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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.

Pre-earthquake signals – Part I: Deviatoric stresses turn rocks into a source of electric currents