Mechanism of transient electric activities associated with rock failure

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Despite the increasing interest in fracto-emission phenomena in rocks, particularly in connection with the electromagnetic anomalies observed possibly related to earthquakes, experimental data reported on the topic has so far been limited. In this study, uni-axial and tri-axial compression tests of rocks have been conducted and fracture induced electric signals were measured as a transient electric charge signal and as an electric potential difference between two electrodes of thin copper plates, between which the tested rock specimens sit. Our experimental results showed that in both types of the experiments : (1) Electric charge signals that appeared prior to the main failure of rock specimens were detected only for granitic rocks of which main constituent mineral is quartz, and (2) At the moment of the main failure, however, electric charge signal and intense electric field change were observed for all tested specimens irrespective to the existence of quartz grains. For quartz-bearing rocks such as granite, the piezoelectric behavior of quartz should contribute to some extent to the electric signals observed during the precursory period prior to fracture of dry rocks. Our results, however, confirmed that there are other important contributing factors.