Voltage changes induced by stick-slip of granites

Tri-axial compression tests of dry granites with a pre-cut shear surface were conducted with an electrode to detect electrical potential. The samples were shaped 19.5 mm in diameter and 60 mm in length, and deformed at a strain rate of 10⁻³ s⁻¹ and confining pressure of 78.5 MPa. Pulse-like electrode voltage changes were detected several times just before (0.5-5 ms) stick-slip motion during compression. Magnitudes of stick-slips (e.g. normal stress drop of 5-160 MPa) and intensities of electrode voltage changes (0.1-10 V) increased whenever stick-slip occurred. Here we propose some scaling laws on the number of stick-slips and relate those statistics to the intensity of the electrical phenomena. Maximum electrode voltage change is proportional to normal stress drop that depends on real contact area of the slip plane. Local charge distributions, fractoemission or both may due to failure of asperities just before stick-slip.