spring-block model simulated the seismogenic fault. It is experimentally shown that the electric triggering of the laboratory ''earthquake'' (sharp slip of a movable block of the spring-block system) is possible only within a range of subcritical state of the system, when the shear stress between the movable and fixed blocks obtains 0.98-0.99 of its critical value. The threshold of electric triggering action is about 20 A/m 2 that is 7-8 orders of magnitude higher than estimated electric current density for Bishkek test site (Northern Tien Shan, Kirghizia) where the seismic response to the man-made electric action was observed. In this connection, the electric triggering phenomena may be explained by contraction of electric current in the narrow conductive areas of the faults and the corresponding increase in current density or by involving the secondary triggering mechanisms like electromagnetic stimulation of conductive fluid migration into the fault area resulted in decrease in the fault strength properties.
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