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

在这项科学研究中，我们开发了利用断层粘土中粘土矿物的本征ESR信号进行断层活动性评价的ESR等时线测年方法。该测年方法基于粘粒周围黏附的^<238>U和^<22>Th浓度随粒径的变化而变化，并通过同时使用多个相同年龄、不同粒径的粘粒获得断层粘土的等时年龄。当我们实际将等时线方法应用于天然断裂带的断层粘土时，必须讨论断裂带的放射性平衡状态是否满足的问题。在本研究中，通过ICP-MS检测U <238>和Th <232>的比值，可以判断断裂带放射性元素的封闭系统。接下来，我们将氡异常现象作为地震前兆进行了讨论，这直接关系到断裂带的放射性平衡问题。在1995年地震后不久的一次调查中，在野岛-平林野岛断裂带露头处检测到强烈的伽马射线强度为^<214>Bi，用原子吸收分光光度法和ICP-MS对其进行了化学分析。因此，从^<238>U、^<232>Th和^<208>Pb的浓度及其比值分析发现，野岛断裂带氡异常可能是由于放射性元素的附着和随着断裂带粘土矿化的进展而释放的。另一方面，利用ICP-MS对野岛断裂带500m钻孔岩心样品进行分析，在断裂带附近可能在1995年地震中移动过的断层粘土中检测到^<208>Pb异常。这个^<208>的Pb异常可能来源于地壳深处大量产生的^<220>的Rn气体。此外，断层表面（宽度0 ~ 3mm）断层粘土中Fe_2O_3、Al_2O_3、k_<133>Cs和^<138>Cs等部分主、次元素含量变化较大。ESR分析表明，断层粘土只是在断层表面受到了加热。这些结果可以通过假设含有特定元素的热流体从野岛断层的较深位置流出来解释。在1997年6月25日山口县北部地震中，沿震源断层的剖面上探测到^<214>Bi的高伽马射线强度。认为地震氡异常可能与断层粘土矿化有关。少

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