Mechanism of frictional discharge plasma at fault asperities

The mechanism of seismic-electromagnetic phenomena (SEP) encouraged as precursors of earthquake forecast remains unrevealed. The previous studies reported that the surface charges of the frictional and fractured quartz are enough to cause electric discharge due to the dielectric breakdown of air. To verify the discharge occurrence, friction experiments between a diamond pin and quartz disk were performed under nitrogen gas with a CCD camera and UV-VIS photon spectrometer (e.g., Muto et al., 2006). The photon emission was observed at the pin-to-disk gap only during the friction. The photon spectra obtained from a friction experiment (normal stresses of 13-20 MPa, a sliding speed of 1.0×10^-2 m/s, and a gas pressure of 2.4×10⁴ Pa) showed that the photon was emitted through the second positive band (SPB) system of neutral nitrogen and the first negative band (FNB) system of ionized nitrogen. The estimated potential difference at the gap gave the breakdown electric field and surface charge density on the frictional surface at a gap, where photon was the most intense. These values were enough to cause dielectric breakdown of air. Therefore, the above results demonstrated that frictional discharge could occur on a fault asperity due to dielectric breakdown of ambient gases by frictional electrification. However, the details of electronic transition during the discharge and its type are unknown.
This study discussed the details of the gas pressure dependency for the photon emission intensity and distribution, and the discharge type using the electronic transition theory. Moreover, we compared the surface charge density estimated from the potential difference with that estimated from electron and hole trapping centre concentrations in the frictional quartz subsurfaces measured by electron spin resonance. From this comparison, we also discussed the possibility for the trapping centres to be the sources of the discharge. We could explain the nitrogen gas pressure dependency for the photon emission intensity and vibration temperature observed during our friction experiments using the electron transition theory. For example, Miura et al. (2004) reported that the gas pressure decreases with increasing vibration temperature of the SPB system and the relative intensity in the SPB system to the FNB system. This result showed that the vibration temperature and the relative intensity were about 2800 K and 0.1 during the friction experiment under a pressure of 2.4×10⁴ Pa. The FNB system is related to negative glow charge and the discharge observed during the friction experiments was spark and/or glow discharges. The gas pressure decreases with increasing vibration temperature and molecule density as shown in several previous studies and decrease with increasing electron temperature and density as explained the electron transition theory. This implies that the increase in the free path of excited molecules as gas pressure decreases can result in the photo emission pattern change. The surface charge density of a frictional quartz surface estimated from the potential difference to be 5.5×10^-5 C/m² included in the range of 6.51×10^-6–6.4×10^-3 C/m² estimated from the trapping centre concentrations. Hence, the trapping centres can be the sources of the frictional discharge.