Electromagnetic and acoustic emission associated with rock fracture

Abstract

Abstract To understand the physical mechanism of the anomalous electromagnetic emissions observed before earthquakes, we carried out some laboratory experiments on electromagnetic and acoustic emission from a rock. Granitic samples were loaded at a constant strain rate and electromagnetic and acoustic emission were simultaneously recorded during deformation of the sample. Ten to 20% of the acoustic emissions detected during the experiment are associated with electromagnetic emission. The onset time of electromagnetic emission is found to coincide with the origin time of the acoustic emission. This demonstrates that the emission of electromagnetic waves is truly associated with microcracking in the sample. The efficiency of generating electromagnetic emission seems to be higher in tensile cracks than in shear cracks. The frequency spectrum of the electromagnetic emission obtained with five coil antennae of different frequency responses indicates that the main power of the electromagnetic emission is in the frequency range > 0.5 MHz. A possible mechanism of electromagnetic emission is electrification of a fresh surface created by subcritical cracking in a rock. If this interpretation is correct, production of new cracks is a necessary condition for generation of electromagnetic emissions. Although we do not completely understand how a natural earthquake occurs, it is generally considered to be a result of shear faulting that connects pre-existing small cracks. Therefore it is reasonable to expect anomalous electromagnetic emission associated with small tensile cracks before an earthquake and to expect that the anomalous electromagnetic emission will be observed only before an earthquake but not at the main shock, which may not be very efficient for creation of a fresh surface.