Experimental and theoretical investigations of electromagnetic radiation induced by rock fracture

There is a general agreement in the literature that the technique of measuring electromagnetic radiation (EMR) emitted from cracked rock is a good candidate for forecasting of earthquakes. Our immediate objective in pursuing this goal is to correlate EMR with crack dimensions in micro-scales (mm-cm), coupling it with the understanding of atomic-scale phenomena for coherently understanding the EMR process. We review some of the results obtained in this laboratory. They include the isolation, both experimentally and theoretically, of an individual EMR pulse. Individual EMR pulse parameters are correlated with crack dimensions: the time from pulse origin up to its maximum is proportional to the crack length, and the frequency of the EMR pulse relates to the crack width. Individual EMR pulses are classified both according to their length and according to their location on the stress-strain curve. We find that the key elastic parameter for EMR characterization during triaxial compression is the Poisson ratio: the lower the Poisson ratio, the higher the EMR activity. Amplitudes of EMR and their changes with loading are shown to be independent of crack mode (tensile vs. shear), they are only dependent on the entire crack area. In order to experimentally overcome load limitations we introduce a new sample shape, the truncated cone, that fails more readily than standard cylindrical samples.