A capacitive coupling model between the ionosphere and a fault layer in the crust with supercritical water
Akira Mizuno, Minghui Kao, Ken Umeno*
* The author to whom correspondence should be addressed.
Department of Applied Mathematics and Physics, Graduate School of Informatics, Kyoto University, Kyoto, Japan
Abstract
Changes in the total electron content, TEC, lowering of ionosphere, and the velocity and direction of TID (Traveling Ionospheric Disturbance) have been reported as potential precursors of major earthquakes. There is a growing interest in the mechanism of the crust-ionosphere coupling. As stratified fractures begin to occur within the crust, rocks can be expected to undergo friction under high pressures, resulting in extremely high temperatures. Recently smectite-rich gouges have been found by boring surveys. Smectite contains water, and with the fracture, the water is squeezed out. In high pressure and temperature, water becomes supercritical state, and the electrical resistivity increases. When the fracture begins and the layer starts to move, fine particles will be generated and charged within the fractured layer. Tribocharging could also possibly take place. Then the voltage across the fractured layer increases, and at a certain voltage, electrical breakdowns take place inside the layer, generating electromagnetic wave. This electrical breakdown limits the maximum voltage generated across the fractured layer to be about 300 V as estimated. Because thickness of the fractured layer is small, the capacitance is large compared to that between the surface of the earth and the ionosphere. The voltage generated in the fractured layer inside the crust, or the electric field to cause perturbation, is then transferred to the ionosphere by the capacitance coupling. A crude model of the capacitance coupling between the crust and ionosphere is proposed. The order estimation of the amount of charge and energy per unit area of the fractured layer inside the crust matches those necessary for the downward drift of electrons in the ionosphere for approximately 20 km. In addition, the 300 V increase of the surface potential could cause approximately 1 mV m-1 electric field in the ionosphere. This value can change the velocity of middle scale TID. To see a possibility of this capacitance coupling, we made a preliminary charging test of clay/water mixture at high temperature and high pressure. Although the experimental results require further verification, the observation suggests that the electrification in supercritical conditions should be considered as one of the possible causes of voltage generation for the perturbation associated with intense earthquakes.
Keywords - Supercritical water, earthquake, pre-earthquake phenomena, lithosphere-atmosphere-ionosphere coupling, ionospheric disturbance.