Although several mechanisms of precursory electromagnetic emissions of earthquakes have been proposed, there has been no in situ observation of electromagnetic phenomena occurring at the frictional contacts during rock frictions. From an experiment simulating the motion of an asperity on a fault surface, we report the generation of photon emissions around frictional contacts between natural rock minerals. Spectroscopic analysis clarified that the photon emissions are caused by dielectric breakdown of ambient gases (electric-discharge plasma) due to frictional electrification. The plasmas were found to occur under normal stress of 4 MPa and sliding speed of 10-2 m/s. This indicates that plasma generations do not require high-normal stresses needed to fracture rocks and sliding speeds as high as seismic rates (∼1 m/s). Thus, frictional discharges could occur at microscopic asperities at the onset of the slips of earthquakes, and might be one of the sources of the seismo-electromagnetic emission.
- Frictional discharge plasma
- Pin-on-disk experiment
- Seismo-electromagnetic phenomena