Measurement of AE and electric potential changes of rock samples under cyclic loading

有一些报道说，在大地震发生之前，已经观察到地电势的变化和地电磁波的异常辐射。如果这些现象是地震前兆，它们可能对地震预报有用。因此，弄清地电势变化的机理是一个重要的问题。在以往的研究中，与岩石破裂有关的电势测量都是在实验室压应力单调递增的情况下进行的。在本实验中，测量了岩石试样在循环荷载作用下的声发射和电势。矩形花岗岩试样在2300次弯曲型循环加载后发生断裂。实验结果总结如下：(1)电位信号与声发射事件同时发生。(2)震波和声发射在主破裂前迅速增加。(3)振幅较大的EP信号与较大的AE事件同时发生。(4)声发射和电位主要发生在应力增大阶段，在每个循环中均达到最高水平。根据我们以往的研究，新的裂缝出现和扩展是在应力增大和达到最高水平的阶段。因此，目前的研究结果表明，EP是由新裂纹的产生和扩展引起的。大地电位在主破裂前迅速增加的结果表明，地电位可能是一种有用的前兆现象。

There are a number of reports that changes in geoelectric potential and anomalous radiation of geoelectromagnetic waves have been observed befor major earthquakes. If these phenomena are earthquake precursors, they may be useful for earthquake prediction. Therefore, clarification of the mechanism of changes in geoelectric potential is an important problem.In previous studies, measurements of electric potential related to rock fracture have been carried out under the monotonously increasing compressive stress in laboratory. In the present experiment, AE and electric potential from a rock sample were measured under the cyclic loading. A rectangular granite sample ruptured after 2300 cyclic loading of bending type.The experimental results are summarized as follows : (1) Electric potential signals (EP) occur simultaneously with AE events. (2) EP and AE rapidly increase just before the main rupture. (3) EP signals with large amplitude occur together with large AE events. (4) AE and EP mainly occur in the stage in which the stress is increasing and reaches at the highest level in each cycle.According to our previous study, new cracks appear and extend in the stage in which the stress is increasing and reaches at the highest level. Therefore, the present result shows that EP occurs by the generation of a new crack and its extension.The result that EP rapidly increase just before the main rupture suggests that the geoelectric potential may be useful as a precursory phenomena.