Earthquakes are massive rock failure events. Many large earthquakes are preceded by transient signals: electromagnetic, magnetic field variations, ionospheric perturbations, changes in soil conductivity, and other pre-earthquake (pre-EQ) signals. To the surprise of observers some of the pre-EQ signals tend to fade, even disappear, shortly before the seismic events. The discovery of stress-activated electric currents in rocks provides a possible explanation. Stresses activate mobile charge carriers in rocks, namely electron vacancy defects (EVD), also known as positive holes, symbolized by h•. The h• are equivalent to O− in a matrix of O2−. They exist in unstressed rocks in an electrically inactive, dormant state. When activated the h• flow down stress gradients, producing an electrical current. At very high stress rates dislocations begin to coalesce into microfractures, initiating catastrophic failure. They can no longer activate h• charge carriers. As a consequence, pre-rupture signals will decrease in magnitude.
Whenlarge-scale tectonic stresses increase in the Earth crust beyond some critical threshold, faults can fail catastrophically and cause earthquakes. The same is true, albeit on a smaller scale, for rock outbursts in mines and other rock failure events. Rock rupture falls well into the realm of classical mechanics and is usually treated in this context. However, the literature is also replete with reports that, prior to major earthquakes, the Earth sends out various pre-earthquake (pre-EQ) signals. They are non-seismic in nature, sometimes strong, more often subtle and fleeting. They include magnetic field variations, electromagnetic (EM) emissions from the VIS through the IR and RF to ULF/ELF, seismo-electric signals (SES), various atmospheric and ionospheric perturbations, and others. There have been numerous attempts to explain these different types of non-seismic pre-EQ signals (for recent reviews see (Uyeda 2009, and Freund 2010).
