http://www.sciencedirect.com/science/article/pii/S1674987110000058

The thermo-electric coefficients of twenty-six magnetite samples, formed eitherby magmatism or metamorphism, were tested by the thermo-electric instrument BHET -06. Results showed that the coefficient is of a constant value of about -0.05 mV/℃. It is emphasized that because every magnetite grain was tested randomly, the coefficient is independent of the crystallographic direction. This fact means the thermal voltage generated from a single magnetite crystal can be accumulated, and as a result a new thermo-electric field can arise when a gradient thermal field exists and is active within the earth's crust. Because magnetite is widespread in the earth's crust (generally appearing more in the middle-lower crust), there is more-thanrandom probability that the additional thermo-electric field can be generated when certain thermal conditions are fulfilled. We, therefore, used the thermo-electric effect of magnetite to study the mechanism responsible for the presence of abnormal geo-electric fields during earthquake formation and occurrence,because gradient thermal fields always exist before earthquakes. The possible presence of additional thermo-electric fields was calculated under theoretical seismological conditions, using the following calculation formula:E= - 0.159(σ×△T×Φ×ρ2× (h2 - 2x2)cos α+3hxsin α]/ρ1 (h2+x2)5/2). In the above formula, σ is thermo-electric coefficient of magnetite, Δtis the temperature difference acting on it,φis a sectional area on a block of magnetite vertically perpendicular to the direction of the thermal current, ρ1 and ρ2 are the respective resistivities of magnetite and the crust, and h, a, and x, respectively, h is the depth of embedded magnetite block, a means the angle created by the horizontal line and ligature of the two poles of magnetite block, and x is the distance from observation point to projective center point of the magnetite block on earth surface. According to simulations calculated with this formula, additional thermo-electric field intensity may reach as high as n to n×102 mV/km. This field is strong enough to cause obvious anomalies in the background geo-electric field, and can be easy probed by earthquake monitoring equipment. Therefore, we hypothesize that geo-electric abnormalities which occur during earthquakes may be caused by the thermo-electric effect of magnetite.