Electric potential changes associated with nucleation of stick-slip of simulated gouges

Our friction experiments using simulated gouges detected premonitory changes in electric potential before stick-slip events. These precursor electric signals have been detected both in piezoelectric quartz and non-piezoelectric gabbroic gouges. The changes in the potential were locally detected by electrodes set very close to the fault surface associated with slow slip prior to stick-slip events. The magnitude of electrification is proportional to fault slip associated with gradual stress releases, indicating that the electrification is slip-dependent process. From the detailed microstructural analyses on a representative sample, Riedel (R₁) shears known to be formed during stable sliding were the most densely developed around the electrode pair which detected the precursory voltage changes. This indicates that local increases in the voltages were likely caused by frictional electrification due to slow slip on R₁ shears in nucleation phases of stick-slip events. Our experimental results imply that natural faults with thicker gouge zones would require greater precursory slips, resulting in larger electric signals in the nucleation phase of earthquakes.