Observational evidence of anisotropic changes of apparent resistivity before strong earthquakes

Abstract

The observation data of seven circumjacent terrestrial resistivity stations before two stronghistorical earthquakes in China were studied through performing the method of normalizedmonthly variation rate.We analysed the relationship between anisotropic variations of apparentresistivity and the corresponding maximum principal stress's direction of the focal mechanismsolution. The results may demonstrate that significant apparent resistivity variation occurs inthe direction nearly perpendicular to the azimuth of the maximum compressive principalstress, while only small fluctuations were recorded in nearly parallel direction. In the first case,a decrease in 1 yr scale and a short-impending increase of apparent resistivity were clear. Wealso suggest that anisotropic variation of apparent resistivity is more substantial in (or near)the seismic fracture zone than at two sides of the fault.We conclude that the most effective wayto estimate the epicentre of an earthquake is to increase the density of observation networkswithin the earthquake fault zone and to reasonably dispose electrode orientations based on thisphysical phenomenon.