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

在这项科学研究中，我们发展了利用断层粘土中粘土矿物的固有ESR信号进行ESR等时线测年的方法，以评价断层的活动性。这种新的测年方法是基于粘附于粘土颗粒周围的^&lt；238&gt；U和^&lt；22&gt；Th的浓度随颗粒大小而变化的事实，同时利用多个相同年龄和不同大小的粘土颗粒获得断层粘土的等时线年龄。当我们将等时线方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。在本研究中，我们可以通过电感耦合等离子体质谱来判断断裂带中放射性元素的封闭系统。其次，我们把氡异常现象作为一种地震前兆来处理，它直接关系到断裂带的放射性平衡问题。断层粘土样品采集…用原子吸收光谱分析和电感耦合等离子体质谱对野岛-广林野岛断裂露头进行了化学分析，发现了~238&U、~(232)Th和~(208)T；~(208)用电感耦合等离子体质谱(ICPMS)对野岛断裂500m钻心样品进行了分析，从1995年地震中可能活动过的断层粘土附近的断层粘土中检出了异常。这种异常可能来源于地壳深部大量产生的放射性气体。断裂面上断层粘土(宽度0~3 mm)中的部分主次元素Fe_2O_3、Al_2O_3、K_2O_3、K_(133)Cs、~(138)Cs等也有极大的变化。ESR分析表明，断层面上的断层粘土已经受热。这些结果可以通过假设具有特定元素的热流体从Nojima断层更深的位置流出来解释。1997年6月25日山口县北部地震中，震源断层沿线的伽马射线强度高达214&gt；Bi。因此，地震中的氡异常可能与断层粘土成矿有关。较少

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