© Author(s) 2007. This work is licensed under a Creative Commons License. Natural Hazards and Earth System Sciences Pre-earthquake signals – Part I: Deviatoric stresses turn rocks into a

Abstract. Earthquakes are feared because they often strike so suddenly. Yet, there are innumerable reports of pre-earthquake signals. Widespread disagreement exists in the geoscience community how these signals can be generated in the Earth’s crust and whether they are early warning signs, related to the build-up of tectonic stresses before major seis-mic events. Progress in understanding and eventually using these signals has been slow because the underlying physi-cal process or processes are basically not understood. This has changed with the discovery that, when igneous or high-grade metamorphic rocks are subjected to deviatoric stress, dormant electronic charge carriers are activated: electrons and defect electrons. The activation increases the number density of mobile charge carriers in the rocks and, hence, their electric conductivity. The defect electrons are associ-ated with the oxygen anion sublattice and are known as posi-tive holes or pholes for short. The boundary between stressed and unstressed rock acts a potential barrier that lets pholes pass but blocks electrons. Therefore, like electrons and ions in an electrochemical battery, the stress-activated electrons and pholes in the “rock battery ” have to flow out in different directions. When the circuit is closed, the battery currents can flow. The discovery of such stress-activated currents in crustal rocks has far-reaching implications for understanding pre-earthquake signals.