Electric potential changes associated with slip failure of granite: Preseismic and coseismic signals

Abstract

Electric potential changes were measured for stick‐slip events in granite samples with a three‐block direct shear arrangement at 8 MPa normal stress. Two electrodes were mounted on the left‐ and right‐hand blocks, and the electric potential difference between each electrode and the ground was measured with a high input impedance recording system of frequency range from DC to 100 Hz. As well as coseismic electric signals of about 1.5 V which appeared the moment of the dynamic slip event, preseismic signals were detected just before the slip event. The coseismic signal rises stepwise with opposite polarities at the two electrodes and exponentially decays with a time constant of ε/s, where ε is the permittivity and s is the conductivity of the rock sample. We conducted a simple test of rapid stress drop without slipping and observed almost the same electric signal as the coseismic signal. This suggests that the electric signal is generated by the piezoelectric effect. We proposed a generation model based on the piezoelectric effect and the resultant relaxation process and obtained a theoretical frequency response, which is in agreement with experimental data. The preseismic signal appears about 2–3 s before the dynamic event with an amplitude of about 50 mV. The local strains along two sliding surfaces were also measured to monitor the growth of the rupture nucleation zone. When the growth of the rupture nucleation zone occurred on the left sliding surface, a clear preseismic signal was detected at the electrode mounted on the left granite block. When the growth occurred on the right‐hand surface, a signal was detected at the electrode on the right block. This shows that the preseismic electric signal is caused by stress change in the rupture nucleation zone. These preseismic and coseismic signals were also detected with an antenna, which was placed away from the sample surface.