According to pre-designed patterns, multiple electrodes are arranged on the surfaces of four samples. Among them three samples have the original resistivity anisotropy which was not formed by ingredient of rock and one sample has that formed by ingredient of rock. The samples are fully saturated with water. The multiple electrodes are combined form arrays of resistivity changing anisotropy in different directions and spacing. The experiments of dynamic rock resistivity change are done with uniaxial compression, low confining triaxial compression and shear, and the variation of resistivity during the whole loading process is observed. The experimental results show that the samples of original resistivity anisotropy are essentially accordant with that of original resistivity isotropy in resistivity changing anisotropy. Namely, for the measuring-points that are located at cracks or rupture bands, the results of resistivity changing anisotropy are good. The four directions deduced from four kinds of combinatorial equation sets of resistivity are essentially identical with each other and coincide with the orientations of cracks or main rupture bands approximately. For the measuring-points without cracks or rupture bands passing through, either the four directions calculated by different combinatorial arrays of resistivity are inconsistent with each other, or the direction can not be determined, especially in the case that the crack plane is parallel to the measuring-surface.Key words Electrical resistivity of rock, Geoelectric resistivity, Anisotropy of rock resistivity, Main fracture of rock.

Key words: electric resistivity of rock earth resistivity anisotropy in resistivity earthquake geologic hazard 1 DefinitionWhen water-bearing rock (rock body) deform or rupture with stress change, its original microcracks changing or new cracks produced cause its water net electric conduction thoroughfare distorting, consequently results in the anisotropy of resistivity change. Above mentioned the whole change process is styled the dynamic rock resistivity change anisotropy for the time being. PrincipleThe mechanism of electric conductivity of water-bearing rock (rock body) is, in general, based on the conductivity of porous water and crack's conductive filling in the connected pore net inside the rock. The water net conducting-thoroughfare of rock will change after the rock is loaded, the change of crack takes the main effect •

The models of uniform and non‐uniform media of 1m×1m×0.3m were made with quartz sand and river sand blend cement under mould pressing. The amplitudes and anisotropic features of apparent resistivity changing were observed at different positions and different orientations on measured planes of the model during shear and frictional slipping. The results of experiments are as follows. The resistivity change is related to the position of measurement lines that are combined with four electrodes. The amplitudes of the resistivity change are as big as several percent to tens percent for the lines near the cracks of shear and frictional slipping, and in reverse are small from several percent to zero for the models of uniform media. For the models of non‐uniform media, the amplitudes of the resistivity change are smaller than that of the models of uniform media, which are only several percent. The resistivity change is related to the orientations of lines besides the positions of lines, and the amplitudes are different in four orientations of a measuring point. Some differences of them are very big. The amplitudes of the resistivity change are the same in order of magnitudes for the two loading manners of shear and frictional slipping. For the measuring points located in the zone near the cracks and the places where cracks passes through, the directions of the four anisotropic main axes of the apparent resistivity change coincide with the real orientations of cracks of shear and frictional slipping very well.

Multiple electrodes are arranged on the surfaces of cubic granite samples of different sizes according to pre-designed patterns. Samples are fully saturated with water in vacuum. Waterproof insulation glue is coated on the measuring surface and a small parts (including two arrises) of the adjacent surfaces of the saturated sample to ensure that the electric current flows only within the sample through the connection between the electrodes. The multiple electrodes are combined form arrays of different direction and spacing with symmetrical four-electrode method according to need of measuring of resistivity changing anisotropy, electric profiling and electric sounding. The samples are placed into container filled with water~ The samples are uniaxially compressed along the direction parallel to the longest dimension of the cubic, and the variation of resistivity during the whole loading process is observed. In the experiments, some samples are loaded to rupture with macro-fractures, some are only loaded to the stage, which shows obvious precursors in variation of resistivity associated with the indication of forthcoming rupture. Finally a quantitative comparison between the dominant orientation of pre-existing cracks in photo-micrography of unruptured samples and those macro-fractures in ruptured sample is made, together with their respective resistivity changing anisotropy behaviors. The experimental results are the following: O For measuring points in areas that are passed by cracks or rupture bands, the directions of principal anisotropy axes deduced from four kinds of combined equation sets are essentially identical with each other, and accord with the orientation of cracks or main rupture bands approximately. For measuring points in areas without crack or rupture band passing through, either the directions of calculated principal anisotropy axes by different combinatorial arrays are inconsistent with each other, or the principal anisotropy axis cannot be determined, especially in the cases where the crack plane is parallel to the measuring surface. ® The dominant orientation of microfractures or rupture bands shown from micrographs is close to the direction of principal anisotropy axis along which the variation in resistivity is the greatest. ® The results of electric profiling can be used for detecting the localization of cracks.