Estimation of surface charge densities supporting seismo-electromagnetic phenomena through the surface charging mechanism

The mechanism of seismo-electromagnetic phenomena (SEP) related to electron and hole trapping centres in quartz has been discussed by estimating surface charge densities involved in the SEP. However, a few studies have estimated the densities from direct measurements of trapping centres in quartz produced by rock fracture and friction experiments. In this study, we estimated surface charge densities in quartz grains of simulated fault gouges produced by friction using electron spin resonance (ESR) for the gouges and a standard material with a known concentration of a trapping centre. We also estimated surface charge densities in fractured quartz grains from the previous data of the trapping centre concentrations obtained by ESR or a radical scavenger. The values in the fractured and sheared quartz were estimated as 6.51×10⁻⁶–6×10⁻¹C/m². These quantities are roughly enough to cause a corona discharge that can be a SEP source. Positive and negative charges are considered to be released from trapping centres and form electric dipoles generating electric and magnetic fields. Therefore, the trapping centres, especially the E₁^′ centre and the NBOHC, can be some of the main SEP sources. These values are divided into two groups depending on the amounts of the surface charge. The trapping centres decrease with time due to their chemical stability or chemical reactions with the air composition. Hence, we consider that the difference exists because the trapping centres are measured during, immediately after, or after a long period since fracture events. From this point of view, we propose two new SEP sources.