Complex resistivity of fault gouge and its significance for earthquake lights and induced polarization

We have measured complex resistivity of 2 water-saturated San Andreas fault gouges from 10⁻³ to 10⁶ Hz and confining pressures of 0.2 to 200 MPa. Consistent with earlier observations of clays and common rocks, large low-frequency permittivities were observed in all cases. Comparisons were made to induced polarization (IP) measurements by inversion of the data into the time domain, where we found that principal features of the IP response curves were due to these large low-frequency permittivities. The results also suggest that following large earthquakes, significant electrical charge could remain for many seconds and could result in a variety of reported electromagnetic effects.