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

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.

Assessment of the Relationship between ESR Signal Intensity and Grain Size Distribution in Shear Zones within the Atotsugawa Fault System, Central Japan

“…ESR spectra in some of the shear zone revealed that the intensity of the radiation induced ESR signals decreased in samples close to the slip plane and increased away from the slip plane (Figure 14). This observation is consisted with the findings of [20] [32] [36]- [38]. [38] reported that at 0 mm from the slip plane, frictional heating could cause the reduction in signal intensity of defect centers in quartz grains.…”

“…This observation is consisted with the findings of [20] [32] [36]- [38]. [38] reported that at 0 mm from the slip plane, frictional heating could cause the reduction in signal intensity of defect centers in quartz grains. Although frictional heating was not investigated in this study, it could not be completely rolled out as a reason for our observation.…”

“…ESR signal intensities measured in relatively fine grain samples showed a decreasing trend as oppose to coarse grain samples taken further from the slip plane. This trend in signal intensity has equally been observed in experimentally sheared samples e.g., [22] [39] [40] [43] and in natural samples e.g., [36] [38] [43]- [45]. This decrease in signal intensity in fine grained samples close to the slip plane could be attributed to two controlling factors; rate of displacement or deformation and the concentration of defect centers in the fine grain samples.…”

High‐velocity frictional strength of Longmenshan fault gouge and its comparison with an estimate of friction from the temperature anomaly in WFSD‐1 drill hole

“…We agree that it is a beautiful foliated gouge zone (Figure 16 of their paper) which must have accommodated many seismic slip events in the past, but none of those features restricts the timing of slip (information is not like resetting of ESR dating signals in the Nojima fault gouge [cf. Fukuchi and Imai , ; Matsumoto et al ., ]). Besides, those features are not shared with the coseismic slip zone on the surface outcrop (Figures and ), where the fault moved during the Wenchuan earthquake, and with about 40 other gouge zones in WFSD‐1 cores (Figure e [ Li et al ., , Figure 8]).…”

ESR Techniques for the Detection of Seismic Frictional Heat

“…ESR signals derived from electrons or holes trapped at lattice defects in paramagnetic minerals commonly decay with time on heating and their decay processes may be expressed by the 1 st , 2 nd order or other kinetic model (Fukuchi, 1989(Fukuchi, , 1992Fukuchi & Imai, 2001;Ikeya, 1993). On the other hand, the chemical kinetics of FMR signals has been studied using the FMR signal of maghemite in the Nojima fault gouge (Fukuchi, 2003;Fukuchi et al, 2005).…”