Carbon deposition during brittle rock deformation: Changes in electrical properties of fault zones and potential geoelectric phenomena during earthquakes

Abstract

To investigate potential mechanisms for geoelectric phenomena accompanying earthquakes, we have deformed hollow cylinders of Sioux quartzite to failure in the presence of carbonaceous pore fluids and investigated the resulting changes in electrical conductivity and carbon distribution. Samples were loaded at room temperature or 400°C by a hydrostatic pressure at their outer diameter, increasing pressure at a constant rate to ∼290 MPa. Pore fluids consisted of pure CO, CO2, and CH4 and a 1:1 mixture of CO2 and CH4, each with pore pressures of 2.0 to 4.1 MPa. Failure occurred by the formation of Mode II shear fractures transecting the hollow cylinder walls. Radial resistivities of the cylinders fell to 2.9 to 3.1 MΩ‐m for CO tests and 15.2 to 16.5 MΩ‐m for CO2:CH4 tests, compared with >23 MΩ‐m for dry undeformed cylinders. Carbonaceous fluids had no discernable influence on rock strength. On the basis of mapping using electron microprobe techniques, carbon occurs preferentially as quasi‐continuous films on newly formed fracture surfaces, but these films are absent from preexisting surfaces in those same experiments. The observations support the hypothesis that electrical conductivity of rocks is enhanced by the deposition of carbon on fracture surfaces and imply that electrical properties may change in direct response to brittle deformation. They also suggest that the carbon films formed nearly instantaneously as the cracks formed. Carbon film deposition may accompany the development of microfracture arrays prior to and during fault rupture and thus may be capable of explaining precursory and coseismic geoelectric phenomena.