Experimental study of AE from rock under cyclic loading - in relation to the effect of oceanic tide to the earthquake swarm

在1980年的伊豆-汉-东-奥基地震群中，发现了地震活动与海潮之间非常明显的相关性（Mogi, 1983）。为了获得澄清这种潮汐效应的线索，对岩石试样在循环荷载下的声发射进行了实验室测量。由于推断伊豆地区的群震发生在扩展应力场中，采用弯曲法对矩形平行六面体岩石试样施加拉伸型循环加载。循环荷载的幅值逐次增大。实验结果表明，声发射事件在加载的上行和下行过程中均有发生。综上所述，循环荷载作用下声发射事件的发生有两种不同的机制：(a)微裂纹的发育机制和(b)微裂纹表面粘滑机制。实验结果表明，在重复荷载作用下，声发射与自然地震具有一些共同的特征。它们是：(1)活动的峰值并不总是与负荷的最大值一致；(2)声发射和地震事件都发生在特定的应力水平上（声发射和地震族的出现）；(3)在海潮上升过程中震群活度较高，在循环荷载幅值较小的情况下，声发射活度在荷载上升过程中也较高。这些结果为弄清潮汐应力对地震发生的影响提供了线索。

In the case of the 1980 Izu-hanto-toho-oki earthquake swarm, a very clear correlation between the seismic activity and the oceanic tide was found (Mogi, 1983). To obtain a clue to clarify this tide effect, laboratory measurements of AE from rock specimens under cyclic loadings were carried out. Since it is deduced that the swarm-type earthquakes in the Izu occurred in an extension stress field, cyclic loadings of a tensile type were applied to a rectangular parallelepipeds rock sample by a bending method. The amplitude of cyclic loadings were increased at successive intervals.According to this experiments, AE events occur both in the upward and the downward processes of loadings. It is concluded that AE events under cyclic loadings occur by the following two different mechanisms, (a) development of micro-fractures and (b) stick-slip along micro-crack surfaces.The experimental results show that there are some common character istics between AE and natural earthquakes under repeated loadings. They are: (1) The peak of activity dose not always coincide with the maximum point of loadings; (2) Both AE and seismic events occur at specific stress levels (the appearance of AE and earthquake families); (3) The activity in the earthquake swarm is high in the upward process of the oceanic tide, and the AE activity is also high in the upward process of loadings for the case of small amplitudes of cyclic loadings. These results may give a clue to make clear the effect of tidal stress to the earthquake occurrence.