ESR and ICP analyses of the DPRI 500 m drill core samples penetrating through the Nojima Fault, Japan

Abstract

Abstract The Disaster Prevention Research Center, Kyoto University (DPRI) 500 m drilling core samples from the Nojima Fault, which caused the 1995 Hyogo‐ken Nanbu earthquake in Japan, were analyzed using electron spin resonance (ESR) and inductively coupled plasma (ICP). The fault gouge of the Nojima Fault zone, measuring approximately 50 mm in width, is located at a depth of about 388.4 m from the surface along the 500 m core samples. Electron spin resonance measurements of the fault gouge have revealed that a quartet ESR signal in montmorillonite and the hyperfine structure of an Al center in quartz were reduced to 31–36% of the initial intensity by heating, while an E′ center in quartz, which is often used for Quaternary fault dating, had been reduced to only 70% of the initial value. According to a computer simulation on the decay of ESR signals by frictional heating, the ESR intensity pattern measured in the fault gouge does not coincide with the calculated pattern for dry faults with a planar slip surface because the temperature on the fault plane must rise above the melting point of the fault blocks. However, for fault gouge containing pore water, the temperature in the fault gouge cannot rise above the boiling temperature of water, approximately 310–370°C under a pore water pressure of 10–20 MPa. If pore water in the fault gouge was boiled to approximately 350°C by frictional heating and the boiled water diffused into the fault gouge, the ESR intensity pattern could be explained. Alternatively, the ESR intensity of Fe3+ increases in the dark gray fault gouge and is consistent with ICP data. This result is also consistent with the increased magnetic susceptibility in the fault gouge. Furthermore, using inductively coupled plasma mass spectrometry (ICP‐MS), a 208Pb anomaly was detected in the fault gouge, which can be probably explained by the generation of 220Rn gases in the Earth’s crust. This 208Pb anomaly means that the fault gouge zone had been in radioactive disequilibria.