Anisotropy of the apparent resistivity variation rate near the epicentral region for strong earthquakes

The apparent resistivity data of 11 stations observed in or near the epicentral regions for 6 strong earthquakes are processed by the normalized monthly rate method. The result show that in the medium-short and short-imminent terms of earthquake preparation the variation rate of apparent resistivity perpendicular (or near perpendicular) to the direction of maximum principal stress is greater than that parallel (or near parallel) to the direction. Such anisotropic phenomenon is interpreted as follow: the predominant alignment of cracks containing conductive fluid in the maximum principal stress direction caused the anisotropy of the fluid migration during the dilatancy process in the late period of earthquake preparation and resulted in the anisotropy of true resistivity variation rate for rock (or solid) medium which induced the variation rate anisotropy of apparent resistivity on the ground. The study of this paper provides some seismic examples to show the apparent resistivity anisotropy which may be helpful to study the stress status in or near the source zone in the late period of earthquake preparation of strong events.