Friedemann T. Freund - Ames Associate, NASA Ames

Earthquakes are feared because they strike suddenly. Yet, there are reports of many kinds of pre-earthquake signals. Widespread disagreement exists how these signals can be generated in the Earth’s crust and whether they are in fact precursory, related to the build-up of stress before major seismic events. Progress in understanding these signals has been hindered by the lack of the underlying physical process or processes. It has now been shown that, when rocks are subjected to stress, they turn into batteries. Stress activates dormant electronic charge carriers: electrons and holes, i.e. defect electrons in the oxygen anion sublattice, also known as positive holes or pholes for short. The boundary between stressed and unstressed rock acts a diode, i.e. as a barrier that lets pholes pass through but blocks electrons. The pholes propagate through unstressed rocks. They are long distance runners. For the purpose of modeling pre-earthquake signals, one can distinguish two conditions in the Earth’s crust: (i) only pholes can spread into the unstressed rocks leading to the build-up of a positive surface charge, perturbations in the ionosphere, stimulated infrared emission and other phenomena that seem to precede major earthquakes by days and weeks; (ii) electrons can also flow out of the stressed rock volume shortly before a major seismic event, closing the battery circuit and generating bursts of electric currents, which lead to bursts of electromagnetic emissions. Every km3 of stressed rock can deliver currents up to 30,000 to 100,000 A for hours and days. Understanding stress-activated electronic charge carriers in common crustal rocks holds the key to deciphering pre-earthquake signals. It opens the door to a future global earthquake early warning system, provided resources are pooled through a concerted and onstructive community effort, including seismologists, with strong international participation.