ESR analysis of the Nojima fault gouge, Japan, from the DPRI 500 m borehole

Electron spin resonance (ESR) analyses of quartz grains in fault gouge were performed for a core sample taken from the Nojima Fault that moved during the 1995 Kobe earthquake (Hyogo-ken Nanbu earthquake). Distribution of radi ation-induced defects in the gouge at a depth of 389.4 m was measured by ex tracting quartz grains from seven discrete positions within 30 mm of the fa ult plane on the granite side. The decrease in E'(1) and Al centers was obs erved within 2 mm of the fault plane, suggesting partial annealing due to f aulting. Partial annealing even at that depth suggested that conventional E SR dating, which is based on the hypothesis of complete annealing during fa ulting, was not applicable. Theoretical calculations of the temperature ris e and of the thermal annealing of defects have been made by assuming a simp le annealing model of heat generation on the fault plane. Thermal energy wa s calculated to have been approximately 8 MJ/m(2) to explain the profile of the heat-affected region. Thermal energy was much larger than the one esti mated from hydrothermal solution, and corresponded to the frictional heat c alculated for a normal stress of 20 MPa, a displacement of 2 m, and a frict ional coefficient of 0.2.