The fault dating method and the assessment of fault activity using fault clay

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In this scientific research, we have developed the ESR isochron dating method using intrinsic ESR signals of clay minerals in fault clay for the purpose of the assessment of fault activity. This new dating method is based on the fact that the concentrations of ^<238>U and ^<22>Th adhered around clay grains change with grain size, and the isochron age of fault clay is obtained by simultaneously using multiple clay grains with the same age and different sizes. When we actually apply this isochron method to fault clay in a natural fault zone, we must discuss the problem of whether the state of radioactive equilibrium in the fault zone is satisfied. In this research, we can have judged a closed system of radioactive elements in the fault zone by cheking the ratio of ^<238>U and ^<232>Th using ICP-MS.Next, we tackled the radon anomaly phenomenon as an earthquake precuosor which is directly concerned with the problem of radioactive equilibrium in the fault zone. The fault clay sample collect … More ed from the outcrop of the Nojima Fault at Nojima-Hirabayashi, where strong gamma-rays intensity of ^<214>Bi was detected by a survey soon after the 1995 earthquake, was chemically analyzed by atomic absorption spectrophotometry and ICP-MS.Consequently, it has been found from the concentrations of ^<238>U,^<232>Th and ^<208>Pb and their ratios that the radon anomaly along the Nojima Fault may be ascribed to the adherence of radioactive elements and the release of radon with progressive clay mineralization in the fault zone.On the other hand, we analyzed the Nojima Fault 500m drilling core sample by ICP-MS and detected ^<208>Pb anomaly from the fault clay in the immediate vicinity of the fault surface which may have moved in the 1995 earthquake. This ^<208>Pb anomaly may be derived from ^<220>Rn gases generated in large quantities at deep positions of the Earth's crust. Furthermore, parts of major and minor elements such as Fe_2O_3, Al_2O_3, k_<133>Cs and ^<138>Cs in the fault clay (0-3mm in width) just on the fault surface showed extreme variations. ESR analysis made it clear that the fault clay just on the fault surface had been subjected to heating. These results may be explained by supposing that a hot fluid with specific elements flowed out of a deeper position of the Nojima Fault. In the 1997 June 25 Northern Yamaguchi Prefecture Earthquake, high gamma-rays intensity of ^<214>Bi was detected over the lineament along the earthquake source fault. We conclude that the radon anomaly in earthquakes may be concerned with fault clay mineralization. Less