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

本研究利用断层粘土中粘土矿物的固有ESR信号，建立了评价断层活动性的ESR等时线测年方法。这种新的测年方法是基于粘粒周围的~（13）<238>U、~<22>（13）Th浓度随粒度的变化而变化的事实，利用多个相同年龄、不同粒度的粘粒同时测定断层粘粒的等时线年龄。当我们把这种等时线法实际应用于天然断裂带中的断层粘土时，必须讨论断裂带中的放射性平衡状态是否满足的问题。本研究利用ICP-MS测定断裂带中的~（13）U/~（13）Th比值，判断断裂带中存在一个放射性元素的封闭体系<238><232>。其次，探讨了断裂带中氡异常现象作为地震前兆，它与断裂带的放射性平衡问题直接相关。断层泥样品采集 关于我们 1995年大地震后，在野岛-平林的野岛断层露头处测得了强γ射线强度的^<214>Bi，用原子吸收分光光度法和电感耦合等离子体质谱（ICP MS）法对该断层的艾德进行了化学分析，结果表明，从^<238>U、^<232>Th和^<208>Pb的浓度及其比值来看，沿沿着野岛断层的氡异常可能是由于放射性元素的吸附和断层带粘土矿化的进行性氡释放所致。利用ICP-MS分析了野岛断层500米钻孔岩芯样品，在<208>1995年地震中可能发生移动的断层面附近的断层粘土中检测到~（13）Pb异常。这种^<208>Pb异常可能来源于<220>地壳深部大量生成的^ Rn气体。在断层泥（0- 3 mm宽）中，Fe_2O_3，Al_2O_3，k_ Cs和γ Cs等部分常量和微量元素<133><138>在断层面上表现出极大的变化。ESR分析表明，断层泥就在断层面上，受到了加热。这些结果可以通过假设具有特定元素的热流体从野岛断层的更深位置流出来解释。在1997年6月25日的北方山口县地震中，在<214>沿震源断层沿着的线性构造上探测到了高γ射线强度的^ Bi。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