Electrical Conductivity Measurement and Anisotropy of Mylonite and Cataclasite

Resent studies of electromagnetic survey reveal the electrical conductivity structure at the deeper part of faults. There seems to be high conductivity region around the focal areas. The high conductivity may be related to the existence of water in rocks, which affect the strength and activity of faults, i.e., earthquake. Investigations on the electrical conductivity structure suggest important clues on the mechanism and conditions of earthquake occurrence. Rocks at the focal zone of a fault are expected to suffer from hard deformation and/or fracturing with alteration. Rocks have characteristic fabrics. They are called fault-related rocks, such as mylonite and cataclasite. We collected mylonite and cataclasite samples from the Hatagawa fracture zone in northeast Japan. The Hatagawa fracture zone is regarded as a major exhumed fault. We can observe the mylonite and cataclasite at a surface now that were produced from granitic rocks at the focal depth region in the past. We conducted measurements of electrical conductivities of the fault-related rocks at ambient condition, as a first step. We connected a frequency response analyzer and a potentiostat to measure AC impedance spectra. The combination of the two instruments performs sine wave correlation between signals passing trough a sample and a reference resistance. In addition, the potentiostat can measure a micro current. As a result, the measurement system is basically resistant to an electrical noise around the sample under severe condition and is able to measure up to 10⁹ Ω . We observed clearly foliations and lineations in mylonite samples. We cut each mylonite sample into three cylindrical chips parallel to three directions: parallel to both the foliation and the lineation (x-axis), parallel to the foliation and perpendicular to the lineation (y-axis), perpendicular to the foliation (z-axis). We did not observe any planar or linear fabric on the cataclasite samples. Porosities of the samples were less than 1 %. The conductivities at 1 Hz of samples under dry condition (samples are heated at 120 degree C for 6 hours) are about 10^-8 1/Ω m and have little difference among samples. Under wet condition at the same frequency (samples were forcibly saturated by distilled water in a vacuum chamber), conductivities of all samples increased by more than one order of magnitude, but conductivities along three direction of mylonite are significantly different. The conductivity along x-axis and y -axis was higher by about one order and several times, respectively, than that along z-axis. This result suggests the distribution and connectivity of cracks in mylonite are anisotropic and more conductive water than rock matrix connected along x-axis. At focal deep region along a fault, cracks must be compacted, but, if thin sheets of water are distributed anisotropically in mylonite zone, some anisotropic conductivity structure may be detected.